P olicy discussions around issues such as border security, drug trafficking, and the opioid epidemic often involve questions about illicit drug flows into the United States. For instance, while U.S. border officials are charged with facilitating the lawful flow of people and goods, they are also responsible for stopping unauthorized entries and preventing illicit drugs and other contraband from being smuggled into the country. Border security policy debates include questions of how to balance sometimes competing priorities and allocate finite border enforcement resources to respond to various threats. For example, some have questioned where to place border enforcement and drug detection resources to best target the flow of illicit opioids such as heroin, fentanyl, and synthetic opioid analogues being smuggled into the United States. Available data that can help policymakers understand how illicit drugs are trafficked into the United States are often estimated, incomplete, imperfect, or lack nuance. And debates about drug flows and how best to counter drug trafficking into the country often rely on selected data on drug seizures by border officials. This report provides a brief discussion of what data are and are not available to help understand the universe of illicit drugs produced globally as well as what data are and are not available to indicate how much of the illicit drugs produced are destined for and trafficked into the United States. The report illuminates available data on illicit drug seizures by U.S. border officials and discusses potential implications of using these data to inform U.S. policy on drug trafficking into and within the country. One way of conceptualizing the flow of illicit drugs into the United States is as a funnel. At the top of this funnel is the universe of illicit drugs produced around the world. These drugs generally fall into two categories: plant-based (e.g., cocaine, heroin, and marijuana) and synthetic (e.g., methamphetamine and fentanyl). Although some illicit drugs are produced in the United States, many originate elsewhere and are smuggled into the country. See Figure 1 for a depiction of the illicit drug supply chain. The illicit supply chain for plant-based drugs ultimately destined for the United States begins in the agricultural fields of cash crop farmers. These farmers cultivate coca bush, opium poppy, and cannabis plants in locations that are often remote, politically unstable, or insecure. Potential cultivation and its measurement are affected by a variety of factors. For instance, illicit drug crop productivity varies with each harvest and in each location where the crops are grown; it can be dependent on a mix of factors that include weather, plant disease, soil fertility, field maturity, and farming techniques. There are also factors that limit officials' and analysts' abilities to detect, measure, and obtain comprehensive data on the universe of illicit drugs. For example, where ground-based measurements of the crop fields are impractical, analysts rely on satellite imagery of varying picture quality to estimate the amount of land used for illicit crop cultivation. These estimates can be hampered by cloud cover and techniques to obscure the true scale of cultivation (e.g., interspersing illicit crops between legitimate crops, cultivating smaller plots in new locations). While coca bush and opium poppy crop surveillance programs are ongoing in most major source countries, they do not capture all global cultivation. And, in the case of drug crops that can be cultivated indoors or grown in small amounts (such as cannabis), cultivation estimates are often unreliable or unavailable. Moreover, due to changes in survey methodologies and in the areas surveyed, cultivation estimates may not be directly comparable over time. Satellite imagery-based crop survey data are coupled with information derived from crop yield studies, drug processing efficiency tests, and government-reported eradication totals to arrive at estimates of illicit drug production. Where reported eradication cannot be independently verified, such data can be prone to errors. In addition, variations in the process of refining illicit crops into finished products introduce a host of variables that limit the accuracy of drug production estimates. The U.S. Department of State notes "differences in the origin and quality of the raw material and chemicals used, the technical processing method employed, the size and sophistication of laboratories, the skill and experience of local workers and chemists, and decisions made in response to enforcement pressures all affect production." Ultimately, drug production estimates are calculated in terms of "potential pure" illicit drugs by volume, which assumes that all harvested illicit drug crops are converted into illicit drugs, though this assumption may not hold in all circumstances. In Asia, for example, where opium poppy is often consumed as opium rather than processed further into heroin, the State Department acknowledges that the proportion of opium ultimately processed into heroin is "unknown." At each stage in the illicit drug development cycle, added variables further complicate the ability of analysts to accurately estimate the true amount of illicit drugs produced. Unlike plant-based drugs, whose cultivation footprint can provide a starting point for estimating potential drug production, the illicit supply chain for synthetic drugs ultimately destined for the United States begins in chemical manufacturing and pharmaceutical facilities. Although the import and export of some chemical inputs (precursors) used in illicit synthetic drug production are internationally regulated, others are notâand the trade data for such chemicals are not necessarily current, available for all countries, or indicative of diversion trends. For example, the Combat Methamphetamine Epidemic Act of 2005 (CMEA; Title VII of P.L. 109-177 ) requires the State Department to conduct annual economic analyses on global production of and demand for three precursor chemicals commonly used in the production of methamphetamine, but its efforts have been hampered by data limitations. The State Department has noted that "[e]phedrine and pseudoephedrine pharmaceutical products are not specifically listed chemicals under the 1988 U.N. Drug Convention. Therefore, reporting licit market trade and demand for ephedrine and pseudoephedrine as well as pharmaceutical products derived from them is voluntaryâ¦. Thus far, the economic analysis required by the CMEA remains challenging because of outdated, insufficient, and unreliable data." Challenges in acquiring and analyzing relevant data on synthetic drug production and precursor chemicals used in illicit drug production are further compounded by the proliferation of new psychoactive substances (NPS)âmolecularly altered variants, or synthetic analogues, of known illicit substances that are not internationally controlled and thus designed to avoid detection by authorities. NPS also include fentanyl analogues destined for the United States. Law enforcement authorities around the world have reported to the United Nations more than 850 uncontrolled NPS as of the end of 2018. The next step in the supply chain of illicit drugs produced abroad and destined for the United States is the transit of these substances toward and into the country, as depicted in Figure 1 . The United States, while a major consumer of illicit drugs, is just one of many drug consumption markets. Of the illicit drugs that are produced around the world, some may be consumed in the country of production, some may be destined for the United States, and some may be intended for an alternate market. Of those drugs intended to be moved to the United States, some may become degraded or lost in transit, some may be seized by law enforcement or otherwise destroyed or jettisoned by traffickers pursued by enforcement officials, and some reach the U.S. border. The challenge of estimating drug flows in transit is a longstanding one. While there are estimates of certain types of illicit drugs produced in certain countries that are subsequently bound for the U.S. market, there is not a comprehensive publicly available dataset detailing the estimated amount of each type of illicit drug produced in each source country that is suspected to be destined for the United States. However, snapshots of these data exist. One of these datasets is the Consolidated Counterdrug Database (CCDB), managed by the Office of the U.S. Interdiction Coordinator. According to the U.S. Government Accountability Office (GAO), the CCDB "records drug trafficking events, including detections, seizures, and disruptions. The database is vetted quarterly by members of the interagency counterdrug community to minimize duplicate or questionable reported drug movements." Specifically, it records drug trafficking events, which helps provide estimates on illicit drugs, particularly cocaine, destined for the United States via the transit zone from South America. Of the unknown total amount of drugs that reach the U.S. border by land, air, or sea, some portion is seized by border officials, and some portion makes its way into the country. While the pro portion of illicit drugs coming into the country that are seized at the border is unknowable, the amount of illicit drugs seized is. It is this snapshot of seizure data that has served as a point of reference for current policy debates surrounding border security and drug flows into the country. There are no exact data on the total quantity of foreign-produced illicit drugs flowing into the United States. Indeed, a fundamental element to understanding drug smuggling is the acknowledgement that the total flow of drugs crossing the borderâat and between ports of entry (POEs) âinto the United States is unknowable. As reflected in Figure 1 , as illicit drugs are brought to the border of the United States, they generally fall into two initial categories: drugs that are detected and seized by officials at the border, and drugs that, whether detected or not, are not seized by officials at the border. Illicit drugs that are detected and seized at the border during inbound inspections are quantifiable. Those drugs that are not seized at the border are generally not quantifiable at the time they enter the country. However, some portion of illicit drugs successfully smuggled across the border may later be seized by law enforcement officers. The largely unknown subset of foreign-produced drugs that enter the country but are not seized by officials during inbound inspections at the border is divided into two categories: drugs that are later detected and seized by federal, state, local, or tribal officials; and drugs that, whether detected or not, are not seized by officials. Illicit drugs not seized at the border enter the United States where there are also domestically produced drugs. As such, drugs that are later seized by federal, state, local, or tribal officials in the United States may be of foreign or domestic origin. These drugs may be seized in the interior of the country or by border officials conducting outbound inspections of people and goods leaving the country. In the absence of data on the flow of all illicit drugs entering the United Statesâboth those that are seized and those that successfully evade enforcement officialsâpolicymakers can use certain drug seizure data to better understand how and where drugs are crossing U.S. borders. While a number of agencies may be involved in seizing illicit drugs in the border regions, the primary agency charged with safeguarding the U.S. border (including seizing illicit drugs and other contraband) is U.S. Customs and Border Protection (CBP). Within CBP, the Office of Field Operations (OFO) is responsible for staffing POEs, and drugs seized by OFO are generally seized at POEs . In addition, the Border Patrol is responsible for patrolling the land borders with Mexico and Canada, and the coastal waters surrounding Florida and Puerto Rico; given its responsibilities, drugs seized by the Border Patrol are generally drugs seized between POEs . CBP publishes selected enforcement statistics, including a snapshot of illicit drug seizuresâof marijuana, cocaine, methamphetamine, heroin, and fentanylâby OFO and the Border Patrol. CBP data indicate that larger quantities by weight of cocaine, methamphetamine, heroin, and fentanyl are seized at POEs than between the ports. Figure 2 illustrates seizures of these four drugs by OFO and the Border Patrol for FY2012âFY2018. Cocaine. From FY2012 to FY2018, CBP reported seizing 388,970 pounds of cocaine at and between POEs. OFO seized 86.1% of this cocaine at POEs, and the Border Patrol seized the remaining 13.9% between POEs. Methamphetamine. From FY2012 to FY2018, CBP reported seizing 266,828 pounds of methamphetamine at and between POEs; 82.2% was seized at POEs and the remaining 17.8% between POEs. Of note, the amount of methamphetamine seized by CBP increased more than three-fold, from 17,846 pounds in FY2012 to 67,676 pounds in FY2018. The consistent increase in methamphetamine seizures during this period was seen both at and between POEs. Heroin. From FY2012 to FY2018, CBP reported seizing 35,193 pounds of heroin at and between POEs. OFO seized 88.0% of this heroin at POEs, and the Border Patrol seized the remaining 12.0% between POEs. Fentanyl. CPB started reporting fentanyl seizures by OFO in FY2015 and by the Border Patrol in FY2016. From FY2015 to FY2018, CBP seized 5,000 pounds of fentanyl at and between POEs; 85.5% was seized at POEs and the remaining 13.5% between POEs. Fentanyl seizures increased from the 70 pounds seized by OFO in FY2015 to 2,173 pounds seized across OFO and the Border Patrol in FY2018. Mar i juana. The landscape for CBP marijuana seizures is different than that for the four drugs discussed above. Whereas intelligence and seizure data indicate that most of these four drugs are moved through the legal POEs, a greater quantity of illicit foreign-produced marijuana is smuggled and seized between the ports (see Figure 3 ). From FY2012 to FY2018, CBP reported seizing 14,023,570 pounds of marijuana at and between POEs. The Border Patrol seized 77.1% of this marijuana between POEs, and OFO seized the remaining 22.9% at the ports. Marijuana seizures dropped from over 2.8 million pounds in FY2012 to 761,319 pounds in FY2018. The bulk of this decline can be seen in Border Patrol seizures, which fell from 2.3 million pounds in FY2012 to 461,030 pounds in FY2018. In current discussions of border security, policymakers and the media have relied on this snapshot of regularly published CBP data on seizures of certain illicit drugs (cocaine, methamphetamine, heroin, fentanyl, and marijuana) at and between POEs. While these data provide a summary view of certain CBP drug seizures and indicate generally where certain types of illicit drugs are most often seized by border officials, CBP's dataset that is the foundation for this regularly updated snapshot of seizure data provides a more nuanced view. For instance, the foundational seizure data provide additional information such as the type of POE (e.g., land, air, sea) where drugs were seized and whether the drugs were seized during inbound inspections, outbound inspections, or in operations away from the POEs. Specifically, CRS analysis of OFO drug seizure data from FY2014 to FY2018 indicate that across those five years, about 65% of seized illicit drugs by weight were confiscated at land POEs. In addition, about 28% of seized drugs were confiscated at air POEs, and about 5% were seized at sea POEs (see Figure 4 ). In addition, CRS analysis of OFO drug seizure data from FY2014 to FY2018 indicate that nearly 97% of seized drugs were confiscated during inbound inspections across those years. While nearly all OFO illicit drug seizures occur during inbound inspections, some are seized during outbound inspections of people and goods exiting the country, some may be seized at a POE but cannot be attributed to an inbound or outbound inspection, and some may be seized during enforcement activities occurring away from official POEs (see Figure 5 ). The enforcement statistics that CBP publishes on its website regarding seizures of cocaine, methamphetamine, heroin, fentanyl, and marijuana do not always distinguish between seizures at northern, southern, and coastal border areas. However, officials have noted that "most illicit drug smuggling attempts occur at southwest [border] land POEs." Consistent with this testimony, CRS analysis of OFO drug seizure data indicates that, on average, over 65% of the illicit drugs seized by OFO from FY2014 to FY2018 were seized during inbound inspections at land POEs within the jurisdiction of the OFO field offices along the Southwest border. CBP is not the only agency that seizes illicit drugs in the United States or even in the border regions. Federal, state, local, and tribal law enforcement agencies are all involved in enforcement actions thatâeven if not focused on drug-related crimesâmay involve drug seizures. Notably, there is no central database housing information on illicit drug seizures from all law enforcement agencies. In addition, there is not a set of discrete, yet comprehensive, drug seizure datasets that, if combined, could tally illicit drug seizures for all of the United States. Rather, there are a number of datasets and systems that contain some information on drug seizures. For instance, law enforcement agencies have case management systems, and case files may have certain information on drug seizures. However, this information may or may not exist in electronic format, and may or may not consistently appear in dedicated data fields that allow agencies to sort and tally drug seizure data. In addition, law enforcement case information, including that on drug seizures, may change throughout the course of an investigation, and there is always a chance that case management systems may not be updated to reflect final information, including results of forensic lab tests, on the drugs seized. For instance, an initial report on a case may contain estimates of quantities of drugs seized as well as suspicions or results from preliminary field testing regarding drug types involved. This information could all change as a case progresses and any drugs seized are more thoroughly measured and chemically analyzed. In addition, the data that are available from law enforcement agencies throughout the United States provide imprecise insight into illicit drug smuggling into the country. Foreign-produced illicit drugs that cross the border into the United States without being seized enter the U.S. market along with domestically produced drugs; as such, seizure data from law enforcement agencies across the country may not in and of itself provide information as to the drug's source countryâand thus cannot always add to an understanding of drug trafficking into the United States. This may be particularly so for marijuana, which has seen increased domestic cultivation coupled with decreased Mexican production and trafficking into the United States. As border officials have noted, CBP seizure data include illicit drugs not just from inbound inspections of goods and people entering the country but from outbound inspections as well. In addition, there is a set of seizures for which it cannot be determined whether the intended flow of drugs seized was into, within, or out of the country. While most drugs flowing across U.S. borders may be coming into the country, some unknown portion of drugs crossing the borders are leaving the country. Drugs leaving the country include those produced in the United Statesânamely marijuanaâas well as drugs that pass through in transshipment. Despite an acknowledged imprecision in the completeness, accuracy, and nuance of seizure data, some systems can provide selected information on illicit drugs seized in the United States. National Seizure System (NSS). The DEA runs the NSS through the El Paso Intelligence Center (EPIC). This system allows law enforcement entities to submit data on illicit drug seizures around the country. Certain federal law enforcement agencies (DEA, FBI, CBP, ICE, and Coast Guard) are required to report drug seizures that surpass certain threshold levels, but reporting by other law enforcement agencies is voluntary. As such, while the NSS contains mandatory reported data on drug seizures of certain sizes made by specific federal agencies as well as other voluntarily reported drug seizure data, this reflects only a subsetâand unknown proportionâof total illicit drugs seized across the country. Nonetheless, these seizure data can provide officials with information on the location and magnitude of seizures to help build knowledge of the U.S. illicit drug market, drug trafficking activity in the country, and enforcement strategies. National Forensic Laboratory Information System (NFLIS). The DEA runs the NFLIS, which "collects results of forensic analysis, and other related information, from local, regional, and national entities." One component of NFLIS is NFLIS-Drug, which collects drug chemistry analysis results from "50 State systems and 104 local or municipal laboratories/laboratory systems, representing a total of 283 individual laboratories." Currently, the NFLIS reports on the number of drug cases submitted to laboratories for testing as well as the number of distinct drug reports made from those cases. It does not report on the total quantity of drugs seized that are associated with those samples submitted for chemical testing. Because the NFLIS records drug reports from specific labs around the country, it is possible for law enforcement and analysts to gain a better understanding of trends in drug reports involving certain drugs or substances in certain areas of the United States. As discussed above, the quantities of illicit drugs produced in various countries around the world that are destined for the United States and that are successfully smuggled into the country are unknown, and are likely unknowable. Instead, U.S. officials look at the set of illicit drugs seized in the United States and, in conjunction with drug intelligence, produce estimates of which countries are the major suppliers of certain types of illicit drugs found in the United States. In formulating these estimates, officials submit samples from selected seizures of illicit drugs for chemical testing and analysis. For certain illicit drugs seized in the United States, this chemical analysis helps determine, among other things, the primary source countries and/or methods of production. The chemical testing reveals different information about plant-based drugs than it does about synthetic drugs. Heroin. The DEA operates a heroin signature program (HSP) and a heroin domestic monitor program (HDMP) that helps identify the geographic source of heroin found in the United States. Chemical analysis of a given heroin sample can identify its "signature," which indicates a particular heroin production process that has been linked to a specific geographic source region. The HSP analyzes wholesale-level samples of "heroin seized at U.S. ports of entry (POEs), all non-POE heroin exhibits weighing more than one kilogram, randomly chosen samples, and special requests for analysis" and the HDMP assesses the signature source of retail-level heroin samples seized in the United States. Of the heroin analyzed in the HSP, 86% was identified as originating from Mexico, 10% had inconclusive results, 4% was from South America, and less than 1% was from Southwest Asia in 2016. Cocaine. The DEA's Cocaine Signature Program (CSP) analyzes cocaine samples from bulk seizures for "evidence of how and where the coca leaf was processed into cocaine base (geographical origin), and how cocaine base was converted into cocaine hydrochloride (processing method)." Analyses of cocaine samples seized in 2017 indicate that 93% originated in Colombia, 4% originated in Peru, and 3% had an unknown origin. Methamphetamine. The DEA's methamphetamine profiling program (MPP) examines methamphetamine samples to help determine trends in production methods. The DEA notes, however, that because methamphetamine is synthetically produced, the MPP cannot determine the original source of the drug. Domestic production of methamphetamine commonly involves pseudoephedrine/ephedrine tablets along with household items like lithium batteries, camp fuel, starting fluid, and cold packs. In contrast, Mexican criminal networks "produce methamphetamine using the reductive amination method, which uses the precursor, Phenyl-2-propanone (P2P) instead of pseudoephedrineâ¦. According to the DEA MPP, 97 percent of samples analyzed were produced using the reductive amination method, using P2P as the precursor chemical." This implies that most of the methamphetamine samples analyzed in the MPP were produced using techniques employed by Mexican criminal networks. Fentanyl. The DEA also has a Fentanyl Signature Profiling Program (FSPP), analyzing samples from fentanyl seizures to help "identify the international and domestic trafficking networks responsible for many of the drugs fueling the opioid crisis." The DEA has indicated that fentanyl shipped directly from China often has purity levels above 90%, while fentanyl trafficked over the Southwest border from Mexico has purity levels below 10% on average. However, it is unclear how much of the fentanyl consumed in the United States is coming directly from China versus Mexico. In the absence of comprehensive and precise data on illicit drugs trafficked into the United States, seizure data can provide some insight into various elements of drug flows such as smuggling points into the United States and target markets within the country. For instance, some have relied on selected border seizure data to help understand the locations at which federal enforcement efforts are stopping a portion of the illicit drugs produced abroad from entering the country and joining the domestic drug market. In current policy discussions regarding border security, CBP drug seizure data can help inform policy decisions that involve the most effective placement of counterdrug resources. In addition, drug seizuresâboth at the border and in the interior of the countryâthat are chemically analyzed can provide information on the likely geographic sources of certain illicit drugs found throughout the United States. Policymakers may ask a variety of questions as they debate how to target finite resources to countering illicit drug flows, including which types of illicit drugs are of the highest concern, what are the means traffickers most often employ to smuggle illicit drugs into and throughout the United States, and where can enforcement officials interdict the greatest quantity of top-priority illicit drugs? Border seizure data can also help inform efforts to act on certain policy priorities. If, for example, lawmakers and enforcement officials are particularly concerned with specific categories of illicit drugs such as illicit opioids, they may examine the sufficiency of existing enforcement efforts in the areas where intelligence and seizure data indicate that the flow of these substances may be the highest. For instance, the most recent DEA National Drug Threat Assessment notes that illicit opioids such as heroin are more often smuggled through than between POEs; likewise, CBP seizures of these substances have also been higher at the ports than between them, as reflected in greater seizures of illicit opioids by OFO than by the Border Patrol. As such, in order to counter threats posed by illicit opioids, and in balancing other law enforcement and counterdrug priorities, Congress may consider whether CBP should maintain or change the amount and types of resources allocated to screening for and interdicting illicit drugs at and between POEs. Notably, as reflected in Figure 1 , a focus on border seizures largely excludes a discussion of drug seizures by law enforcement officials throughout the interior of the country. As such, border seizures cannot speak to drug transportation and distribution throughout the U.S. market or law enforcement priorities in the interior of the country. A focus on border seizures also largely excludes a discussion of illicit drugs that are produced domestically. This is, in part, because border seizures largely reflect drugs detected during inbound inspections (and thus are more likely to reflect foreign-produced drugs being moved into the United States). However, drugs detected and seized during outbound inspections may reflect both foreign-produced drugs that were not seized when they entered the country as well as domestically produced drugs being taken out of the country. If policymakers are interested in having a more comprehensive view of drug seizures throughout the United States, they could move to enhance and consolidate data collection. With respect to federal agencies, Congress could take a variety of steps to enhance data availability on drug seizures, both at the border and in the interior of the country. As noted, the NSS at EPIC contains data on drug seizures of certain sizes by specific federal agencies, as well as additional voluntary reports from additional law enforcement entities, but these data are not comprehensive. Lawmakers could ask GAO to conduct a study on agencies' collection and reporting of drug seizure data; this could provide a better understanding of the portion of drug seizures currently included in the NSS. Another option is that Congress could require that all federal law enforcement agencies report information on a greater portion ofâor allâdrug seizures to a central database like the NSS. Congress could also direct the NSS to enhance outreach to state and local law enforcement agencies to encourage them to submit drug seizure data. Yet another option would be for policymakers to incentivize statesâfor example, by providing or withholding grant fundingâto collect and report such data to help establish a more robust view of seizures in the United States. Enhanced data on drug seizures away from the border may not illuminate how these drugs entered the country; however, these data could help provide a more nuanced picture of the domestic drug market. To counter threats at U.S. borders, the Department of Homeland Security (DHS) uses a risk management approach, which the department defines as "the process for identifying, analyzing, and communicating risk and accepting, avoiding, transferring, or controlling it to an acceptable level considering associated costs and benefits of any actions taken." Border threats are continually evolving and include those posed by a wide range of actors, from terrorists who may have weapons of mass destruction and transnational criminals smuggling drugs and other contraband to migrants entering the country without authorization. Risks associated with various threats can be seen as a function of the likelihood that the threat will be realized and its potential consequences. However, threats are complex, threat actors are strategic and adaptive in their behaviors, and assessing the likelihood and gauging potential consequences of the various threats can be challenging. For instance, in understanding the risks posed by threat actors smuggling drugs into the United States, one may consider the likelihood of drugs successfully flowing into the country. This likelihood may be complicated by a variety of factors including past and expected frequencies. As the true frequency of illicit drug smuggling is unknown, officials may rely on a combination of intelligence and known drug seizure levels to inform their expectations. Notably, seizure data reflect illicit drugs that were not successfully smuggled into the country; they reflect known, unsuccessful smuggling attempts. In addition, seizures vary across sectors of the border, differ on whether they were made at or between POEs, and are diverse in the associated modes of land, air, or sea transport; as such, they can help inform, along with intelligence, the likelihood of smuggling attempts at various locations and via a host of transport modes. However, seizure data do not speak to the portion of drugs successfully smuggled into the country. Moreover, expectations of future drug flows may combine knowledge about past flows with intelligence and analysis of additional information such as drug market forces in source and destination countries. Policymakers may question how border officials use intelligence about drug flows and data on drug seizures to assess the risks posed by drug trafficking and appropriately allocate resources to counter the threat. Because there is a need to balance resources for sometimes competing priorities, some may also question whether DHS's risk management approach to countering threats at the borders is able to effectively evaluate and reduce threats posed by drug traffickingâand whether the data to make this evaluation exist. The United States has a number of strategies aimed, at least in part, at reducing drug trafficking into and within the country, and data on drug flows can help evaluate progress toward achieving goals outlined in them. For instance, the 2019 National Drug Control Strategy outlines that one of three key elements in the overarching goal of building a stronger, healthier, drug-free society is reducing the availability of illicit drugs in the United States. The strategy notes that some measures of performance are to "significantly reduce the availability of illicit drugs in the United States by preventing their production outside the United States, disrupt their sale on the internet, and stop their flow into the country through the mail and express courier environments, and across our borders." It also notes that some measures of effectiveness are that "[t]he production of plant-based and synthetic drugs outside the United States has been significantly reduced, illicit drugs are less available in the United States as reflected in increased price and decreased purity, and drug seizures at all U.S. ports of entry increase each year over five years." A robust picture of drug production and movement toward and into the United States can help inform, for instance, whether changes in drug seizures at POEsâas outlined in the strategyâmay be attributable to the effectiveness of U.S. drug control efforts. Intelligence and data on drug flows and seizures could also inform whether changes in seizures may be influenced by other factors such as the amount of drugs arriving at U.S. borders, the means by which traffickers attempt to smuggle drugs into the country, or the staffing levels at and between POEs. For instance, policymakers and officials may question whether fluctuations in drug seizures at ports of entry by OFO, as shown in Figure 2 and Figure 3 , taken with intelligence about other drug supply and demand factors, reflect progress toward meeting goals outlined by the National Drug Control Strategy. Other strategies, such as the National Southwest Border Counternarcotics Strategy and the Strategy to Combat Transnational Organized Crime, also provide action items that involve reducing drug trafficking. While these strategies do not outline specific effectiveness measures, as does the National Drug Control Strategy, the action items and goals could potentially be better evaluated with more specific data such as that on illicit drug production (both domestic and foreign), flows, and seizures. 

Summary: Policy discussions around issues such as border security, drug trafficking, and the opioid epidemic include questions about illicit drug flows into the United States. While there are numerous data points involved in understanding the trafficking of illicit drugs into the United States, these data are often estimated, incomplete, imperfect, or lack nuance. For example, debates about drug flows and how best to counter drug trafficking into the country often rely on selected drug seizure data from border officials, which do not reflect all drug flows into the United States. One way of conceptualizing the flow of illicit drugsâboth plant-based and syntheticâinto the United States is as a funnel. At the top of this funnel is the universe of illicit drugs produced around the world, both foreign and domestic. Factors affecting actual illicit cultivation and/or production are numerous and diverse, as are those affecting analysts' and officials' abilities to measure total worldwide production. Of all the illicit drugs that are produced around the world, some portion is destined for the United States. Of the total amount of illicit drugs that reach the U.S. border by land, air, or sea, some portion is known because it was seized by border officials, and an unknown portion is successfully smuggled into the country. While the proportion of illicit drugs coming into the country that are seized is unknowable, the amount of drugs seized is. And, data on drug seizures at the U.S. borders have sometimes served as a reference for policy debates on border security and drug trafficking into the country, in part because it is a knowable portion of drug trafficking problem. The primary agency charged with safeguarding the U.S. borders (including seizing illicit drugs and other contraband) is the U.S. Customs and Border Protection (CBP). Within CBP, the Office of Field Operations (OFO) is responsible for managing ports of entry and seizes drugs being smuggled into the United States at ports of entry; the Border Patrol is responsible for securing the border between ports of entry and seizes drugs being smuggled into the country between ports of entry. CBP data from OFO and Border Patrol indicate that for cocaine, methamphetamine, heroin, and fentanyl, larger quantities by weight are seized at legal ports of entry than are seized between the ports. Conversely, a larger quantity by weight of illicit marijuana is seized between the ports of entry. CRS analysis of OFO drug seizure data from FY2014 to FY2018 indicate that across those five years, about 65% of seized illicit drugs, by weight, were seized at land ports of entry at the border, about 28% of seized drugs were seized at air ports of entry, and about 5% were seized at sea ports of entry. CRS analysis of these data also indicate that nearly 97% of drugs were seized during inbound inspections across those years. CBP is not the only agency that seizes illicit drugs in the United States or even in the border regions. Federal, state, local, and tribal law enforcement agencies are all involved in enforcement actions thatâeven if not focused on drug-related crimesâmay involve drug seizures. Notably, though, there is no central database housing information on illicit drug seizures from all law enforcement agencies, federal or otherwise. Even though the quantity of total illicit drugs produced around the world that is destined for the United Statesâand successfully smuggled into the countryâis unknown, the likely source of the drugs seized may, in some instances, be knowable. U.S. officials chemically analyze a portion of illicit drugs seized to identify the source and, in conjunction with drug intelligence, assess which countries may be the major suppliers of certain illicit drug types found in the country. In the absence of precise data on illicit drugs moving toward and into the United States, seizure data can provide insight into various elements of drug flows such as smuggling points into the United States and target markets within the country. If policymakers are interested in having a more robust view of drug seizures throughout the country, they could move, through mandates or incentives, to enhance data collection and consolidation of drug seizure data by law enforcement officials. Policymakers may also question how border officials use intelligence about drug flows and data on drug seizures to assess the risks posed by drug trafficking and appropriately allocate resources to counter the threat. They may also evaluate how well available data on drug seizures can help measure progress toward achieving goals outlined in national strategies aimed, at least in part, at reducing drug trafficking into and within the country.