Number 348 May 2008

Traffic Safety Facts

Traffic Tech – Technology Transfer Series

U.S. Department of Transportation

National Highway Traffic Safety Administration

1200 New Jersey Avenue SE., Washington, DC 20590

Multiple Medications and Vehicle Crashes: 

Analysis of Databases

By 2030, over 70 millionAmericans will be over age 65. Older

adults have a higher rate of fatality and injury in motor vehicle crashes per mile driven than any other age group except

for teenagers. Since driving is a complex behavior, many

age-related factors may impair driving ability, including agerelated decrements in cognitive and physical functioning,

increased prevalence of medical conditions or age-related

medical conditions, and increased use of multiple medications. The National Highway Traffic Safety Administration

implemented a study to examine factors relating to crashinvolvement among older adults to devise strategies to

address these concerns.

This studyexaminedthe associationofthe impairingeffectsof

multiple medication use, drug/drug interactions, and drug/

disease interactions on motor vehicle crashes in individuals

age 50 and older. Fifty was selected as a cut-off age (a) to

maximize sample size, and (b) because age-related changes

and corresponding increased medication use begins to occur

at this age (i.e., drugs to treat ailments like high blood pressure, osteoarthritis, hypercholesterolemia, and adult-onset

diabetes), along with taking multiple medications.

Methodology

This study examined two databases: (1) the National Ambulatory Medical Care Survey (NAMCS) database, and (2) the

PharMetrics database. The NAMCS provided information

on patients’ demographic characteristics and medical services received, including information on diagnostic procedures, patient management, and planned future treatment.

This database could not link, longitudinally, the medical and

pharmaceutical data on a patient-specific basis. From 1998-

2000, the NAMCS data included 71,468 physician/patient

visits. The PharMetrics database is an anonymous, proprietary patient-level medical insurance claims database, which

contains longitudinal medical and pharmaceutical data for

thousands of individuals with motor vehicle crash diagnoses

and the medications and diseases in a proximal time period

to the crash. From January 1998 to March 2002, the PharMetrics patient-level database included 81,408 cases (patients

with a diagnosis code for motor vehicle crashes) and 244,224

age-, sex-, and date-matched controls. With this database,

hundreds of drugs and diseases, along with combinations

of drugs and diseases, were examined using a case-control

matched pair design or logistic regression analysis of possible associations of medication use among older adults and

motor vehicle crashes.

The main objectives of this study were to determine the

relative frequency of various combinations of medications

used by those who have experienced motor vehicle crashes

(MVCs) and those who have not by analyzing proprietary

and non-proprietary databases; and to conduct a case-control study and regression analysis of possible associations

between the use of medications (and combinations thereof)

and MVCs amongst older drivers.

Results

The results of the study showed an association between

the kinds and number of medications taken by older adults

and the risk of having MVCs. Drugs considered to impair

the driving ability of older drivers were the most commonly

used by older adults involved in MVCs. Over one-third of

the NAMCS group over 50 years old mentioned the use of

two or more drugs. Over half of the over-50 group mentioned using one or more potentially driver impairing (PDI)

medications. To qualify as a PDI medication, the medication

had to be associated with known effects on the central nervous system, blood sugar levels, blood pressure, vision, or

otherwise have the potential to interfere with driving skills.

Possible PDI effects include sedation, hypoglycemia, blurred

0%

10%

20%

30%

40%

50%

60%

Number of Potentially Driver Impairing (PDI) 

Medication Use by Age

Age 16-49 (N=12,042639) Age >50 (N=4,457.588)

0 1 2

52%

20%

28%

42%

27%

42%
vision, hypotension, dizziness, fainting (syncope), and loss

of coordination (ataxia).

The descriptive analysis of the proprietary database identified higher rates of drug use than the NAMCS results. Over

half of the older adult study subjects received two or more

medications. Almost 65 percent of the older adult study subjects received PDI medications. Over half of the older adult

study subjects suffered from a PDI condition, and almost a

quarter of the older adult study subjects concomitantly used

medications that are known to interact. Almost a tenth of the

older adult study subjects appeared to be using medicines

that presented therapeutic conflicts with diseases/conditions

for which they were being treated.

The Case Control Analysis suggested an association between

motor vehicle crashes and many PDI medications, PDI diseases, and various combinations of drugs and diseases.

Thirty-five of the ninety PDI drug classes had odds ratios

over 1.2 (p < .05). Seven of the fifteen medication classes with

the highest odds ratios are classes that have been reported to

be especially problematic in older patients.

Seventy-nine of 200 driver impairing disease classes had

statistically significant odds ratios over 1.4. The results supported previous studies linking Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), Angiotensin-Converting Enzyme

(ACE) Inhibitors, Anticoagulants, Antidepressants, and Benzodiazepine use with motor vehicle crashes, corroborating

previous studies linking depression, alcoholism, arthritis,

history of falls, back pain, diabetes, heart disease, stroke,

arrhythmias, coronary artery disease, and sleep apnea with

MVCs. Most of the drug interaction pairings had higher odds

ratios for MVCs than when the drugs were used alone. Some

drug-disease conflicts had a statistically significant elevated

odds ratio; however, these were such small numbers of cases

and controls, making it difficult to conclude risk increases.

The number of total medications, PDI medications, PDI diseases, drug-drug interactions, and drug-disease conflicts

were used in a regression analysis examining their role in

MVCs. For patients with one or two PDI diseases, the risk

was 1.49 times greater than for older adults without any PDI

diseases. For those with three or more PDI conditions, the

risk for MVCs increased to 2.20 times. Drug interactions were

also associated with a statistically significant increased risk

of MVCs (odds ratio of 1.47 for 1-2 drug interactions and 1.92

with patients with 3 or more drug interactions). The risk for

MVCs among study subjects with at least one drug-disease

conflict was 1.2 times that for older adults without any drugdisease conflicts.

Limitations 

While older drivers are at increased risk of a crash when they

take multiple PDI medications, this study cannot isolate the

cause of these crashes and determine the relative contributions of the medication, medical condition, and age. This

study also did not address the association of MVCs with de

novo exposure (initiation of drug therapy when individuals are most likely to experience side effects) and prolonged

exposure to prescribed medications (when individuals may

become tolerant to side effects).

Furthermore, this analysis cannot predict whether an individual with an underlying medical condition who takes multiple PDI medications can drive safely. The limitations of the

data, which include sample size and inherent weaknesses

of administrative claims data, make it difficult to gauge the

strength of the association between PDI medications and

MVCs.

Discussion and Conclusions

This study suggests that both the kind and number of medication exposures, and the characteristics of diseases/disorders present among study subjects, are associated with

an increase in risk for MVCs among older adults. As the

population continues to age, many factors will impact driving safety. Older adults may develop chronic diseases that

may have driver impairing characteristics. Layered on to

the underlying chronic diseases are the medications used to

treat those conditions along with their potential to exacerbate

other coexisting conditions, induce side effects, and promote

dangerous drug interactions. By demonstrating the potential

link between multiple drug therapies and MVCs, this study

serves to highlightthe need for a thorough examination ofthe

relationships among drugs, diseases, and the older driver.

This study suggests the need forfurtherresearch on the interplay of factors affecting the aging adult and driving ability.

The results of this study supports the development of educational programs to increase awareness about the potential

driver impairing effects of increasingly complex medical and

pharmaceutical therapies in older adult drivers.

How to Order

To order Multiple Medications and Vehicle Crashes: Analysis 

of Databases (55 pages plus appendices), prepared by Iatrogen, LLC, write to the Office of Behavioral Safety Research,

NHTSA, NTI-130, 1200 New Jersey Avenue SE., Washington,

DC 20590, fax 202-366-7096, or download from www.nhtsa.

dot.gov. John Siegler, Ph.D., was the Contracting Officer’s

Technical Representative for this project.

TRAFFIC TECH is a publication to disseminate information about

traffic safety programs, including evaluations, innovative programs, and new publications. Feel free to copy it as you wish. If you

would like to receive a copy, contact Patricia Ellison-Potter, Ph.D.,

Editor, fax 202-366-7096, e-mail: traffic.tech@dot.gov.

1200	New	Jersey	Avenue	SE.,	NTI-130

Washington,	DC	20590