Misuse of Drugs Act 1971
The Misuse of Drugs Act 1971 is an Act of Parliament, by which the United Kingdom aims to control the possession and supply of numerous drugs and drug-like substances, as listed under the Act, and to enable international co-operation against illegal drug trafficking. As passed in 1971 the Act updated UK legislation to bring it in to line with the requirements of the Single Convention on Narcotic Drugs.
The Act is presented often as little more than a list of proscribed drugs and of penalties linked to their possession and supply. In practice however the Act establishes the Home Secretary as a key player in a drug licensing system. Therefore, for example, various opiates are available legally as prescription-only Controlled Drug medicines and cannabis (hemp) may be grown under licence for 'industrial purposes'.
The Act creates three classes of "controlled substances", and ranges of penalties for illegal or unlicensed "possession" and "possession with intent to supply" are graded differently within each class. The lists of substances within each class can be amended "by order", so the Home Secretary can list new drugs and upgrade, downgrade or delist previously-controlled drugs with less of the bureaucracy and delay associated with passing an Act through both Houses of Parliament.
The Act does not cover all drugs or drug-like substances. Although, for example, cannabis is listed under the Act (as a class C drug), tobacco, another herb or plant source of drug material, is not listed.

# International cooperation
The Act provides for cooperation with other nations in implementing drug control treaties. Specifically, the Act makes it a crime to assist in, incite, or induce, the commission of an offense, outside the UK, against another nation's "corresponding law" on drugs. A "corresponding law" is defined as another country's law "providing for the control and regulation in that country of the production, supply, use, export and import of drugs and other substances in accordance with the provisions of the Single Convention on Narcotic Drugs" or another drug control treaty to which the UK and the other country are parties. An example might be lending money to a U.S. drug dealer for the purpose of violating the Controlled Substances Act.

# History and criticism
The Drugs (Prevention of Misuse) Act 1964 controlled amphetamines in the UK in advance of international agreements and was later used to control LSD.
Before 1971, the UK had a relatively liberal drugs policy and it was not until U.S. influence had been brought to bear  particularly in United Nations circles  that drugs use was generally criminalised. Before the passage of the Act, it was possible for heroin addicts to be prescribed enough of the drug to manage their addiction without being forced to buy from the black market, for example.
Supporters of the newer drugs policy tend to believe that criminalising both drug use and possession (it is not illegal to be under the influence of illegaly supplied drugs once ingested/injected unless operating machinery/driving) is the best way to handle the social problems caused by drugs, whereas opponents tend to suggest that criminalising users and dealers alike is counterproductive and detrimental to the health of users.
The Science Select Committee said in 2006 that the present system was based on historical assumptions, not scientific assessment.

## Nutt, King, Saulsbury and Blakemore's research
In March 2007, The Lancet published the results of a study that, in their opinion, highlighted the inadequacy of the current drugs classification system in the UK, calling it "not fit for purpose". The research, which compared 20 substances, some classified and some not classified, concluded that alcohol was the fifth most harmful drug, behind heroin, cocaine, barbiturates and methadone. Tobacco came ninth, resulting more harmful than cannabis, and even than some Class-A drugs like LSD and ecstasy.
The researchers said that "the exclusion of alcohol and tobacco from the Misuse of Drugs Act is, from a scientific perspective, arbitrary".

# Penalties
The penalties for drug offences depend on the class of drug involved. It should be noted that these penalties are enforced against those who do not have a valid prescription or license to possess the drug in question. Thus it is not illegal for someone to possess oxycodone, a class A drug, so long as it was administered to them legally (by prescription).
Class A drugs are deemed to be the most dangerous and attract the highest penalty. The maximum penalties possible are as follows:

# Class A drugs
This list is not exhaustive
- 2C-B (4-Bromo-2,5-dimethoxybenzeneethanamine)
- Acetorphine
- Allylprodine
- Alphacetylmethadol
- Alphameprodine
- Alphamethaclol
- Alphaprodine
- Anileridine
- Benzethidine
- Benzylmorphine (3-benzylmorphine)
- Betacetylmethadol
- Betameprodine
- Betamethadol
- Betaprodine
- Bezitramide
- Bufotenine
- Cathinone
- Clonitazene
- Coca leaf
- Cocaine
- Desomorphine
- Dextromoramide
- Diamorphine (heroin)
- Diampromide
- Diethylthiambutene
- Dihydrocodeinone O-carboxymethyloxime
- Dihydromorphine
- Dimenoxadole
- Dimepheptanol
- Dimethylthiambutene
- Dioxaphetyl butyrate
- Diphenoxylate
- Dipipanone
- Ecgonine, and any derivative of ecgonine which is convertible to ecgonine or to cocaine
- Ethylmethylthiambutene
- Etonitazene
- Etorphine
- Etoxeridine
- Fentanyl
- Furethidine
- Hydrocodone
- Hydromorphinol
- Hydromorphone
- Hydroxypethidine
- Isomethadone
- Ketobemidone
- Levomethorphan
- Levomoramide
- Levophenacylmorphan
- Levorphanol
- Lysergamide
- Lysergic acid diethylamide (LSD)
- Lysergide and other N-alkyl derivatives of lysergamide
- MDMA Commonly called Ecstasy (3,4-methylenedioxymethamphetamine)
- Mescaline
- Metazocine
- Methadone
- Methadyl acetate
- Methamphetamine (Crystal Meth)
- Methyldesorphine
- Methyldihydromorphine (6-methyldihydromorphine)
- Metopon
- Morpheridine
- Morphine
- Morphine methobromide, morphine N-oxide and other pentavalent nitrogen morphine derivatives
- Myrophine
- Nicodicodine (6-nicotinoyldi-hydrocodeine)
- Nicomorphine (3,6-dinicotinoyl-morphine)
- Noracyinethadol
- Norlevorphanol
- Normethadone
- Normorphine
- Norpipanone
- Opium, whether raw, prepared or medicinal
- Oxycodone
- Oxymorphone
- Pethidine
- Phenadoxone
- Phenampromide
- Phenazocine
- Phenomorphan
- Phenoperidine
- Piminodine
- Piritramide
- Poppy-straw and concentrate of poppy-straw
- Proheptazine
- Properidine (1-methyl-4-phenyl-piperidine-4-carboxylic acid isopropyl ester)
- Psilocin
- Psilocybe mushrooms, and any other 'fungi containing psilocin' (after the Drugs Bill 2005)
- Racemcthorphan
- Racemoramide
- Racemorphan
- Thebacon
- Thebaine
- Trimeperidine
- 4-Cyano-2-dimethylamino-4, 4-diphenylbutane
- 4-Cyano-1-methyl-4-phenyl-piperidine
- N,N-Diethyltryptamine
- N,N-Dimethyltryptamine
- 2,5-Dimethoxy-a,4-dimethyl-phenethylaniine
- 1-Methyl-4-phenylpiperidine-4-carboxylic acid
- 2-Methyl-3-morpholino-1
- 1-diphenylpropanecarboxylic acid
- 4-Phenylpiperidine-4-carboxylic acid ethyl ester
- "Any preparation designed for administration by injection which includes" a Class B drug.

# Class B drugs
- Acetyldihydrocodeine
- Amphetamine
- Codeine
- Dexamphetamine
- Dihydrocodeine
- Ethylmorphine (3-ethylmorphine)
- Flunitrazepam
- Methaqualone
- Methylphenidate
- Nicocodine
- Norcodeine
- Phenmetrazine
- Pholcodine
- Temazepam (Becomes Class A if prepared for injection)
Note that preparing a class B drug for injection makes it a Class A drug.

# Class C drugs
- Benzphetamine
- Benzodiazapines (Except flunitrazepam and temazepam)
- Cannabinol or a cannabinol derivative (downgraded from Class A from January 29 2004)
- Cannabis and cannabis resin (downgraded from Class B from January 29, 2004)
- Cathine
- Chlorphentermine
- Fencamfamin
- Gammahydroxybutyrate (added in July 2003)
- Ketamine (added in January 2006)
- Mephentermine
- Pemoline
- Phendirnetrazine
- Phentermine
- Pipradrol
- Prolintane
Some drugs, including some benzodiazepines (for example chlordiazepoxide and diazepam) are controlled under the Misuse of Drugs Act as Class C drugs but the possession offence is waived so that it is not illegal to possess or use them without a prescription. It is an offence to sell or supply them to another person. The exceptions are temazepam and flunitrazepam; it is illegal to possess either of these drugs without a valid prescription.