Class A

Class A is the highest tier in the three-level classification scheme established by the United Kingdom's Misuse of Drugs Act 1971. Parliament created the scheme to rank controlled substances by relative harm; Class A marks those judged to carry the greatest risk.

The Act creates two parallel systems that are often confused. The class (A, B, or C) governs criminal liability — what it means to possess or supply a substance. The schedule (1 through 5, set by the Misuse of Drugs Regulations 2001) governs who may lawfully handle it for medical or research purposes. A substance can be Class A and still be licensed for clinical use, because the two systems answer different questions.

Some other national systems use identical words — Class A, Class B — but these labels do not travel. The UK framing is specific to the Misuse of Drugs Act, and the term as used on these pages means only that.

What it means in practice

Placing a substance in Class A makes possession and supply criminal offences under UK law. The classification applies to the substance itself and typically extends to its salts, isomers, and specified preparations.

The list of Class A substances is set in the Act's schedules. It can be amended by Parliament or, in certain circumstances, by order of the Home Secretary following advice from the Advisory Council on the Misuse of Drugs (ACMD). New psychoactive substances that fall outside the existing list are addressed by separate legislation — the Psychoactive Substances Act 2016 — rather than automatically acquiring a class.

Class A status also shapes research access. Most Class A substances sit in Schedule 1 of the Misuse of Drugs Regulations, which requires a specific Home Office licence to possess or supply them for scientific purposes. A handful of Class A substances are placed in lower schedules because they retain accepted medical use, meaning the two-system structure matters to clinicians and researchers even where criminal liability is not in doubt.

How jurisdictions vary

The Class A label is specific to UK law and does not correspond to any tier in other major systems. The United States organises controlled substances into Schedules I through V under the Controlled Substances Act 1970; Schedule I is the most restrictive US tier, but it is not equivalent in scope, membership, or legal effect to Class A.

Ireland operates a broadly similar A/B/C/D structure under its own Misuse of Drugs Acts, but the two systems are separate legislation with different substance lists and different offence structures. Shared nomenclature should not be read as shared law.

The international drug conventions — the Single Convention on Narcotic Drugs 1961 and the Convention on Psychotropic Substances 1971 — use Roman-numeral schedules (I through IV), and their criteria and listings differ from any national class system. A treaty Schedule I designation is not Class A, and the two are not interchangeable.

What is contested

The criteria used to assign substances to Class A — based on judgments of relative harm — have been contested by independent researchers and, at times, by members of the ACMD itself. Published analyses of drug-related harm, using frameworks that weigh harm to the user alongside harm to others, have found that the distribution of substances across the three classes does not consistently track those rankings.

The statutory phrase describing Class A substances as the most harmful is a legal designation, not a settled scientific finding. It reflects the judgment made at the time of classification or reclassification. Reclassification is possible and has occurred, but the process is legislative and advisory rather than automatic when new evidence emerges.

Scope and currency

This entry describes the general structure of the Class A designation as of August 2026. It is not legal advice and does not describe the law as it applies in any particular place or to any particular person.

Laws change; classifications are amended; individual circumstances vary. For the legal status of a specific substance, consult the legal-status section of that substance's record, which carries its own jurisdiction, date, and source.