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 medicines, and cannabis (hemp)[6] may be grown under licence for 'industrial purposes'.

The lists of substances within each class can be amended by Order in Council, 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.

Any preparation designed for administration by injection which includes a substance or product for the time being specified in any of paragraphs 1 to 3 of Part II of this Schedule.

(ca)[31] any compound (not being clonitazene, etonitazene, acemetacin, atorvastatin, bazedoxifene, indometacin, losartan, olmesartan, proglumetacin, telmisartan, viminol, zafirlukast or a compound for the time being specified in sub-paragraph (c) above) structurally related to 1-pentyl-3-(1-naphthoyl)indole (JWH-018), in that the four sub-structures, that is to say the indole ring, the pentyl substituent, the methanone linking group and the naphthyl ring, are linked together in a similar manner, whether or not any of the sub-structures have been modified, and whether or not substituted in any of the linked sub-structures with one or more univalent substituents and, where any of the sub-structures have been modified, the modifications of the sub-structures are limited to any of the following, that is to say— (d)[30] 1-Phenylcyclohexylamine or any compound (not being ketamine, tiletamine or a compound for the time being specified in paragraph 1(a) of Part 1 of this Schedule) structurally derived from 1-phenylcyclohexylamine or 2-amino-2-phenylcyclohexanone by modification in any of the following ways, that is to say, (i) by substitution at the nitrogen atom to any extent by alkyl, alkenyl or hydroxyalkyl groups, or replacement of the amino group with a 1-piperidyl, 1-pyrrolidyl or 1-azepyl group, whether or not the nitrogen containing ring is further substituted by one or more alkyl groups; (ii) by substitution in the phenyl ring to any extent by amino, alkyl, hydroxy, alkoxy or halide substituents, whether or not further substituted in the phenyl ring to any extent; (iii) by substitution in the cyclohexyl or cyclohexanone ring by one or more alkyl substituents; (iv) by replacement of the phenyl ring with a thienyl ring.

Class C drugs, supposedly the least harmful drugs, include the following substances:—[11][non-primary source needed] (a) (b) (c) any compound (not being Trilostane or a compound for the time being specified in sub-paragraph (b) above) structurally derived from 17-hydroxyandrostan-3-one or from 17-hydroxyestran-3-one by modification in any of the following ways, that is to say, (i) by further substitution at position 17 by a methyl or ethyl group; (ii) by substitution to any extent at one or more of positions 1, 2, 4, 6, 7, 9, 11 or 16, but at no other position; (iii) by unsaturation in the carbocyclic ring system to any extent, provided that there are no more than two ethylenic bonds in any one carbocyclic ring; (iv) by fusion of ring A with a heterocyclic system; (d) any substance which is an ester or ether (or, where more than one hydroxyl function is available, both an ester and an ether) of a substance specified in sub-paragraph (b) or described in sub-paragraph (c) above; (e) (f) 1–benzylpiperazine or any compound (not being 1–(3–chlorophenyl)piperazine or 1–(3–chlorophenyl)–4–(3–chloropropyl)piperazine) structurally derived from 1–benzylpiperazine or 1–phenylpiperazine by modification in any of the following ways (i) by substitution at the second nitrogen atom of the piperazine ring with alkyl, benzyl, haloalkyl or phenyl groups; (ii) by substitution in the aromatic ring to any extent with alkyl, alkoxy, alkylenedioxy, halide or haloalkyl groups; 2.

Where unspecified it is thought to indicate derivatives which can be made from the specified compound in a single synthetic step, although such a definition would indicate that alkyllysergamide analogues would be uncontrolled.

[40] The maximum penalties possible are as follows:[41] The act makes it a crime to assist in, incite, or induce, the commission of an offence, outside the UK, against another nation's corresponding law on drugs.

An example might be lending money to a United States drug dealer for the purpose of violating that country's Controlled Substances Act.

It is noted that bar the smoking of opium and cannabis; Section 8, part d, under the 1971 Act was not an offence (relating to the prosecution of the owner of a premises/building inside of which controlled drugs were being used).

[46][47] The Current Section 8 covers: people knowingly allowing premises they own, manage, or have responsibility for, to be used by any other person for: Notable criticism of the act includes: The Transform Drug Policy Foundation offers rational criticism of the harms caused by the Government's current prohibitionist drug policy.

[52] The Drug Equality Alliance (DEA) has launched legal actions against the UK Government's partial and unequal administration of the Act's discretionary powers, making particular reference to the arbitrary exclusion of alcohol and tobacco on the subjective grounds of historical and cultural precedents contrary to the Act's policy and objects.

[54] Jacqui Smith, then Home Secretary, was also widely criticised by the scientific community for bullying Professor David Nutt into apologising for his comments that, in the course of a normal year, more people died from falling off horses than died from taking ecstasy.

I cannot have public confusion between scientific advice and policy and have therefore lost confidence in your ability to advise me as Chair of the ACMD.

This incorrect view may be further re-enforced by R&D chemical suppliers often stating and asking scientists to confirm that anything bought is for research use only.

A further misconception is that the Misuse of Drugs Act simply lists a few hundred substances (e.g. MDMA, Fentanyl, Amphetamine, etc.)

However, the reality is that in most cases all ethers, esters, salts and stereo isomers are also controlled and it is impossible to simply list all of these.

The act contains several "generic statements" or "chemical space" laws, which aim to control all chemicals similar to the "named" substance, these provide detailed descriptions similar to Markushes, a good example of a few of these are found in the Misuse of Drugs Act 1971 (amendment) order 2013.

[59] Due to this complexity in legislation the identification of controlled chemicals in research is often carried out computationally, either by in house systems maintained a company's sample logistics department or by the use commercial software solutions.

However, the associated Misuse of Drug Regulations 2001[62] does exempt products containing less than 1 mg of a controlled substance (1 μg for lysergide and derivatives) so long as a number of requirements are met, including that it cannot be recovered by readily applicable means, does not pose a risk to human health and is not meant for administration to a human or animal.

In 2017 the Home Office also confirmed that the 1 mg limit applies to the total of all preparations across the entire container in the case of sample microtitre plates.