The Federal Analogue Act and "Substantially Similar"

This page is analysis and advocacy — it is not legal advice

Read this before you read anything else on the page. What follows is a research and policy brief. It is written to be used by regulators, legislators, counsel and industry researchers as an argument, and the argument is stated at full strength because a hedged brief is useless to the people who need it. It is not a legal opinion, it is not a clearance, and it is not advice to you. Three things make that more than a formality. First, cannabinoid law varies enormously by state: a compound that is lawful under federal law can be a felony in the jurisdiction where you live, ship, store or sell, and state analogue statutes, total-intoxicant caps and compound-by-name bans operate independently of the federal scheme described here. Second, this area of law is changing quickly — the federal definition of hemp itself was rewritten in late 2025 with a delayed effective date, state legislatures have moved in both directions in every recent session, and agency positions have shifted. Anything written here can be overtaken by an amendment, a rule, or a decision. Third, an argument that a substance is not a controlled substance analogue is a defence, not a permission: it is something a defendant says after being charged, and the fact that it is a good argument does not mean nobody will be charged. Do not make a business decision, a product decision or a personal decision on the basis of this page. Get your own lawyer, licensed in your own jurisdiction, and give them the actual facts. On this page specifically: a non-analogue argument is a defence raised after charging. Understanding the doctrine is not the same as being safe under it.

• Not legal advice, and no lawyer-client relationship arises from reading it.
• Federal lawfulness does not imply state lawfulness — check your own state, and the states you ship to.
• The law described here is in motion; a delayed federal effective date sits in the middle of it.
• A strong non-analogue argument is a defence to a charge, not immunity from one.
• Nothing here is medical advice either, and nothing here is an endorsement of using any compound.

What the Act does

The Controlled Substance Analogue Enforcement Act of 1986 exists because scheduling is slow and chemistry is fast. Rather than list compounds, it creates a category: 21 U.S.C. § 813 provides that a controlled substance analogue shall, to the extent intended for human consumption, be treated for the purposes of federal law as a controlled substance in schedule I. There is no rulemaking, no notice, no Federal Register entry — the status attaches by operation of the definition, and it is decided for the first time in the courtroom, after the conduct. That is the structural feature from which every criticism of the Act follows: it is a criminal prohibition whose scope is determined retrospectively, by expert disagreement about a phrase Congress never defined.

Sources: United States Code 1986 · United States Code 1986 · Stackhouse TP 2012

The definition, set out properly

The definition at 21 U.S.C. § 802(32)(A) describes a substance with three characteristics, and precision about them matters because arguments are won and lost on the connectives. In substance: (i) the chemical structure of which is substantially similar to the chemical structure of a controlled substance in schedule I or II; (ii) which has a stimulant, depressant, or hallucinogenic effect on the central nervous system that is substantially similar to or greater than the stimulant, depressant, or hallucinogenic effect on the central nervous system of a controlled substance in schedule I or II; or (iii) with respect to a particular person, which such person represents or intends to have a stimulant, depressant, or hallucinogenic effect on the central nervous system substantially similar to or greater than the stimulant, depressant, or hallucinogenic effect on the central nervous system of a controlled substance in schedule I or II. Note what prong (ii) actually says: substantially similar TO OR GREATER THAN. Greater potency is inside the prong on its face. Note also that prong (iii) is person-specific and speaks to representation or intent, not to the molecule at all.

• Prong (i) is about the molecule: structural similarity to a scheduled substance.
• Prong (ii) is about pharmacology, and its text expressly includes effects GREATER than the scheduled substance.
• Prong (iii) is about the defendant, not the compound: what this person represented or intended.
• Section 813 adds a limit of its own — the treatment applies only to the extent the substance is intended for human consumption.

Sources: United States Code 1986 · United States Code 1986

The structural question: is prong (i) a mandatory conjunct contested

The three prongs are separated by "or", and on a literal reading of that connective satisfying any one of them would be enough — which would mean that a substance with no structural relationship whatever to a scheduled drug could be an analogue on the strength of its effects, or of what a seller said about it. Courts have largely refused that reading and have construed the definition to require prong (i) as a baseline, plus either prong (ii) or prong (iii). The Eighth Circuit in Washam stated that construction in terms, reading the statute to require part A(i) and either A(ii) or A(iii). The Third Circuit in Hodge reached the same place from the other direction and by the cleanest possible facts: a mixture of candle wax and flour sold to undercover agents as crack cocaine was held not to be a controlled substance analogue, which is only explicable if representation under prong (iii) cannot by itself supply the structural element. The Seventh Circuit in Turcotte treated structural similarity as a required element in a prosecution over 1,4-butanediol. Forbes, at district-court level, required structural similarity and found it absent. The prevailing reading is therefore conjunctive as to prong (i). It should still be described as contested rather than settled: the text says "or", some courts have reasoned from the ordinary disjunctive force of a final "or" in a series, the Supreme Court has not resolved it, and the government's charging position has not always matched the prevailing construction.

Contested — caveat. The conjunctive reading is the prevailing one but is not authoritatively settled, and the case summaries here are compressed to the proposition each decision is cited for. Forbes is a district-court decision and is persuasive only; its vagueness holding has not been followed by the courts of appeals that have considered the question. Read the opinions before relying on any of them.

Sources: United States Code 1986 · United States Court of Appeals for the Eighth Circuit 2002 · United States Court of Appeals for the Third Circuit 2003 · United States Court of Appeals for the Seventh Circuit 2005 · United States District Court for the District of Colorado 1992 · United States Court of Appeals for the Fourth Circuit 2003 · Supreme Court of the United States 2015

Void for vagueness: what the challenges achieved and what they did not

The vagueness attack on the Act is the oldest and the most obvious. A criminal statute must give ordinary people fair notice of what is prohibited and must not invite arbitrary enforcement, and a prohibition whose scope depends on whether a chemical structure is "substantially similar" to another — a comparison for which the statute supplies no metric, no threshold and no method — is a natural target. Forbes agreed, holding in 1992 that the definition provided neither fair warning nor effective safeguards against arbitrary enforcement as applied to alpha-ethyltryptamine. That remains the high-water mark, and it is a district-court decision. The courts of appeals that have taken up the question have upheld the Act, generally by narrowing it: they read prong (i) as mandatory, they hold that a person of ordinary intelligence has sufficient notice on the particular facts, and they treat the scienter requirement as curing much of the notice problem. The honest summary is therefore that the vagueness argument has repeatedly failed as a facial attack and has succeeded once, at district level, as an applied one. Its practical residue is not invalidation but pressure: it is why courts construe the Act narrowly, why they insist on structural similarity, and why the government generally charges analogue cases against compounds with an obvious structural parent. That last point is where the homolog argument on the next page gets its purchase.

Sources: United States District Court for the District of Colorado 1992 · United States Court of Appeals for the Eighth Circuit 2002 · United States Court of Appeals for the Fourth Circuit 2003 · United States Court of Appeals for the Seventh Circuit 2005 · Stackhouse TP 2012

Scienter after McFadden v. United States

In McFadden v. United States, 576 U.S. 186 (2015), the Supreme Court resolved a split over what the government must prove a defendant knew in an analogue prosecution. The holding is that the knowledge requirement can be satisfied in either of two ways: by proof that the defendant knew the substance he was dealing with was some controlled substance or controlled substance analogue regulated under the Act, even without knowing which one; or by proof that the defendant knew the specific features of the substance that make it a controlled substance analogue — that is, knew its structure was substantially similar to that of a scheduled substance and that it had or was represented to have the relevant effects. That second route is the one that carries weight for compounds sold openly under their own systematic chemical names, with certificates of analysis, by registered businesses paying tax on the transaction. It does not make such a defendant safe; the first route remains available to the government, and knowledge can be proved circumstantially. But it means the government has to prove a state of mind about legal status or about chemistry, and the evidentiary posture of a labelled, tested, openly sold product is materially different from that of a compound sold in foil sachets marked "not for human consumption".

Sources: Supreme Court of the United States 2015 · United States Code 1986 · United States Code 1986

"Substantially similar" has never been given a chemical definition

This is the central defect and it should be stated flatly. Congress did not define "substantially similar" in the Act. No regulation defines it. No agency has published a method, a similarity metric, a threshold, or a list of structural features that count. There is no reference to any of the quantitative molecular-similarity measures that cheminformatics has used for decades, no specified comparison of scaffolds, no rule about whether to compare two-dimensional connectivity or three-dimensional conformation or pharmacophore, and no guidance on how much of a difference is enough. The consequence is that in each prosecution the question is put to competing expert witnesses, and expert chemists reach opposite conclusions on the same molecule — including, as commentators have documented, chemists within the enforcement agencies themselves. For a definition that operates as a criminal prohibition, that is not a detail. It is the reason the vagueness argument keeps being made, the reason courts narrow the Act when they uphold it, and the reason an argument about a specific structural difference is worth making at all: there is no standard against which to lose it in advance.

Sources: United States Code 1986 · Stackhouse TP 2012 · Shulgin AT 1992*

The Shulgin critique contested historical / ethnographic

The best-known statement of the problem comes from Alexander Shulgin, the chemist whose work on phenethylamine and tryptamine pharmacology made him the expert most often called on the question. His critique is semantic and it is sharper than it first appears. "Similar" means pretty much the same; "substantially identical" would also mean pretty much the same; so what is the residual content of "substantially similar" — a phrase built from a word of degree qualifying a word of resemblance, with no reference point for either? He illustrated it with two analogies that have been quoted ever since. Is the right foot substantially similar to the left foot? They are mirror images: identical in composition, identical in every measurable dimension, and not superimposable — which is exactly the relationship between two enantiomers, one of which may be a medicine and the other inert. And a salt shaker and a pepper shaker: to a collector they are a matched pair, effectively identical; to a cook they are opposites, because the only thing that matters about them is the one respect in which they differ. The point is not wordplay. It is that similarity is not a property of a pair of objects; it is a property of a pair of objects plus a purpose, and the statute never states the purpose. Two things must be said about the standing of this argument. It is an expert-witness argument and a piece of advocacy, widely cited in the analogue literature and persuasive to some courts. It is not a holding, and no court has adopted it as the rule of decision. It is also not attributable to a primary source located in this pass — it circulates through quotation, and it is cited here as unverified for that reason.

Contested — caveat. Advocacy, not authority. The Shulgin critique is an expert-witness argument that has never been adopted as a holding, and the quotations circulate through secondary sources; the primary text was not located in this pass, so the citation is marked unverified.

Sources: Shulgin AT 1992* · United States Code 1986 · Stackhouse TP 2012

What the Analogue Act is not

A brief that wins the analogue argument and stops there has answered one question out of four, and the industry reading it will be misled. The Analogue Act is one of several independent routes by which a cannabinoid can be unlawful, and each route has to be defeated separately. A compound can be scheduled by name, by Congress or by the DEA through the statutory scheduling process, in which case similarity is irrelevant. It can fall outside the definition of hemp — because it did not come from the plant, or, after 12 November 2026, because it is not naturally produced by the plant or was synthesised outside it — in which case it is a tetrahydrocannabinol in schedule I and the analogue question never arises. It can exceed a quantitative threshold in a finished product. It can be prohibited by state law, by name or by a total-intoxicant cap, or captured by a state analogue statute drafted more broadly than the federal one. And the article in which it is sold can be unlawful for reasons that have nothing to do with scheduling at all — food-additive and new-dietary-ingredient law, labelling, and state consumable-hemp licensing. The analogue argument is necessary for the homologs. It is nowhere near sufficient.

• Scheduling by name — defeats the similarity argument entirely.
• Falling outside the hemp definition — then it is a schedule I tetrahydrocannabinol on its own terms.
• Quantitative product thresholds — 0.4 mg total THC per container from 12 November 2026.
• State schedules, state total-intoxicant caps, and state analogue statutes.
• Food, dietary-supplement, labelling and licensing law, which is a separate body of regulation again.

Sources: United States Code 1986 · United States Code 2018 · Drug Enforcement Administration 2020 · United States Congress 2025 · Minnesota Statutes 2022 · Texas Legislature 2025