Since 1986, a law called the Federal Analogue Act has given prosecutors a tool to deal with so-called "designer drugs," which crop up occasionally as a road around prohibited drugs. Take an existing structure, tweak it a little bit, and put it on the legal market as "bath salts" or "synthetic pot" and let word of mouth do the marketing work.

The little-known law to fight this practice was written poorly, and as a result has been interpreted very broadly by American law enforcement. However, a Supreme Court decision last week, McFadden vs. US, narrowed the scope of the Federal Analogue Act, and closed off one of the worst facets of law enforcement's interpretation.

So, where did this law come from? Ironically, it was sparked not by a new drug discovery, but by an impurity. Back in the early 80s, amateur chemists commonly synthesized an opioid drug called MPPP — similar to morphine, and discovered back in the 40s. However, due to the reaction pathway and shoddy lab technique, they sometimes contaminated the product with something called MPTP. This chemical turned out to cause very serious brain damage, and induced Parkinson's disease-like symptoms in several MPPP users. (In a double irony, MPTP was used in animal trials to develop what is now the standard Parkinson's treatment.)

At any rate, after the usual moral panic, Congress passed a law banning drug analogues. They defined an analogue this way:

(1) The substance has a "chemical structure" which is "substantially similar" to an already-controlled substance.

(2) It is intended for human consumption.

(3) It has an effect that is "substantially similar to or greater than" an already-controlled substance or is represented as such.

This raises several difficulties.

First of all is sheer vagueness. What is meant by "substantially similar?" Neither courts nor scientists can agree. Adding or removing a single carbon atom can drastically change the behavior of a drug, making such a phrase practically meaningless in a pharmaceutical context.

Second is the wide latitude granted by the "human consumption" part. A chemical which is "substantially similar" in structure need only meet the same vague standard in its bodily effect, or be represented as such.Thus it ought to be possible to prosecute someone presenting oregano as marijuana — after all, they're both made mostly of carbon and meant for consumption.

Third is the question of intent, the one on which McFadden turned. As part of the Controlled Substances Act (CSA), the Analogue Act requires the government to prove criminal intent — that the defendant actually knew he was breaking the law. That's easy when it comes to something like assault, but difficult when it comes to complex chemicals which are not listed as prohibited anywhere.

McFadden was selling "bath salts," and was convicted on nine counts under the Analogue Act. On appeal, he argued that his jury should have been told that proving criminal intent would require showing that he knew that his bath salts were chemically similar to a controlled substance and that they had a similar pharmacological effect. The Fourth Circuit disagreed, arguing that proving intent for human consumption was enough.

The 9-0 opinion, written by Justice Clarence Thomas, partially sides with McFadden. Here's the major summary:

Held: When a controlled substance is an analogue, §841(a)(1) requires the Government to establish that the defendant knew he was dealing with a substance regulated under the Controlled Substances Act or Analogue Act.

Thomas writes that there are two ways to go about this: Either prove that a person knew that he was distributing some controlled substance, even if he didn't know the exact identity of it; or prove that he knew the particular identity of the substance. The second method can hold even if he does not know the substance is banned, since ignorance of the law is not generally an excuse for lawbreaking.

Put this in the context of a heroin dealer. You can convict him if you can prove he knew he was dealing a controlled substance, even if he wasn't sure which one it was. (Imagine he was hiding from the cops carrying packages of some type of white powder.) Or, you can prove he knew it was heroin, since you don't get to commit crimes just because you didn't know that the law bans heroin.

Now consider the vitamin store owner. If he's openly selling something he think is vitamins but is actually "bath salts," then he should get off under this logic, since he didn't know it was controlled, and he didn't know what it was. That does make for a higher burden of proof when it comes to analogues.

But that doesn't mean McFadden gets off scot-free. The court did not accept his version of jury instructions, which would make it all but impossible to convict people without training in chemistry. Instead, they kicked it back down to the lower courts, and instructed them to analyze whether their mistakes would have resulted in a different verdict.

Overall, this is only a small part of a much larger law which badly needs an overhaul (in the CSA, marijuana is classed as more dangerous and less medically useful than meth). However, the Supreme Court did clarify and restrict one of its most objectionable facets. The oregano scenario mentioned above is no longer possible. Better still, it will be much harder to prosecute someone for not knowing about a secret and potentially infinite list of banned substances.