Federal Analogue Act

The Federal Analogue Act, , is a section of the United States Controlled Substances Act passed in 1986 which allowed any chemical "substantially similar" to a controlled substance listed in Schedule I or II to be treated as if it were listed in Schedule I, but only if intended for human consumption. These similar substances are often called designer drugs.
Case law.
"United States v. Forbes".
"United States v. Forbes", 806 F. Supp. 232 (D. Colo. 1992), a Colorado district court case, considered the question of whether the drug alphaethyltryptamine (AET) was a controlled substance analogue in the United States. The controlled drugs to which it was alleged that AET was substantially similar were the tryptamine analogues dimethyltryptamine (DMT) and diethyltryptamine (DET).
 
AET
DMT
DET
In this case, the court ruled that AET was not substantially similar to DMT or DET, on the grounds that (i) AET is a primary amine while DMT and DET are tertiary amines, (ii) AET cannot be synthesized from either DMT or DET, and (iii) the hallucinogenic or stimulant effects of AET are not substantially similar to the effects of DMT or DET. Furthermore, the court ruled that the definition of controlled substance analogue given in the Federal Analogue Act was unconstitutionally vague, in that
“Because the definition of 'analogue' as applied here provides neither fair warning nor effective safeguards against arbitrary enforcement, it is void for vagueness.”
The common law principle that the people should have the right to know what the law is, means that the wording of laws should be sufficiently clear and precise that it is possible to give a definitive answer as to whether a particular course of action is legal or illegal. However, despite this ruling the Federal Analogue Act was not revised, and instead AET was specifically scheduled to avoid any future discrepancies.
As a district court decision, this case is no binding precedent.
"United States v. Washam".
"United States v. Washam" (2002) 312 F.3d 926, 930 was an appellate decision for the eighth judicial circuit in which it was considered whether the drug 1,4-butanediol (1,4-B) was a controlled substance analogue in the United States. The controlled drug which it was alleged 1,4-B was substantially similar to was gamma-hydroxybutyrate (GHB). 
1,4-B
GHB
In this case the court ruled that 1,4-B was substantially similar to GHB, on the grounds that (i) “1,4-Butanediol and GHB are both linear compounds containing four carbons and that there is only one difference between the substances on one side of their molecules”, and (ii) that 1,4-B is metabolized into GHB by the body and so produces substantially similar physiological effects.
It was raised in defense that 1,4-B and GHB contain different functional groups, and that the food additive monosodium glutamate (MSG) also metabolizes into GHB in the body, but these were not held to be grounds to consider 1,4-B not substantially similar to GHB.
It was also raised in the case of "Washam" that the Federal Analogue Act was unconstitutionally vague, but in this case the court rejected this argument on the grounds that the defendant's actions in concealing her activities and lying to DEA agents showed that she knew her actions were illegal, and furthermore that “…a person of common intelligence has sufficient notice under the statute that 1,4-Butanediol is a controlled substance analogue.” The court in "Washam" construed the Analogue Act to require parts A(i) and either A(ii) or A(iii), and concluded the Act was constitutionally permissible upon this construction.
As a result of "Washam", the Federal Analogue Act has been upheld (at least for the states and territories comprising the eighth judicial circuit) and can be considered valid at the present time.