Discussed in detail here on SCOTUSblog re: McFadden v. United States: http://www.scotusblog.com/2015/04/argument-preview-the-tricky-mens-rea-requirements-for-federal-narcotics-cases/
[DNM relationship obvious]
Edit (since nobody wants to read court docs on a Saturday):
Summary of the defense's argument:
The purpose of this amicus brief is to highlight problems associated with the term "substantially similar" within the Controlled Substance Analogue Enforcement Act of 1986. 21 U.S.C. §§ 802, 813 (2013) ("CSAEA"). The issues are significant and relevant to this Court's analysis of the prosecutorial requirements under the statute.
The CSAEA provides that, to the extent a controlled substance analogue is intended for human consumption, it is to be treated as a schedule I controlled substance for purposes of any federal law. Id. The CSAEA was designed to combat the problem of persons who seek to avoid the reach of federal criminal drug laws by manufacturing or distributing substances that are not listed as controlled substances, but which are specifically designed to provide effects that are "substantially similar" to the effects of listed substances.
The primary challenge of an alleged analogue evaluation is that the statutory provision set out in 21 U.S.C. § 802(32)(A)2 does not define "substantially similar." The classification of two chemical substances as "substantially similar" for the use in the legal system is concerning for two reasons.
First, the phrase "substantially similar" has no quantifiable meaning and, thus, no objective criteria for its measurement exist. There simply is no established and agreed upon rubric in the scientific community to measure structural similarity. Additionally, neither the field of forensic chemistry, nor any major academic, government, or technical forensic science entity has developed a standard definition for "substantially similar" or a standard method for the evaluation of alleged analogs.
Second, when attempting to compare either chemical structures or the effects of the two substances, scientists do not agree on what should be compared, nor the weight each comparison should be given. In other words, similarity depends on what is being compared, whether it is core structure, functional group, or some other factor. Further, even if scientists were to agree on what to compare within the chemical structure of two molecules when comparing the structural similarity, the CSAEA fails to provide a defined threshold where ordinary similarity ends and substantial similarity begins.
Stopped reading after this