Exceptions to Federal Ban
"Only two limited exceptions to the federal ban on marijuana have been made. The first, a compassionate use program created under President Carter, is superficially analogous to extant state medical use programs; it allows patients to use marijuana legally for therapeutic purposes. The marijuana for the program is supplied by a federally approved grow-site at the University of Mississippi (the only federally approved grow-site in the United States). However, the program stopped accepting new applications in 1992, and only eight (yes, eight) patients currently receive marijuana through it. Over its entire history, only thirty-six patients have been enrolled.52 The second and only other way to obtain marijuana legally under federal law is by participating in an FDA-approved research study. But since the federal government approves so few marijuana research projects—eleven since 200053—only a small fraction of the population that currently qualifies for state exemptions could participate."
Miklos, Robert A., "On the Limits of Supremacy: Medical Marijuana and the States’ Overlooked Power to Legalize Federal Crime," Vanderbilt Law Review (Nashville, TN: Vanderbilt University Law School, March 9, 2009), p. 113.