At worst it should be states' rights issue. It took a constitutional amendment to ban alcohol, the product of processed plants. The federal government, as per the constitution, has no right to impose such bans on the people.
I'm sure the sale of marijuana for use in the study would probably run afoul of the good old commerce clause that lets the federal government regulate sale of just about everything.
Basically because of the atrocity that was Wickard .v. Filburn, 317 US 111, 1942, if interested.