I have come to doubt whether the FDA rules should apply to cannabis. There is no question about its safety. It is one of humanity's oldest medicines, used for thousands of years by millions of people with very little evidence of significant toxic effects. More is known about its adverse effects than about those of most prescription drugs.
The consciousness-expanding drugs - the hallucinogens, such as cannabis, mescaline, LSD, Psylocybin - I think are useful to a writer up to a certain point. That is, they open psychic areas that would not otherwise be available to the writer. But I feel that once these areas have been opened and the writer has reached them, he is able to get back there in the future without the drug.