In the armed forces, while you may not use any drug that is illegal, the issue surrounding drugs is more than the legal aspects. The question you have to ask yourself is: Can I rely on the quick thinking and judgement of the person next to me when my life is in danger? If the person in question is having flashbacks, has blown too many synapses thru overuse of pot, etc., etc., the answer is NO. The prohibition against drugs is partly legal, but mostly has to do with being unfit for duty. And it's the second part that ticks other members off.
If it ain't from the base pharmacy, or prescription given to you by a doctor, stay away.