How do you prove it?
Yes it's a scheduled II controlled substance (same as cocaine), but there's a jump from it being in his system and you proving he bought it illegally.
If he has high doses then we all 'know' he did, but knowing (or being sure of just because) and proving are not the same thing.
Let's take this scenario that however unlikely *could* be true.
A buddy of his who was legally prescribed the drug crushed them up finely and slipped a couple into his drink to help him party.
Martin realizes what has happened and also realizes his buddy will go to jail if he is honest. So he blames himself and takes the rap when he tests positive.
Of course that almost never happens, but almost is the operative word.
Can you prove it?
There is nothing either good or bad, but thinking makes it so.