@methyl - I was a regional union rep for several years. I wouldn't advise anyone to do what you have said.
''Get the letter signed by a responsible person who knows you well (ideally your doctor, but a good friend will do)''
This is like asking your doctor or a friend to write down that you do not have firearms in your house or that you do not shoplift. How do they know for sure? Would you commit yourself by stating in writing that one of your friends hasn't done something (ie never used crack cocaine)?
If boabauld is innocent and has not smoked cannabis, they should put it in their employers hand to prove it.
By boabauld saying they commit to the company doctor to do drugs testing would certainly make them think twice as to the guilt. If they do the test and it comes back negative, they will trust and respect boabauld more than those that reported them. That would be the positive outcome.