Legally get fired with a false reason, like you were told
to go see the doctor for other ppls protection and refused. I think your
boss is just covering his a$$. Do yourself a favor, if you ever did drugs and it shows up anywhere in your medical history, be able to give a good explanation. If it doesn't show anywhere in your history, keep it to yourself, don't admit to using drugs unless you absolutely have to. My guess is that you didn't expect that from him, that really sux.