Employment Law Questions? Ask an Employment Lawyer.
It would be tough to say you have a claim because most employment is at will meaning a person can be fired at any time with or without cause. The only basis for which you might have a claim is their refusal to give a second drug test. They obviously thought you were doing a good job hence the promotion. They will argue that your prior history made them think this drug test was accurate. Even though I think you were treated unfairly I do not see a basis to say you were treated illegally. I am happy to talk via phone if you like in case there is some aspect I am missing.