Not to keep butting in here, but I don't think the onus should be on the OP to prove why he is on a prescription medication for a recognized medical condition. If he is screened out at this point (after he's hired) for being on methadone, he shouldn't have to beg and plead for a job, he should seek legal action. This is a valid human rights issue.
I'm also not convinced about the pot. There's a chance that they claimed to find weed on the lab report, to demonstrate illegal drug use, to cover their asses.
I really, truly believe that there should be laws in place to prevent companies for screening for methadone. If private labs are getting these contracts, it should be relegated to the public sector for maximum oversight.
I believe companies can and should use every legal means of discrimination available to pick the best possible candidate. Although, I disagree with both criminal background checks, and drug testing on principle alone. That aside, they cannot use illegal means of discrimination, nor can they be trusted to simply 'strike from the record' knowledge that they should not have.