The US is one of the worst, (if not the worst), countries in the world as far as drug testing of employees. In Canada, the Human Rights Act prohibits all pre-employment drug/alcohol testing and prohibits random drug/alcohol testing of employees unless they are deemed to hold legitimately safety-sensitive positions (one in which incapacity due to drug or alcohol impairment could result in direct and significant risk of injury to the employee, others, or the environment, and has been proven as such). Of course, some private employers don't always follow the rules and there has been a lot of pressure from the US to adopt similar drug testing policies to their's. Drug/alcohol testing is really only supposed to be performed due to reasonable cause, and only in people in positions where there is a significant safety risk. Then the employer is supposed to do everything within reason to accomodate the person, with dismissal being the last resort. The problem is that it can be hard to prove that you were dismissed due your drug/alcohol use, many employers will just dismiss someone for other reasons rather than give them a drug test. But not all are like that, for example I have an acquaintance with a drinking problem that is noticeably affecting his work and his company has given him multiple chances to stop drinking on the job, offered to send him to detox, changed his responsibilities so that he wouldn't be operating any heavy machinery, etc, and as of yet he still hasn't been fired. But I think there is a lot more stigma attached to hard drug use, especially heroin, than to alcohol use.
In my situation where I was fired essentially for admitting I was on methadone, I think it was more likely that my boss just believed that I was likely still using heroin, I don't think it was that she thought the methadone itself would impair my ability to do my job, just that she didn't trust people who were former heroin addicts. Plus I think she wanted an excuse to hire her friend to replace me.
If I were in Kid Amine's situation I would NOT have considered it justifiable unless you were newly on a high level of methadone that was causing noticeable physical effects. But it's such a tough situation because taking it to court could be a huge ordeal and there is such a huge stigma against drug users (even former ones) and MMT that it might be more stress than it's worth. I would have at least gotten a second opinion from another doctor and a letter stating that there appeared to be no impairment due to your medication.
thisisabsurd - I really don't think you should have the same problems since you are just taking methadone for pain, especially if the job is not one where being a little sleepy could seriously diminish your capacity to safely perform your job. The only problem I really foresee is that you lied to them on your drug disclosure form. So hopefully the methadone is out of your system, and if it isn't, hopefully they give you a chance to explain.