In so far as Title I of the ADA (the part which deals with employment issues) is concerned, someone currently using illegal drugs is not regarded as a
'qualified individual with a disability', to the extent that the employment decision in question is based on the illegal drug use. Generally speaking, an employer can test for illegal drug use and fire an employee on the basis there of. From
42 U.S.C. § 12114:
To the question of whether an employer has to provide an alternate position to an employee with a disability that can no longer perform their original position's functions, that's a trickier question and depends on facts and conditions specific to their situation. Generally speaking, an employer has to be willing to make
'reasonable accommodations' for the employee to be able to continue working. What does that mean? Well, for
starters:
The problem with such terms, and with legislative provisions like many of the ones included in the ADA, is that, no matter how many hundreds of pages of regulations the bureaucrats write to facilitate the application of the legislation, interested parties sometimes can't know for sure what they mean until some judge decides. These kinds of laws, and terms, beg for endless litigation. In the alternative, employers have to go beyond what might actually be required in order to avoid litigation.
The ADA, to include subsequent amendments there to, is one of the worst pieces of legislation we've ever passed - and I'm not just referring to the part that deals with employment considerations. It's a nightmare for businesses and society in general, and it's debatable whether or not it even helps those with genuine disabilities in the aggregate.
I should note that there's separate legislation, the Rehabilitation Act (
29 U.S.C. § 701 et seq.), which applies to, among other entities, federal contractors. I suppose it's possible that it weighs differently on the issue of firing illegal drug users rather than offering them rehabilitation, but I'm not inclined to believe it does. That said, I'm not really familiar with it.