Is it legal for a former employer to give out false information about me to someone other than another employer?

If your former boss told anyone else (even another employer) that you were fired for drug use when in fact that is not true, he/she committed the tort of slander and both he and his employer may be liable to you for damages in a civil suit.  In most jurisdictions, there are some types of slander for which you can recover even if you cannot show actual monetary damage, and others in which you can only recover for actual money damages you suffered - for example, if you can prove that you did not get a job because of the lie your former boss told.    I believe this case involves the first type, slander per se, which would be a much easier case to prove, however, if you did not suffer any actual money damages, it may be very difficult to get a jury to award you any substantial sum.  You should also be aware that there are many factors which could change this situation.  For example, slander (oral defamation) and libel (defamation in writing) only apply to statements of fact, not opinion; if your former boss told your stepfather that he THOUGHT you were doing drugs, this would not be slander, as it would only be his opinion.  There are also situations where statements which can be privileged, although it does not appear from the facts in your question that any privilege would apply here.  In short, it's a complicated situation.