I do not believe actors have to submit to random drug tests, and presumably a good agent could make sure that was NOT in the contract. However, many contracts - Disney included - will include a morality clause of some kind stating that the actor will refrain from breaking the law or doing various things which could generate negative publicity and hurt the show, production company or network. If you gained a Miley Cyrus level of fame and did something that is technically against the clause (like getting caught doing drugs or having a wild affair) you mich get away with a slap on the wrist in a private meeting and a public statement of support, but if you were merely a barely known bit player, you'd likely be terminated.
In short, if you get a good part in a successful show, don't risk it by doing drugs.
