What is forbidden in Facebook ads?

Facebook bans all ads depicting or selling drugs, even if they're technically legal. So don't try to sell prescription medication on Facebook. Illegal drugs or drug-related products. Any recreational or illegal drugs are strictly forbidden from Facebook ads, and so are images or videos that depict their use.

Borderline bullying & harassment, hate speech, and violence & incitement: Content that comes close to but does not cross the line of our Bullying and Harassment, Hate Speech, and Violence and Incitement policies.

If a Facebook ad account has been disabled, it's usually because of a repeated offense. Your ad account can get banned because of your ads, or “suspicious” activity on your account. You can read the entire list of advertising policies here.