If you have had any problems with substance abuse in the past are you allowed to fly planes?

Yes: Employers are not allowed by federal law to discriminate due to past substance abuse.

It depends: If you're 35 now and used marijuana a few times in your teens without any consequences, you'd probably be okay. If you've had a more serious problem with addiction including arrests or frequent relapses, or you've used a lot of hard-core drugs (heroin, meth, etc.), it probably isn't safe for you to be a pilot.