I don't turn anyone away for being on medication. Still, what you come to the session with does make a difference.
On strong psychiatric medication, some clients respond noticeably more dampened to suggestions — hypnosis still works, but it's weaker and harder to access. Illegal drugs and alcohol affect the outcome in a similar way, usually depending on the dose: if someone is at their usual "working level" of being slightly drunk, it's often still fine to work with.
As a general rule, though: if it's possible for you, come to the session sober — hypnosis works more directly that way.