I don't think it is personally safe to go to a therapist in an impaired state -- although the one I see suggested I drink or get a friend to give me a pill (this particular therapist seems to believe everyone takes abilify or xanax) before going to an appointment to help with the enormous anxiety I deal with in going to the therapist appointment.
She also said clients come in stoned or drunk all the time. I don't think it surprises them although some of them seem to handle it better than others.