What drug(s) did you take? If it was weed or something else that has a certain degree of a trippy/mind altering quality, I'd say maybe you wanted to further explore some type of insight that you had during the therapy. Of course, there are healthier and far more effective ways to do this than getting messed up. If it was something that was just mind numbing, then I'd say maybe you were trying to run from whatever was discussed in the session. I would also ask yourself if something triggered this in the therapy session or if it was just a random impulse you had upon leaving the session? I'll say that I have never come from a counseling session with any of the counselors I've been to with an urge to get drunk or stoned. Maybe it didn't even have anything to do with the therapy, and it was just acting on a random impulse, which is something that you and anyone else should be mindful of trying to avoid.