I find it much more believable that Michael would take drugs, than for someone to drug him. But that's just my opinion. I have no idea if drugs are even involved here.
I do think people tend to be a little naive about drug use. You really can't tell by how someone looks, socio-economic status, grades, family, etc. I think I already shared this earlier in the thread, but my group of friends in college were routinely doing cocaine for a while. There were several of us who went out together a lot, but they kept their usage from myself and one other person in our group. We were always out together and the two of us never knew. I was shocked when they told us years later. Again; classy, beautiful, brilliant women.