Job Interview Question?

I was at an interview recently,at a Rehab Facility, When the interviewer asked me if I could pass a drug test, I told him yes, then he asked me if I was on prescription drugs, and I told him yes, but non-narcotics, he told me If I could name them, and told him I would feel more comfortable if I just gave him a list, instead of blurting them out loud. Then he told me it wouldn't work out, and ended the interview. My question being.....is this grounds of discrimination? Isn't it a little personal to ask someone if they are on prescription drugs. I understand this is a Rehab place, but why would I bring my prescription drugs to work with me? Any advice or answers would be great!