Intolerance to medicine?

Hi everyone!

Has anyone ever seen or experienced this?: My boyfriend was once able to take 80 mg of Paxil for OCD and was doing really well on it, but then after about 6 months, his body started to reject the medication and he had to start lowering the dosage to the point where he was no longer taking it. Once he completely stopped taking Paxil, he was deeply depressed and had severe OCD and anxiety. The doctors tried prescribing all different medications - Zoloft, Celexa, Prozac, Lexapro, and a few others that I can't remember - but his body was not able to tolerate any of these. He would only feel "relief" for the first 2 weeks or so but then he would start to feel really sick ...

Any ideas? Thanks!