This link seems to have a lot of information on different medications and how caffeine and nicotine can interact with them.
This is a hot topic in research. I gave a presentation on caffeine and antipsychotics last semester in abnormal psychology, the verdict from the study I examined was that caffeine had a slight but insignificant effect of the mental health of well-medicated patients with schizophrenia. It differs from drug to drug, though. I just finished my first cup myself!