Pharmacokinetic interactions involving the cytochrome P450 system have been a recent focus of clinical and research interest in psychopharmacology. The authors reviewed 100 consecutive patients seen on a consultation-liaison (C-L)psychiatry service for potential drug-drug interactions. The patients were taking a mean of 8.8 medications; for those undergoing organ transplantation the mean was 12.9. While both enzyme inhibitors (n = 87) and substrates (n = 89) were commonly used, only rarely were they used in the same patient (n = 14). The authors conclude that the potential for drug interactions on a C-L psychiatry service is significant, necessitating awareness of this potential complication.