This article reviews the reported pharmacokinetic interactions between cyclosporin and other drugs. Both rifampicin and the majority of anticonvulsants can decrease cyclosporin concentrations to levels that are at or below the limit of detection for most assays. There have been no reports of any interaction between valproic acid and cyclosporin. Other drugs that have been reported to decrease cyclosporin concentration include sulfadimidine and trimethoprim, nafcillin and octreotide. Erythromycin, ketoconazole and some calcium channel blockers have been clearly shown to increase the concentration of cyclosporin. Other less well documented interactions have been reported with other macrolide antibiotics, other azole antifungal drugs, high dose methylprednisolone, metoclopramide, fluoroquinolones, imipenem/cilastatin, oral contraceptives/danazol, sulindac, methyltestosterone, colchicine, acetazolamide, alcohol and cimetidine. Although the most commonly reported mechanism is inhibition of cyclosporin metabolism, there is increasing evidence that erythromycin, metoclopramide and probably other drugs increase the bioavailability of oral cyclosporin. Two calcium channel blockers which have not been reported to interact with cyclosporin are nifedipine and nitrendipine. With increasing use of cyclosporin, the number of drugs reported to interact will rise. Prudent clinicians should monitor the concentration of this agent more frequently when another drug is added or discontinued and cyclosporin dosage should be adjusted when appropriate. Sustained changes in cyclosporin concentration can result in graft rejection (or graft-versus-host disease) or renal toxicity. Further studies are needed to determine the mechanism of most of these interactions.