BACKGROUND Seven percent of acute hospital admissions result from adverse drug reactions, of which 25% are due to drug interactions. Adverse effects of statin drugs occur in 3% of patients, mainly due to co-prescribing with other lipid-lowering agents or agents that alter their metabolism. AIM The aim of this study was to investigate co-prescribing of the frequently-used statin medications with interacting drugs. METHODS Data from the General Medical Services (GMS) scheme of the Eastern Health Board from January to December 1998 were used in this study. Using the coding index for statins, co-prescribing was identified when concomitant medications were administered under the same GMS claim number. RESULTS Of 7,602 patients prescribed statins, co-prescribing of simvastatin, atorvastatin and fluvastatin with competing substrates or inhibitors of their metabolism occurred in 32, 26 and 13.4% of prescriptions issued. Thirty-four per cent of patients on simvastatin, 28% on atorvastatin and 16% on fluvastatin were prescribed medications with drug interaction potential. CONCLUSION Co-prescribing of statins with competing substrates or inhibitors of their metabolism occurred in up to one-third of prescriptions issued. When statins are co-prescribed with recognised inhibitors of drug metabolism, pravastatin, which does not undergo significant hepatic metabolism, is the statin of choice.