The objectives of this study were to determine the extent of coprescribing of statins with medications that may increase the risk of adverse events and to identify associated patient and prescriber characteristics. The authors conducted a retrospective cohort study in a large database representative of the U.S. prescription-filling population. The authors studied the U.S. retail prescription environment as reflected in prescriptions dispensed. Patients filling a prescription for a statin between October 1, 2001, and September 30, 2002 were studied. A coprescription for medications of interest as reflected in the warnings and precautions of statin product package inserts was studied: fibric acid, niacin, azole antifungal, macrolide antibiotics, nefazodone, protease inhibitors, verapamil, or warfarin within 3 weeks of a statin prescription. Patient and prescriber characteristics and proportion of coprescribing events attributable to the same prescriber were examined.A total of 5,637,918 patients filled a statin prescription during the 12-month study period. Nearly 19% had concomitant use of a medication with warnings and/or precautions for coprescribing (coprescription event). Coprescription was highest for fibrates, macrolides, and niacin accounting for 19.3%, 17.4%, and 9.4%, respectively, and did not differ across statins. One-third of coprescribing events involved same-day prescriptions, 57% occurred within 7 days of the index statin prescription, and 71.2% of all coprescription events originated from the same prescriber. Coprescribing of medications not compatible with statins occurs frequently. The extent of coprescribing indicates the need for additional intervention to assure that the risk of untoward effects is mitigated.