In order to improve patient safety, systematic analysis of common and repetitive patterns of preventable adverse drug reactions (pADRs) in the clinical setting should be performed regularly in order to propose adequate prevention strategies. Our aim is to evaluate the preventability of all ADRs collected in a drug information research center database, by spontaneous reporting and clinical surveillance in two internal medicine departments. One reviewer systematically reevaluated all the cases stored in the database. ADRs were deemed preventable if they were due to: a contraindication, an inadequate dose, a drug interaction, an inappropriate prescribing decision for the patient's condition, inadequate monitoring, self-medication, or non-adherence to therapy. Out of 251 ADRs evaluated, 103 (41 %) were considered preventable. Out of the total pADRs, 86.4 % were serious. The most frequent adverse outcomes affected the gastrointestinal system (21.4 %), followed by the renal (11.6 %), metabolic (10.7 %), vascular (10.7 %) and hepatic (6.8 %) systems. Acenocoumarol (28 %), diclofenac (12.6 %), digoxin and furosemide accounted for more than 50 % of all preventable reports. One of up to three factors was involved in the preventability of the analyzed reports. Drug-drug interactions were the cause of 49.5 % of the pADRs. Inappropriate dose accounted for 17.5 % reports out of the total pADRs, inappropriate monitoring for 9.7 % reports, history of allergy to drug or drug class for 5.8 % reports and administration of a contraindicated drug for 4.8 % reports. Identifying prevalent pADRs in this study indicates a clear target for prevention strategies: drug prescription, with a special emphasis on drug interactions.