<jats:title>Abstract</jats:title><jats:sec><jats:title>Purpose</jats:title><jats:p>Adverse drug reactions (ADRs) are a major cause of hospital admission and inpatient morbidity. The department of internal medicine is not an exception to this issue. This study was performed to determine the nature and frequency of ADRs in an internal medicine ward in Iran.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>This survey was a prospective observational study based on admissions of 400 patients to the internal medicine ward over a 15‐week period. Patients were intensively followed in order to assess any ADR as a cause of admission or occurring during hospitalization. Any suspicious ADR was confirmed by a pharmacist/pharmacologist.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>There were 47 patients of 400 patients (11.75%) that experienced at least one ADR. ADR leading to the admission was seen in seven cases (1.75%) and in 40 (10%) it occurred during hospitalization. ADRs were identified as preventable reactions in 50% of cases and as predictable in 94.3%. The severity of 18.6% of the ADRs was identified as mild, 62.9% as moderate, 14.3% as severe and 4.3% as lethal. Gastrointestinal system disorders (44.3%) represented the most frequent ADRs. The therapeutic groups that most commonly associated with suspected ADRs were antineoplastic, immunosuppressive and medicines used in palliative care (54.8%).</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>ADRs are common among hospitalized patients in department of internal medicine and can be severe and even lethal. Since most ADRs occurred during hospitalization in studied patients and half of them were preventable, prevention strategies should be considered in hospitals. Also, our findings confirmed the role of hospital pharmacists in the reducing ADRs. Copyright © 2008 John Wiley & Sons, Ltd.</jats:p></jats:sec>