Abstract Objectives: To determine the frequency of adverse effects related to the use of the tuberculosis treatment regimen designated Regimen I and the need for regimen alterations due to these effects. Methods: A retrospective analysis of 329 medical charts of patients who were treated with Regimen I and discharged after cure between March 2000 and April 2006 was carried out at the Tuberculosis Outpatient Clinic, Department of Pulmonology of the Santa Casa de Misericórdia de São Paulo Hospital in the city of São Paulo, Brazil. Adverse effects and the timing of their appearance, as well as subsequent modifications in the treatment regimen, were investigated. Results: We included 297 patients, 146 (49.1%) of whom presented one or more adverse effects related to antituberculosis medications. The frequency of minor side effects was 41.1%, and that of major side effects was 12.8%. The most common reactions were those involving the gastrointestinal tract (40.3%) and the skin (22.1%). Adverse effects were more common in the first and second months of treatment (58.4%). Modification of the treatment regimen was necessary in 11 cases (3.7% of the total sample). Drug-induced hepatitis was the adverse effect that demanded the most regimen changes. Conclusions: In this group of patients, the frequency of adverse effects related to treatment with Regimen I was 49.1%. However, in most of the cases, it was not necessary to modify the treatment regimen due to side effects.

Keywords: Tuberculosis/therapy; Antitubercular agents/adverse effects; Hepatitis, toxic.