OBJECTIVES: This study aimed to characterize and estimate the frequency of adverse reactions to antituberculosis drugs in the population treated at the Centro de Sau´ de Escola Germano Sinval Faria, a primary health care clinic in Manguinhos, Rio de Janeiro City, and to explore the relationship between adverse drug reactions and some of the patients’ demographic and health characteristics. METHODS: This descriptive study was conducted via patient record review of incident cases between 2004 and 2008. RESULTS: Of the 176 patients studied, 41.5% developed one or more adverse reactions to antituberculosis drugs, totaling 126 occurrences. The rate of adverse reactions to antituberculosis drugs was higher among women, patients aged 50 years or older, those with four or more comorbidities, and those who used five or more drugs. Of the total reactions, 71.4% were mild. The organ systems most affected were as follows: the gastrointestinal tract (29.4%), the skin and appendages (21.4%), and the central and peripheral nervous systems (14.3%). Of the patients who experienced adverse reactions to antituberculosis drugs, 65.8% received no drug treatment for their adverse reactions, and 4.1% had one of the antituberculosis drugs suspended because of adverse reactions. ‘‘Probable reactions’’ (75%) predominated over ‘‘possible reactions’’ (24%). In the study sample, 64.3% of the reactions occurred during the first two months of treatment, and most (92.6%) of the reactions were ascribed to the combination of rifampicin + isoniazid + pyrazinamide (Regimen I). A high dropout rate from tuberculosis treatment (24.4%) was also observed. CONCLUSION: This study suggests a high rate of adverse reactions to antituberculosis drugs.