Nifurtimox has been used to treat Chagas disease for 40 years, but tolerance and safety data in adults are scarce. We aimed to evaluate nifurtimox tolerance and safety in a cohort of Trypanosoma cruzi-infected adult patients in a country of nonendemicity.This observational study included all consecutive adults patients who were given a diagnosis of T. cruzi infection from June through December 2008. Eligible patients received nifurtimox at 10 mg/kg/day for 60 days, with regular medical and biological follow-up. Adverse events (AEs) were recorded according to Common Terminology Criteria for Adverse Events, version 3.0.Eighty-one patients received nifurtimox. Eight were lost to follow-up during treatment, and 41 (56.2%) completed the 60-day course. All premature treatment terminations were caused by AEs; 97.5% of patients suffered from AEs, mostly expected (90.5%) and not severe. Gastrointestinal symptoms predominated. Six (7.4%) patients presented with a suspected unexpected serious adverse reaction: drug reaction with eosinophilia and systemic symptoms (n = 3), Quincke edema (n = 1), acute myocarditis (n = 1), and anaphylaxis (n = 1). Patients with 3 or more AEs had an increased risk of premature treatment termination (hazard ratio, 8.42; 95% confidence interval, 1.6-45.5).Nifurtimox is poorly tolerated among adults with chronic Chagas disease, resulting in a low treatment completion rate. Considering the significant risk of serious AEs, close monitoring is required, which may be difficult to implement in poor rural areas of countries of endemicity. The safety and efficacy of nifurtimox and benznidazole should be compared to improve current therapeutic recommendations, and pharmacovigilance systems should be enhanced.