The objective of this study was to analyze adherence and side effects of three iron supplement regimens (ferrous sulfate) on anemic pregnant women. The clinical trial involved 150 women between the 16th and 20th gestational weeks, at low obstetric risk and with hemoglobin concentration of between 8.0 and 11.0 g/dL. Treatment was provided by ferrous sulfate with 60 mg of elemental iron during 16 (+/- 1) weeks, in three regimens: single tablet a week (n = 48); single tablet twice a week (n = 53) or single tablet a day (n = 49). The outcomes were adherence, assessed through interviews and by counting tablets, and side effects, according to patient information. The adherence showed a declining trend (92%, 83% and 71%; p = 0.010) and the side effects revealed a growing trend (40%, 45% and 71%; p = 0.002) as the dosage increased. Diarrhea and epigastric pain were significantly associated with the dose administered (p = 0.002). These results suggest that in anemic pregnant women, complaints are directly proportional and the compliance is inversely proportional to the amount of medicinal iron.