This cross-sectional study in a sample of 887 primigravidae in Rio Branco, Acre State, Brazil aimed to analyze factors associated with the use of medicines during the first pregnancy. Information was obtained from interviews and prenatal cards. Medicines were classified according to the Anatomical Therapeutic Chemical (ATC) Classification System of the World Health Organization and risk categories according to the U.S. Food and Drug Administration. Mean age was 21 years and mean number of medicines used was 2.42. The most frequently consumed medicines were antianemics (47.5%), supplements and vitamins (18.7%), analgesics (13.8%), and antibiotics (10.5%). In the risk categorization, 69.3% belonged to category A, 22.3% to B, 7.6% to C, and 0.8% to D. The odds of having used risky medicines were higher among primigravidae in unit B (OR = 2.10; 95%CI: 1.26-3.50), in the 19 to 24-year age bracket (OR = 2.79; 95%CI: 1.58-4.93), and in the presence of a medical prescription (OR = 1.86; 95%CI: 1.18-2.95). Essential drugs were less used by women with higher family income (OR = 0.63; 95%CI: 0.42-0.96) and those who had received private prenatal care (OR = 0.53; 95%CI: 0.38-0.74).