Breastfeeding women may experience various health issues that require medication. This survey aimed to gain insights into the use of nonprescription and prescription drugs by breastfeeding women in Japan. A cross-sectional study involving women with children aged under two years was conducted in Fukuoka, Japan. Nonprescription drugs were used by 26% of participants in the breastfed-only group, 41% in the breastfed more than half the time group, 55% in the formula-fed more than half the time group, and 82% in the formula-fed-only group. We found that when breastfeeding rates decreased, the use of nonprescription drugs increased (p < 0.05, Cochran–Armitage test for trend). There were significant differences in the use of nonprescription cold medicines and oral analgesics between the formula-fed and breastfed groups, but a nonsignificant difference in prescription drugs use between the groups. These results indicated breastfeeding had a significant influence on use of nonprescription drugs, which was not observed with prescription drugs. Breastfeeding women commonly used the Internet to obtain information on both nonprescription and prescription drugs; however, this did not influence medication use.