Breastfeeding is universally recognized as the preferred method of infant nutrition, but is sometimes abbreviated because of fear of harm to the infant from maternal medication. The amount of medication that enters breast milk varies based on the maternal serum concentration and the pharmacologic properties of the medication. When prescribing medications for a breastfeeding patient, those with the lowest risk to the infant should be selected, and dosing should be before the infant’s longest sleep interval. Prescribers should use current, accurate resources. LactMed is a convenient, government-sponsored, authoritative resource that lists safety information for many medications and is available free online. When mental health conditions occur during lactation, priority should be given to effectively treating the mother, often with medications that were effective during pregnancy. Most antidepressants are compatible with breastfeeding. Stimulant medications may decrease milk supply. Insulin, metformin, and second-generation sulfonylureas are generally preferred to treat diabetes mellitus during breastfeeding, but newer agents require caution because they have not been studied in lactation. Inhaled and nasal treatments for asthma and allergic rhinitis are unlikely to affect breastfed infants. Acetaminophen and ibuprofen are preferred analgesics during lactation. Maternal opioid use can cause infant sedation. Herbal supplements are concerning for risk of impurities and lack of study of effects on breastfed infants. Nonhormonal and progestin-only contraceptives are preferred over combination oral contraceptives. Contrast for computed tomography or magnetic resonance imaging is not concerning during lactation, but use of radiopharmaceuticals, such as iodine 131, can accumulate in the lactating breast and increase risk to the infant.