Medication use by the nursing mothers is a controversial topic, with the potential for infants to be exposed via breastmilk. This ethical dilemma has led to health professionals, authorities, and parents considering infant formula to be a reasonable alternative to mitigate risk. As a consequence, there is a lack of evidence about the safety and efficacy of many medications during lactation, including lithium1. Decisions to use lithium during the perinatal period are challenging because information about the efficacy and safety for the mother-child dyad come from observational studies of lactation outcomes, which have important limitations and biases