Lithium excretion into breastmilk and concentrations in infant serum are highly variable. Although lithium appears on some lists of drugs contraindicated during breastfeeding,[1] most sources do not consider it an absolute contraindication in healthy-full-term infants, especially in infants over 2 months of age and during lithium monotherapy.[2-9] Numerous reports exist of infants who were breastfed during maternal lithium therapy without any signs of toxicity or developmental problems. Most were breastfed from birth and some infants continued to nurse for up to 1 year of maternal lithium therapy. Reports suggest that lithium in milk can adversely affect the infant acutely when its elimination is impaired, as in dehydration or in newborn or premature infants. Neonates may also have transplacentally acquired serum lithium levels. Lithium levels in these infants decline whether they are breastfed or not, although serum levels may fall more slowly in exclusively breastfed infants.[10,11] The long-term effects of lithium on infants are not certain, but limited data indicate no obvious problems in growth and development.[9,10,12] Lithium may be used in mothers of full-term infants who are willing and able to monitor their infants.[13] Because maternal lithium requirements and dosage may be increased during pregnancy, maternal serum levels should be monitored frequently postpartum and dosage reduced as necessary to avoid excessive infant exposure via breastmilk.[14] Infants with higher lithium levels at birth have more side effects and are more likely to be admitted to the NICU.[15] Discontinuing lithium 24 to 48 hours before Cesarean section delivery or at the onset of spontaneous labor and resuming the prepregnancy lithium dose immediately after delivery should minimize the infant's serum lithium concentration at birth.[16] Some investigators recommend monitoring infant serum lithium, serum creatinine, BUN, and TSH in intervals ranging from "periodic" to every 4 to 12 weeks during breastfeeding and maternal lithium therapy.[4,13,17] One group recommends monitoring maternal and infant serum lithium at 2 and 10 days postpartum in mixed-fed infants with additional monitoring at 30 and 60 days postpartum for exclusively breastfed infants.[10] A systematic review recommends infant lithium serum level, thyroid and renal function tests only at 10 days postpartum, then only if the infant’s serum lithium is 0.3 mEq/L or greater or if clinical signs of toxicity appear.[18] However, others recommend close pediatric follow-up of the infant and only selective laboratory monitoring (i.e., serum lithium, creatinine, TSH, BUN) if clinically indicated by unusual behavior, restlessness, feeding difficulties, sedation or abnormal growth and development. Infants who are preterm, dehydrated, or have an infection, should receive hydration and be assessed for lithium toxicity.[6,7] If the infant serum lithium level is elevated, reducing the percentage of breastfeeding can decrease it.[9]