Acyclovir excretion in human breast milk

OBJECTIVE: To measure acyclovir concentrations in the breast milk of a lactating woman using the drug for herpes zoster while nursing her 7-month-old infant. METHODS: The maternal dosage of acyclovir was 800 mg five times daily for seven days. Three random breast milk samples collected on the fifth and sixth days of therapy were analyzed for acyclovir concentrations with radioimmunoassay (RIA). RESULTS: Acyclovir concentrations in breast milk ranged from 18.5 μmol (4.16 μg/mL) to 25.8 μmol (5.81 μg/mL). An estimate of the infant’s dosage ingested through nursing was 0.73 mg/kg/d, or approximately 1 percent of the maternal dose in milligrams/kilograms/day. The baby was nursed without any signs of adverse effects. CONCLUSIONS: Acyclovir was measured in clinically insignificant concentrations in the milk of a woman receiving large dosages for herpes zoster. Breast feeding continued without adverse effects to the nursing infant.