The excretion of loratadine, a new nonsedating antihistamine, into human breast milk was studied in six lactating nonpregnant volunteers. Each volunteer received one 40-mg loratadine capsule. Milk and blood were collected before and at specified times (to 48 hours) after dosing. Plasma and milk loratadine concentrations were determined by a specific radioimmunoassay, and those of an active but minor metabolite, descarboethoxyloratadine, by high performance liquid chromatography (HPLC). Breast milk concentration-time curves of both loratadine and descarboethoxyloratadine paralleled the plasma concentration-time curves. For loratadine, the plasma Cmax was 30.5 ng/mL at 1.0 hour after dosing and the milk Cmax was 29.2 ng/mL in the 0 to 2 hour collection interval. Through 48 hours, the loratadine milk-plasma AUC ratio was 1.2 and 4.2 micrograms of loratadine was excreted in breast milk, which was 0.010% of the administered dose. For descarboethoxyloratadine, the plasma Cmax was 18.6 ng/mL at 2.2 hours after dosing, whereas the milk Cmax was 16.0 ng/mL, which was in the 4 to 8-hour collection interval. Through 48 hours, the mean milk-plasma descarboethoxyloratadine AUC ratio was 0.8 and a mean of 6.0 micrograms of descarboethoxyloratadine (7.5 micrograms loratadine equivalents) were excreted in the breast milk, or 0.019% of the administered loratadine dose. Thus, a total of 11.7 micrograms loratadine equivalents or 0.029% of the administered dose were excreted as loratadine and its active metabolite. A 4-kg infant ingesting the loratadine and descarboethoxyloratadine excreted would receive a dose equivalent to 0.46% of the loratadine dose received by the mother on a mg/kg basis.(ABSTRACT TRUNCATED AT 250 WORDS)