Atenolol in human plasma and breast milk.

To assess the problem of continuing atenolol in a breast-feeding mother with hypertension associated with thyrotoxicosis, studies were performed to determine simultaneous plasma and breast milk concentrations of atenolol after a single dose of 50 mg and on continuous therapy of varying weekly dosages. Peak atenolol concentrations in breast milk were 3.6 times higher than simultaneous plasma atenolol concentrations after single-dose administration and 2.9 times higher after continuous-dose administration. A single plasma concentration obtained in the infant following a peak level feeding was less than 10 ng/ml. Although it would seem unlikely that the amount of atenolol excretion in breast milk should warrant interruption of breast-feeding because of maternal medication with ordinary doses, the infant should be evaluated for signs of beta-blockade.