Accumulation of atenolol and metoprolol in human breast milk

Summary

Passage of the cardioselective beta adrenoceptor antagonists atenolol and metoprolol from serum to breast milk was assessed in 7 lactating women treated with atenolol due to hypertension developing during pregnancy, and in 3 healthy women who agreed to take metoprolol at cessation of lactation. For both drugs, the concentration in breast milk was higher than that in serum at every time studied, and the resulting AUC values were 1.5–6.8 times (atenolol) and 2.6–3.7 times (metoprolol) greater in milk than in serum. Assuming ingestion of 75 ml milk per meal, and as the maximum milk concentrations recorded were 6.35 µmol/l (atenolol) and 2.58 µmol/l (metoprolol), the data indicate that the dose following a meal at the time of maximum maternal drug concentration would not exceed 0.13 mg atenolol and 0.05 mg metoprolol, and would be considerably less after the other meals. In the only infant from whom serum samples could be obtained, the plasma atenolol concentration ranged between 0 and 0.26 µmol/l. None of the atenolol-exposed infants had any sign of an effect of the beta blocker. It would seem likely that, unless renal (atenolol) or hepatic (metoprolol) function in the infant were pronouncedly impaired, breast feeding need not be interrupted due to maternal medication with ordinary doses of either of these drugs. However, the infants should be observed for signs of beta blockade.