Drugs and breast feeding

Abstract

The characteristics of a drug which influence the rate and amount excreted in breast milk include its protein binding capacity — only those molecules not bound to protein are available for transfer. Other factors include the degree of ionisation of the drug at its physiological pH. Drugs may gain access to the milk by one of three mechanisms:

• 1. Most drugs pass into milk by simple diffusion (for examples, ethanol, tetracycline).

• 2. For drugs that ionise, diffusion is governed by the pH of plasma (74) and that of milk (69), bases passing into milk much more freely than acidic ions.

• 3. There may be active transport into the milk (for example, thiouracil).