A novel index for expressing exposure of the infant to drugs in breast milk.

1. We present a novel index for expressing the exposure of the infant to drugs in breast milk, which unifies two independent factors: a pharmacokinetic parameter, drug clearance, and a physiochemical parameter, i.e. milk‐to‐maternal plasma drug concentration ratio (M/P ratio). 2. During breast‐feeding by a woman receiving therapeutic doses of a drug at steady state this index is given by: Exposure index = A × (M/P ratio)/CLI where A is a coefficient (10 ml kg‐1 min‐1) and CLI is drug clearance in the infant (ml kg‐1 min‐1). 3. This equation indicates a hyperbolic relationship between drug clearance and the exposure level of the breast‐fed infant at a given M/P ratio of drug, emphasizing the importance of drug clearance as a determinant of infant exposure. 1994 The British Pharmacological Society