Table 1 Definitions and clinical relevance of the pharmacokinetic parameters used to assess the lactation risk following maternal intake of medications

Half-life or “T ½”

The half-life of a substance is the time it takes for its plasma concentration to halve. If the half-life is long (>12-24 hrs), drugs may accumulate in maternal plasma and the time of drug transfer from plasma to breast milk is longer.

Maternal plasma protein binding (PB)

This parameter is expressed in percentage. The higher the percentage of the drug bound to the maternal plasma proteins, the less the drug passes into breast milk. An ideal drug to be taken during breastfeeding should have a plasma protein binding > 80%.

Milk-to-plasma ratio (M/P)

It denotes the ratio of the drug concentration in the mother’s milk (M) divided by its concentration in the mother’s plasma (P). It is an indicator of drug transfer into breast milk. A M/P ratio greater than 1.0 suggests that the drug may be present in breast milk in high concentrations.

Oral bioavailability

It describes the fraction of one orally administered dose of a drug that reaches the systemic circulation. It is expressed as a percentage of the administered dose. When the intestinal absorption of a drug is impaired, the risk of adverse effects may be lower.