Pharmacokinetic definitions

Peak time

Peak time describes the time at which the medicine reaches the highest level in the mother’s serum. As most medicines transfer into the milk via passive diffusion, it is also the time when the medicine is at the highest concentration in the breast milk. If possible, avoid breastfeeding at this time to minimise infant exposure.


The medicines’ half-life value refers to the period of time required for the concentration or amount of medicine in the maternal circulation to be reduced by one-half.  It takes approximately five half-lives for medicines with linear pharmacokinetics to be eliminated from the body. Medicines with long half-lives have the potential to accumulate in the breast milk.

Protein binding

The protein binding is the percentage of medicine bound to maternal plasma protein. Medicines that are minimally protein bound may be more likely to enter the milk compartment than those that are highly protein bound, but they are excreted much faster. 

Molecular weight

The molecular weight is a determinant for the entry of a medicine into the placenta or breast milk. Medicines with a lower molecular weight are likely to transfer cross the human placenta or breast milk compartments.

Iodine content

The amount of iodine (percentage by weight) bound within the compound.  


The ionisation constant (pKa) of a medicine is the pH at which the medicine is equally in the ionised and non-ionised form.

If a medicine is in the ionised form in breast milk, it is less able to transfer from the milk compartment back to the maternal compartment. This results in the medicine being trapped in the milk (ion trapping). Medicines with a pKa of greater than 7.2 are more likely to be trapped in breast milk compared to a medicine with a lower pKa.

Oral bioavailability

Oral bioavailability is the fraction of an oral administered medicine that reaches the bloodstream.

Last Updated: 7 November 2018

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Practical recommendations

The Therapeutic Goods Administration (TGA) pregnancy categories and manufacturer’s product information are sometimes difficult to interpret. The Women’s PBMG has developed a series of practical recommendations for medicines used during pregnancy and breastfeeding.