In plain words
Half-life is the time it takes for the level of a substance in the blood to fall by half. It is used for caffeine, alcohol, and many medicines. A short half-life means a substance clears quickly. A long half-life means it stays longer. In pregnancy, the body clears some substances more slowly, so half-life can stretch. For breastfeeding, half-life helps explain how long a substance may remain in milk. It is a rough average, and it varies from person to person. It does not tell a mother exactly how much reaches her baby. Those questions are usually best answered with a provider or a reference built for breastfeeding.
The deeper version
Elimination half-life is the time for plasma concentration to fall by half during the terminal elimination phase. It equals ln 2 divided by the elimination rate constant, and it depends on volume of distribution and clearance. Clearance for many compounds depends on hepatic enzymes, such as CYP1A2 for caffeine. CYP1A2 activity falls progressively in pregnancy under estrogen influence, so caffeine half-life lengthens in later pregnancy. For lactation, compounds with short half-lives and low milk-to-plasma ratios are generally considered more compatible, as summarized in Hale and Rowe's Medications and Mothers' Milk (2019) and the database. Pharmacokinetic reasoning is extrapolated largely from adults. Infant exposure data are sparse, and infant elimination capacity changes with age, especially in the first weeks.
Where it comes up
- Alcohol, caffeine, and breastfeedingFeeding Your Baby
- Coffee and caffeinePregnancy: Eating
- Medications and breastfeeding: how to checkFeeding Your Baby
What mothers here say about half-life
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Sources
- Hale TW and Rowe HE — Medications and Mothers' Milk (2019 edition)
- US National Library of Medicine — LactMed: Drugs and Lactation Database