Iannella 2026: naltrexone levels in pregnancy, cord blood and breast milk

What they tested

Pharmacokinetics is the study of how the body absorbs, spreads and clears a drug. Researchers enrolled 7 pregnant people on naltrexone: 4 on the extended-release injection and 3 on tablets.

They took the mother's blood and urine up to 6 times in pregnancy. At delivery they collected cord blood, the mother's blood and the baby's blood and urine. Four weeks after birth they collected breast milk and blood from mother and baby. Samples were tested for naltrexone and its main breakdown product, 6β-naltrexol.

What they found

Naltrexone stayed detectable in the mother's blood during pregnancy across the dosing interval with both the shot and the tablets.

At delivery, the median ratio of cord blood to maternal blood concentration was 1.11 in the shot group and 0.74 in the tablet group.

Of the 7 infants, 1 was still breastfed at 4 weeks. That baby's relative infant dose of naltrexone through breast milk, measured 31 days after delivery, was 0.83%.

Limits

• With 7 people, and breast milk data from a single baby, the numbers are a first look rather than reliable averages. The authors note the small sample themselves.
• It measured drug levels, not whether the babies had any health effects.

What it means if you're considering naltrexone

This study confirms that naltrexone crosses to the baby during pregnancy and passes into breast milk in small amounts. The site's pregnancy guide notes that only two breastfed babies have been measured in published reports, so these figures help with counselling but can't rule out rare effects.