Methenamine Pregnancy and Breastfeeding Warnings

Brand names: Hiprex, Mandelamine, Urex

Medically reviewed by Drugs.com. Last updated on May 19, 2023.

Methenamine Pregnancy Warnings

This drug should be used during pregnancy only if the benefit outweighs the risk to the fetus.

AU TGA pregnancy category: A
US FDA pregnancy category: C

Animal studies using the hippurate salt have failed to reveal evidence of fetal harm. Animal studies have not been reported with the mandelate salt. This drug crosses the placenta. In 2 studies, use during pregnancy did not show an increased adverse fetal outcome when compared to controls. This drug has been in widespread use for many years without apparent harmful effects.

The Collaborative Perinatal Project reported 49 pregnancies involving first-trimester exposure to this drug. Birth defects were reported in 4 infants. In 299 pregnancies involving exposure to this drug any time during pregnancy, 12 malformations were reported (5.34 expected).

In a review of 229,101 deliveries to US Michigan Medicaid patients, 209 first-trimester exposures to this drug were recorded and 778 exposures any time during pregnancy. A total of 8 birth defects were reported with first-trimester exposure (8 expected) and included (observed/expected) 1 cardiovascular defect, 1 oral clefts, 1 polydactyly, and 1 limb reduction.

AU TGA pregnancy category A: Drugs which have been taken by a large number of pregnant women and women of childbearing age without any proven increase in the frequency of malformations or other direct or indirect harmful effects on the fetus having been observed.

US FDA pregnancy category C: Animal reproduction studies have shown an adverse effect on the fetus and there are no adequate and well-controlled studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks.

See references

Methenamine Breastfeeding Warnings

Use is considered acceptable.

Excreted into human milk: Yes (small quantities)

Comments: This drug has been used without apparent harmful effects in the nursing infant.

This drug is excreted in breast milk in quantities insignificant to the infant.

Drug levels averaged 7 mg/L in milk 5 hours after a 1 g oral dose (hippurate salt) was administered to 6 mothers nursing newborns. In 2 other women, milk levels averaged 9.1 mg/L at 2 to 3 hours and 4.3 mg/L at 6 to 7 hours after a 1 g oral dose (hippurate salt). Authors calculated the dose received by infants as 0.05 to 0.1 mg/kg (about 1% of adult dose) based on the amount of milk ingested.

In 1 study, 4 neonates were allowed to breastfeed after a 1 g maternal dose (hippurate salt); no side effects were reported.

At a dose greatly exceeding the amount found in a normal dose of methenamine mandelate, 6 mothers were given mandelic acid (3 g orally 4 times a day). According to author estimation, their exclusively breastfed infants received about 273 mg of mandelic acid daily in breast milk, which amounted to an average dose of 86 mg/kg/day (about 48% of adult dose) in the 6 infants; mandelic acid was measurable in the urine of these infants. No signs of harm (clinical or laboratory) observed in the infants.

See references

Further information

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