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Methenamine Pregnancy and Breastfeeding Warnings

Medically reviewed by Last updated on May 19, 2023.

Methenamine is also known as: Hiprex, Mandelamine, Urex

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

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.

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.

See references

References for pregnancy information

  1. Heinonen O, Shapiro S; Kaufman DW ed., Slone D. Birth Defects and Drugs in Pregnancy. Littleton, MA: Publishing Sciences Group, Inc. 1977;297.
  2. Product Information. Hiprex (methenamine). Hoechst Marion Roussel. 2002.
  3. Product Information. Mandelamine (methenamine). Parke-Davis. 2002.
  4. Gordon SF. Asymptomatic bacteriuria of pregnancy. Clin Med. 1972;79:22-4.
  5. Furness ET, McDonald PJ, Beasley NV. Urinary antiseptics in asymptomatic bacteriuria of pregnancy. N Z Med J. 1974;81:417-9.
  6. Briggs GG, Freeman RK, Yaffe SJ. Drugs in Pregnancy and Lactation. Baltimore, MD: Williams & Wilkins. 1998.
  7. Cerner Multum, Inc. UK Summary of Product Characteristics.
  8. TGA. Therapeutic Goods Administration. Australian Drug Evaluation Committee. Prescribing medicines in pregnancy: an Australian categorisation of risk of drug use in pregancy. 2007.
  9. the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists (ASCEPT), Royal Australian College of General Practicioners (RACGP), the Pharmaceutical Society of Australia (PSA). Australian Medicines Handbook. 2007.
  10. Christiansen K, et al. Therapeutic Guidelines: Antibiotic. Melbourne, Australia , AK: Therapeutic Guidelines Ltd. 2006.

References for breastfeeding information

  1. Allgen L-G, Holmberg G, Persson B, Sorbo B. Biological fate of methenamine in man. Acta Obstet Gynecol Scand. 1979;58:287-93.
  2. Product Information. Mandelamine (methenamine). Parke-Davis. 2002.
  3. Cerner Multum, Inc. UK Summary of Product Characteristics.
  4. United States National Library of Medicine. Toxnet. Toxicology Data Network. 2013.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.