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Zanamivir Pregnancy and Breastfeeding Warnings

Medically reviewed by Last updated on Sep 15, 2022.

Zanamivir is also known as: Relenza

Zanamivir Pregnancy Warnings

Animal studies have failed to reveal evidence of embryofetal toxicity, teratogenicity, or maternal toxicity. No adverse developmental effects observed with IV (rats and rabbits) or subcutaneous (rats) administration at exposures 300 and 150 times, respectively, the systemic exposure at the maximum recommended human inhalation dose (MRHID); while no adverse maternal or embryofetal effects were observed at 27 mg/kg/day subcutaneously (resulting in systemic drug exposure 150 times the exposure at the MRHID), an increased incidence of skeletal and visceral changes and variants was observed at 240 mg/kg/day subcutaneously. This drug has been shown to cross the placenta in animal studies; there is no data regarding placental transfer in humans. There are no controlled data in human pregnancy.

A study of population-based registers reported outcomes of pregnant women who filled a prescription for a neuraminidase inhibitor compared with outcomes in unexposed pregnant women in the general population. This study included 1560 women who filled a prescription for this drug (including 321 first trimester exposures). No specific analyses were conducted for this drug, but exposure to neuraminidase inhibitors in utero was not associated with major birth defects, preterm birth, low birth weight, small for gestational age, stillbirth, or neonatal morbidity/mortality.

Some studies compared outcomes of pregnant women exposed to this drug with outcomes in various comparator cohorts; such studies indicated no increased risk of major birth defects, preterm birth, or low birth weight. Limitations of available studies include lack of specific analyses for this drug, possible exposure and outcome misclassification, and small sample sizes (ranged from 50 to 180 pregnant women exposed to this drug, 15 to 44 with first trimester exposure); such limitations prevent definitive risk assessment.

Pregnant women have higher risk of severe complications from influenza, which may lead to adverse pregnancy and/or fetal outcomes (including maternal death, stillbirths, birth defects, preterm delivery, low birth weight, and small for gestational age).

AU TGA pregnancy category B1: Drugs which have been taken by only a limited number of pregnant women and women of childbearing age, without an increase in the frequency of malformation or other direct or indirect harmful effects on the human fetus having been observed. Studies in animals have not shown evidence of an increased occurrence of fetal damage.

US FDA pregnancy category Not Assigned: The US FDA has amended the pregnancy labeling rule for prescription drug products to require labeling that includes a summary of risk, a discussion of the data supporting that summary, and relevant information to help health care providers make prescribing decisions and counsel women about the use of drugs during pregnancy. Pregnancy categories A, B, C, D, and X are being phased out.

This drug should not be used during pregnancy (especially during the first trimester [according to some authorities]) unless the benefit outweighs the risk to the fetus.

AU TGA pregnancy category: B1
US FDA pregnancy category: Not assigned.

Risk summary: Malformative risk with use of this drug in pregnant women is unlikely.

-Maternal and embryo/fetal risk due to the mother's underlying condition should be considered.

See references

Zanamivir Breastfeeding Warnings

Benefit to the mother should outweigh risk to the infant.
-According to some authorities: A decision should be made to discontinue breastfeeding or discontinue the drug, taking into account the importance of the drug to the mother.

Excreted into human milk: Unknown
Excreted into animal milk: Yes

-No data available regarding use of this drug during breastfeeding.
-Developmental and health benefits of breastfeeding should be considered as well as the mother's clinical need for this drug.
-The effects in the nursing infant are unknown; potential side effects in the breastfed child due to this drug or the mother's underlying condition should be considered.

Adult data have shown low oral bioavailability of this drug. In limited data from postmarketing reports, no harmful effects suggested in infants exposed to breast milk of mothers using this drug.

According to author estimation, an exclusively breastfed infant (5 kg) would receive about 0.075 mg/day in breast milk after an inhaled 10 mg maternal dose; this is less than 1% of the dose in older children. Also, due to poor oral absorption, this drug is not likely to reach the infant's bloodstream in clinically significant amounts.

See references

References for pregnancy information

  1. "Product Information. Relenza (zanamivir)." Glaxo Wellcome (2001):
  2. Cerner Multum, Inc. "UK Summary of Product Characteristics." O 0
  3. "Antiviral drugs prophylaxis and treatment of influenza." Med Lett Drugs Ther 48 (2006): 87-8
  4. TGA. Therapeutic Goods Administration. Australian Drug Evaluation Committee "Prescribing medicines in pregnancy: an Australian categorisation of risk of drug use in pregancy." (2007):
  5. Cerner Multum, Inc. "Australian Product Information." O 0

References for breastfeeding information

  1. "Product Information. Relenza (zanamivir)." Glaxo Wellcome (2001):
  2. Cerner Multum, Inc. "UK Summary of Product Characteristics." O 0
  3. Cerner Multum, Inc. "Australian Product Information." O 0

Further information

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