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Safety Data on Ondansetron in Pregnancy

Finally, we’ve got some robust evidence to answer the question – is ondansetron safe to take for morning sickness.

Published in JAMA, a very large retrospective study involving over 1.8 million pregnancies, almost 90,000 of which included exposure to ondansetron in the first trimester has found that taking the drug did not increase the risk of cardiac malformations or congenital malformations overall. However, first trimester ondansetron was associated with a very small increased risk of oral clefts (three additional cases per 10,000 women treated).

Interestingly the increased risk for oral clefts was confined to cleft palate, there was no evidence for an increased risk of cleft lip.

The information will be eagerly received by the thousands of pregnant women who experience severe nausea and vomiting, and the clinicians who care for them many of whom have been prescribing ondansetron because of its effectiveness, despite the lack of detailed safety data.

“Although not formally approved for the treatment of nausea and vomiting during pregnancy, ondansetron, a 5-HT receptor antagonist, has rapidly become the most frequently prescribed drug for nausea and vomiting during pregnancy in the United States because of its perceived superior antiemetic effects and improved adverse effect profile compared with treatment alternatives,” the study authors said.

“In 2014, an estimated 22% of pregnant women used ondansetron in the United States,” they said.

The major strengths of this study lie in the size of the cohort and the fact that the information on ondansetron exposure was based on filled prescriptions, thereby negating any possible recall bias. Both these factors are particularly important given how rare these abnormalities are and how many possible confounders there could be.

As for limitations of the study, of course just because a prescription has been filled doesn’t always mean the medication has been taken, but even if the exposure wasn’t as great as calculated, the risk would be only lessened rather than raised. There is also the possibility that there might have been some other unrecognised factor involved especially since all the women in the study were uninsured and treated under Medicaid insurance and therefore included a higher percentage of women from disadvantaged communities. However, given the detailed information collected on these women and their pregnancies, and the multiple analyses conducted on this data, the likelihood of unmeasured confounders affecting the findings was thought to be low.

Overall the results of this study should provide reassurance for clinicians and pregnant women, according to an accompanying editorial, written by a US obstetrician and gynaecologist.

“As clinicians and pregnant women engage in informed, shared decision-making surrounding treatment for nausea and vomiting, the current information is important for contextualising risks in light of the potential benefits,” he concluded.


Huybrechts KF, Hernández-Díaz S, Straub L, Gray KJ, Zhu Y, Patorno E, et al. Association of Maternal First-Trimester Ondansetron Use With Cardiac Malformations and Oral Clefts in Offspring. JAMA. 2018 Dec 18; 320(23): 2429-37. DOI: [10/1001/jama.2018.18307]

Haas DM. Helping Pregnant Women and Clinicians Understand the Risk of Ondansetron for Nausea and Vomiting During Pregnancy. JAMA. 2018 Dec 18; 320(23): 2425-6. DOI: [10.1001/jama.2018.19328]