Ondansetron Use in Pregnancy: Safety and Risk Assessment

Position in Treatment Algorithm

The 2024 BJOG guideline recommends ondansetron as a second‑line anti‑emetic after failure of first‑line agents such as antihistamines, phenothiazines, or doxylamine‑pyridoxine combinations. 1

In a pooled analysis of >97,000 first‑trimester exposures, ondansetron was not associated with an increase in overall congenital malformation rates when compared with disease‑matched or unexposed controls (large observational data). 1
The largest cohort study (≈1.8 million pregnancies, 88,467 ondansetron exposures) found no significant association with cardiac defects after multivariable adjustment (adjusted RR 0.99, 95 % CI 0.93‑1.06). Evidence level: high‑quality population‑based cohort. 1

A modest increase in orofacial cleft risk was observed (adjusted RR 1.24, 95 % CI 1.03‑1.48). Evidence level: moderate (adjusted observational data). 1
The absolute risk translates to ≈3 additional cleft cases per 10,000 births (14/10,000 with ondansetron vs 11/10,000 without). This small absolute increase should be balanced against the harms of untreated hyperemesis gravidarum. 1

Observational studies comparing women taking ondansetron for severe nausea/vomiting with healthy, non‑symptomatic women are limited by confounding by indication. 1
When women with hyperemesis gravidarum were compared to similar women receiving alternative anti‑emetics, no increase in major malformations was detected. Evidence level: moderate (matched cohort). 1
In the same hyperemesis cohort, ondansetron use was associated with higher live‑birth rates and lower pregnancy‑termination rates compared with non‑use. Evidence level: moderate. 1

A multinational cohort of 456,963 pregnancies showed no association between ondansetron exposure and fetal death (adjusted HR 0.91, 95 % CI 0.67‑1.23). Evidence level: high (large cohort). 1
Ondansetron exposure was linked to a reduced risk of spontaneous abortion relative to no anti‑emetic use (adjusted HR 0.82, 95 % CI 0.73‑0.91). Evidence level: moderate. 1
No increased risk of stillbirth was identified across multiple large studies. Evidence level: high. 1

No significant associations were found between ondansetron use and preterm birth, small‑for‑gestational‑age infants, or neonatal developmental abnormalities. Evidence level: moderate (meta‑analyses). 1
Meta‑analyses that excluded controversial studies reported no link to neural‑tube defects or cardiac defects. Evidence level: moderate. 1

When counseling patients, disclose the small absolute increase in orofacial cleft risk (≈3 per 10,000) and emphasize that untreated hyperemesis carries risks such as dehydration, electrolyte imbalance, weight loss, and possible pregnancy termination. 1
Highlight that women with hyperemesis who receive ondansetron have better pregnancy continuation outcomes than those who do not. Evidence level: moderate. 1

The 2024 BJOG guideline does not recommend a blanket restriction on first‑trimester ondansetron; use is acceptable when clinically indicated. 1
The UK Teratology Information Service systematic review supports first‑trimester use when benefits outweigh potential risks. Evidence level: moderate. 1

If ondansetron is declined, metoclopramide can be used, but clinicians should be aware of extrapyramidal side‑effects and administer it as a slow IV bolus over ≥3 minutes to reduce dystonic reactions. Evidence level: moderate. 1
The recommended maximum dose for metoclopramide in pregnancy is 30 mg per 24 hours or 0.5 mg/kg per 24 hours, whichever is lower. Evidence level: moderate. 1