Are ondansetron (Zofran) and dimenhydrinate (Gravol) safe for use during pregnancy?

Safety of Ondansetron (Zofran) and Dimenhydrinate (Gravol) in Pregnancy

Both ondansetron (Zofran) and dimenhydrinate (Gravol) are safe for use in pregnancy, with dimenhydrinate recommended as first-line therapy and ondansetron as second-line when first-line agents fail. 1

Treatment Algorithm

First-Line Antiemetics (Start Here)

• Dimenhydrinate (Gravol) is a first-line agent with established safety data showing no increased risk of congenital malformations or adverse pregnancy outcomes 1
• Other first-line options include antihistamines (H1 receptor antagonists), phenothiazines, and doxylamine-pyridoxine combinations
• These should be prescribed initially for nausea and vomiting in pregnancy (NVP) and hyperemesis gravidarum (HG) 1

Second-Line: Ondansetron (When First-Line Fails)

• Ondansetron is safe and effective as second-line therapy when first-line antiemetics are ineffective 1
• Its use should not be discouraged if first-line agents fail 1

Key Safety Data for Ondansetron

The Orofacial Cleft Concern (Minimal Absolute Risk)

The most important counseling point: The absolute risk of orofacial clefting increases from 11 cases per 10,000 births (background) to 14 cases per 10,000 births with ondansetron use in the first trimester 1. This represents only 3 additional cases per 10,000 births—a very small absolute increase that must be balanced against the risks of poorly managed hyperemesis gravidarum.

Comprehensive Safety Profile

• No increased risk of overall major congenital malformations across multiple large studies including over 97,000 first-trimester exposures 1, 2
• No increased risk of fetal death, spontaneous abortion, or stillbirth in a multinational cohort of 456,963 pregnancies 2
• No increased risk of preterm delivery, low birth weight, or small-for-gestational-age infants 3
• FDA Pregnancy Category B: Animal studies show no harm, though adequate human studies are limited 4

Recent Evidence Strength

The 2024 BJOG Green-Top Guideline (Grade B recommendation) represents the most authoritative current guidance, synthesizing data from the UK Teratology Information Service systematic review 1. This guideline explicitly states that ondansetron use should not be discouraged as second-line therapy.

Important Clinical Nuances

Regulatory Controversy

The European Medicines Authority issued warnings to avoid ondansetron in the first trimester based on conflicting epidemiological studies. However, the UK Medicines and Healthcare products Regulatory Agency (MHRA) did not issue such a warning 1. The UK Teratology Information Service concluded that available data do not provide evidence of increased overall malformation rates 1.

Risk-Benefit Balance

The small absolute increase in orofacial clefting risk must be weighed against the significant maternal and fetal risks of untreated severe NVP/HG, including:

• Dehydration and electrolyte imbalances
• Weight loss and malnutrition
• Wernicke's encephalopathy (if thiamine deficient)
• Maternal psychological distress
• Potential pregnancy termination due to intolerable symptoms 1

Practical Prescribing Approach

1. Start with dimenhydrinate (Gravol) or other first-line antihistamines for initial NVP management 1

2. Escalate to ondansetron if first-line agents fail to control symptoms 1

3. Counsel patients about the minimal absolute risk increase (3 additional orofacial clefts per 10,000 births) while emphasizing the safety data showing no overall increase in major malformations 1

4. Consider combination therapy if single agents are ineffective 1

5. Ensure thiamine supplementation (100 mg three times daily orally or IV as Pabrinex) for all women with severe vomiting or reduced dietary intake before administering dextrose 1

Common Pitfalls to Avoid

• Do not withhold ondansetron due to fear of teratogenicity when first-line agents have failed—the absolute risks are minimal and the consequences of untreated HG are significant 1
• Do not use ketonuria to assess severity of NVP/HG—it is not an indicator of dehydration 1
• Do not forget thiamine supplementation in women with prolonged vomiting to prevent Wernicke's encephalopathy 1