Is ondansetron safe in pregnancy?

The safety evidence for ondansetron in pregnancy is the subject of ongoing research. Earlier studies raised concerns about a small potential increase in cardiac septal defects; subsequent larger studies have been largely reassuring about structural cardiac risk. Some data suggest a possible association with oral cleft at high doses, though absolute risks remain low. The decision to use ondansetron in pregnancy involves weighing the severity of nausea and vomiting against available evidence — this is a decision to make with your GP or obstetrician, not one to make independently based on a news article.

Why isn't ondansetron on the PBS for pregnancy?

Ondansetron was developed as an antiemetic for chemotherapy-related nausea and has not been formally submitted for PBS listing in pregnancy, despite extensive off-label use. PBS listing requires a formal application and health technology assessment. The Australian research recommends the government initiate this process. Until it happens, access remains uneven and outside formal PBS oversight.

GPs bridging a PBS gap on pregnancy nausea with off-label prescribing

What just happened

Australian researchers have published data exposing a quiet but structurally significant gap in how pregnancy nausea is managed and funded in this country. Ondansetron — an antiemetic that is widely used for nausea and vomiting of pregnancy, including hyperemesis gravidarum — accounts for 53.5% of total PBS antiemetic expenditure in pregnancy, despite having no approved PBS indication for this use.

Put differently: more than half of what the PBS spends on antiemetics for pregnant women goes to a drug that is not formally PBS-listed for pregnancy. The workaround that has made this possible is off-label prescribing, with GPs and obstetricians writing ondansetron on private prescriptions or under related PBS items in situations where standard first-line options have not adequately controlled symptoms. Associate Professor Luke Grzeskowiak, one of the researchers, described it as an “acceptable workaround, because it’s helping vulnerable women get necessary treatment they would otherwise not be able to afford.”

That framing — the Robin Hood metaphor — is doing real work here. It describes a clinical system that has quietly outsourced the problem of inadequate PBS coverage to individual prescriber judgment. Doctors have been making pragmatic, patient-centred decisions in consultation rooms, and those decisions have added up to more than half of the PBS antiemetic spend in this space. The policy infrastructure has not kept pace with clinical practice.

Both-and

The clinical case for ondansetron in pregnancy

Nausea and vomiting of pregnancy affects up to 80% of pregnant women. For most, it is manageable with dietary modifications and over-the-counter interventions in the first trimester. For a smaller group — roughly 1-3% of pregnancies — it escalates to hyperemesis gravidarum: severe, persistent vomiting that causes dehydration, weight loss, electrolyte disturbance, and in some cases hospitalisation.

The PBS-listed first-line antiemetics for pregnancy nausea — metoclopramide and prochlorperazine — are effective for mild-to-moderate presentations. RANZCOG guidance positions them as appropriate first steps. For women with severe disease who do not respond adequately to those agents, ondansetron has become the practical next rung, and the clinical evidence supports it in that role. The 5-hydroxytryptamine-3 antagonist mechanism of ondansetron is different from metoclopramide’s dopamine antagonism, which explains the differential response in some patients.

What the PBS does not yet reflect is this clinical reality. Ondansetron was developed for chemotherapy-induced nausea. Its use in pregnancy has grown substantially over two decades of off-label clinical experience without a formal PBS submission process following it. The result is a situation where the drug’s use is common, its off-label prescribing is accepted as clinically reasonable by specialists, and yet its costs and regulatory status remain in a grey zone.

The equity dimension

The research’s finding about socioeconomic distribution is the part that deserves the most attention. The poorest 50% of women accounted for roughly 70% of total PBS antiemetic expenditure in pregnancy. That is not a finding about rich women accessing treatment. It is a finding about women who cannot afford to go without treatment, navigating a system that makes accessing it harder than it should be.

The PBS co-payment system is designed to make medicines affordable at the point of dispensing. When a drug is not PBS-listed for the relevant indication, that protection disappears: the cost falls on the patient, or on the prescriber’s willingness to navigate system workarounds. Off-label prescribing of ondansetron at private script prices costs significantly more than a standard PBS co-payment.

Women with severe pregnancy nausea who cannot afford private script prices face a stark option set: go without adequate treatment, present to an emergency department, or rely on having a GP who knows the workarounds and is willing to use them. The researchers argue that this perpetuates inequality and “incentivises prescribing outside standard regulatory safeguards.” Both claims are supported by the data.

2 cents

If you are pregnant or planning to be pregnant, and you are experiencing nausea that is not controlled by first-line measures, this is a conversation worth having with your GP or obstetrician directly. Ondansetron is a real clinical option for more severe presentations and your doctor can discuss whether it is appropriate for your situation.

The broader point — that PBS coverage of antiemetics in pregnancy does not match clinical practice — is a systemic issue that individual GPs cannot fix in their consulting rooms. The researchers are calling on government to initiate a formal PBS assessment of ondansetron for this indication. That process, if it starts, takes years. Until then, the workaround remains the workaround.

Verdict: yes — an important equity and policy finding that clarifies a gap most pregnant women and many GPs navigate without the full picture.