This information is for women who have severe nausea and vomiting in early pregnancy. It tells you about ondansetron, a treatment used to relieve these symptoms. It is based on the best and most up-to-date research.
Does it work?
We're not sure. There isn't enough evidence from research to tell us whether ondansetron (brand name Zofran) reduces severe nausea and vomiting in pregnancy (called hyperemesis gravidarum).
What is it?
Ondansetron is a drug often used to stop sickness caused by chemotherapy, radiotherapy, anaesthesia, and surgery.
Ondansetron can be taken as a tablet, or as an injection given by a doctor or nurse. Occasionally, doctors might try this drug if your nausea and vomiting is so bad it might threaten your health or that of your baby.
How can it help?
We're not sure it can help. It may reduce your vomiting but there isn't enough evidence to say for sure. 
How does it work?
Often, ondansetron is used to treat nausea and vomiting not associated with pregnancy. For example, it works well to reduce the nausea and vomiting that people can get from cancer chemotherapy. 
Research shows that cancer treatments such as chemotherapy and radiation lead to increased amounts of a chemical called serotonin in the brain, which may cause nausea and vomiting.  Ondansetron was developed to counter this effect. It stops serotonin working in certain parts of the brain.
But there's no evidence that the amount of serotonin in your brain increases when you're pregnant.  So we're not sure how, or if, ondansetron works to reduce severe nausea and vomiting when you're pregnant.
Can it be harmful?
In a small study of pregnant women who took ondansetron, no bad effects were found.  But we need more research before we know for sure that this drug is safe to take in early pregnancy. 
Doctors are advised to avoid prescribing this drug for pregnant women unless the benefits outweigh the risks to the baby. 
Minor side effects from ondansetron can occur, but they are not common.  They include:
How good is the research on ondansetron for severe nausea and vomiting?
There's very little evidence that ondansetron helps reduce severe nausea and vomiting during pregnancy. We didn't find any research summaries (called systematic reviews) or good-quality studies (called randomised controlled trials) that compared ondansetron with a tablet containing no medicine (called a placebo).
We did find one study that compared ondansetron with an antihistamine drug called promethazine (brand name Phenergan).  It looked at 30 pregnant women who were admitted to hospital with severe vomiting. The study found no difference in how sick the women were after taking ondansetron or promethazine.
But the study was too small to be a fair test of the two treatments.