Some women take ondansetron in pregnancy to control morning sickness. Classified by the U.S. Food and Drug Administration (FDA) as a Pregnancy Category B drug, it is generally considered safe for easing nausea in pregnancy. It should, however, only be taken when necessary, such as when the woman is not able to keep food down because of extreme nausea. In such cases, any possible negative effects of ondansetron are typically considered better than the harm that may come to the fetus if it does not get the appropriate nutrients from the mother. For this reason, pregnant women are encouraged to discuss the pros and cons with their doctor before taking this drug.
Ondansetron is classified as a Pregnancy Category B drug because, as of 2011, no controlled studies have been completed with pregnant women. Researchers have, however, run studies on animals to find out the effects of ondansetron in pregnancy. According to the results, this drug has not caused harmful effects on animal fetuses. That being said, animals do not always respond to drugs the same way humans do, which is why it is difficult to say for sure that ondansetron in pregnancy will not harm the fetus. With that in mind, doctors will not offer this drug unless they are sure their pregnant patients need it for health reasons.
The main reason many doctors still prescribe ondansetron despite a lack of studies involving humans is that the small chance of negative effects is often better than the alternative. For example, some women suffer from hyperemesis gravidarum during pregnancy, which means they may throw up almost everything they eat. Women affected by this condition cannot typically keep fluids down, either, which poses a risk of dehydration. This, combined with the lack of nutrients caused by vomiting most foods, can severely harm both the mother and the fetus. Dehydration and malnourishment are both known for being harmful, while ondansetron in pregnancy has no known harmful effects, so taking the latter is usually considered the best option.
Most women taking ondansetron in pregnancy only need it for the first trimester, when they are most likely to suffer from morning sickness. Some women, however, need it for the entire pregnancy. In such cases, their doctor will likely reevaluate the situation at each prenatal appointment to determine whether the benefits of ondansetron in pregnancy still outweigh the possible risks. Most women cease to need this drug once the pregnancy ends, which is likely a good thing for breastfeeding mothers, because it is unknown whether the medicine passes through breast milk.