Ondansetron (brand name Zofran) is generally well tolerated, but its most common side effects are headache and constipation. In clinical trials, about 24% of adults taking ondansetron for chemotherapy-related nausea experienced headaches, compared to 13% on placebo. Constipation affected roughly 9% of users. Beyond these everyday effects, there are rarer but more serious risks involving heart rhythm changes and interactions with other medications.
How Ondansetron Works
When your body encounters certain triggers, like chemotherapy drugs, radiation, or anesthesia, cells lining the gut release a flood of serotonin. That serotonin activates receptors on nerves running from the gut to the brain, which is what makes you feel nauseated and triggers vomiting. Ondansetron blocks those specific serotonin receptors, cutting off the signal before it reaches the brain. This is why it’s effective but also why it can cause side effects: serotonin plays roles throughout the body, and blocking its receptors in the gut can slow digestion and cause constipation.
Common Side Effects
Headache is the single most frequent complaint. In trials involving patients on chemotherapy, nearly one in four people taking ondansetron reported headaches. Even in surgical settings, where patients received a single dose, headache rates ran about 9%, roughly double the placebo rate.
Constipation is the other standout side effect. Because ondansetron dampens serotonin signaling in the gut, it slows things down. About 9% of chemotherapy patients in trials developed constipation, compared to less than 1% on placebo. If you’re taking ondansetron for more than a day or two, increasing fiber intake, drinking 8 to 10 glasses of non-alcoholic fluids daily, and staying physically active can help. A stool softener taken once or twice a day is a reasonable preventive step. If you haven’t had a bowel movement in two to three days, that’s worth bringing up with your care team.
Other commonly reported effects include fatigue, dizziness, and mild warmth or flushing, though these occur less frequently than headache and constipation.
Heart Rhythm Changes
Ondansetron can prolong the QT interval, a measurement on an electrocardiogram that reflects how long the heart takes to reset between beats. When the QT interval stretches too far, it raises the risk of a dangerous irregular rhythm called Torsade de Pointes, which can be life-threatening.
This effect is dose-dependent. In a controlled study of healthy adults, an 8 mg intravenous dose extended the QT interval by about 6 milliseconds beyond placebo, a change considered clinically insignificant. A 32 mg IV dose, however, stretched it by roughly 20 milliseconds, which prompted the FDA to cap the maximum single IV dose at 16 mg. At 16 mg IV, the predicted QT prolongation sits around 9 milliseconds.
People at higher risk for this effect include those with congenital long QT syndrome, heart failure, very slow heart rates, or anyone already taking other medications that prolong the QT interval. If you have a known heart condition, your doctor should be aware before you take ondansetron.
Serotonin Syndrome Risk
Because ondansetron acts on the serotonin system, combining it with other serotonin-boosting drugs can, in rare cases, trigger serotonin syndrome. This is an excess of serotonin activity throughout the body that ranges from uncomfortable to life-threatening.
Mild symptoms include agitation, restlessness, rapid heart rate, sweating, shivering, and diarrhea. More severe cases involve high fever, muscle rigidity, seizures, irregular heartbeat, and loss of consciousness. The condition typically develops within hours of starting or increasing a serotonin-affecting drug.
Medications that raise the risk when combined with ondansetron include common antidepressants (SSRIs like sertraline and fluoxetine, SNRIs like duloxetine and venlafaxine), tricyclic antidepressants, MAOIs, migraine medications like sumatriptan, opioid painkillers including tramadol and fentanyl, lithium, and even over-the-counter cough medicines containing dextromethorphan. St. John’s wort, a popular herbal supplement, also increases serotonin levels. If you take any of these, the risk doesn’t mean you can’t use ondansetron, but it does mean your provider needs to weigh the combination carefully.
Effects in Children
Ondansetron is widely used in children, both for chemotherapy-related nausea (approved for ages six months and up) and off-label in emergency departments for vomiting from stomach bugs. It’s effective at stopping vomiting and reducing the need for IV fluids, but it comes with a trade-off: it can worsen diarrhea. In five clinical trials studying children with acute gastroenteritis, three found significantly more diarrhea episodes in children given ondansetron compared to placebo.
This matters because the whole point of treating a vomiting child is to prevent dehydration. If ondansetron stops the vomiting but ramps up diarrhea, fluid losses may continue through a different route. For most children, the net benefit still favors using the drug, but parents should be prepared for loose stools and keep up with oral rehydration.
Apomorphine Interaction
One combination is strictly off-limits. Ondansetron must not be taken with apomorphine, a medication used for Parkinson’s disease. Reports of profound drops in blood pressure and loss of consciousness led the FDA to list this as a hard contraindication, not just a caution. If you or a family member takes apomorphine, ondansetron is not an option.
Use During Pregnancy
Ondansetron is frequently prescribed for severe morning sickness, which has raised questions about birth defect risks. A large 2022 meta-analysis pooling data from over 100,000 exposed pregnancies found no significant increase in orofacial clefts. Initial analyses suggested small increases in cardiac defects and neural tube defects, but when researchers ran sensitivity analyses excluding studies with methodological concerns, those associations disappeared.
The overall conclusion from the most comprehensive review to date: ondansetron use during pregnancy was not associated with abnormal pregnancy outcomes. That said, the data isn’t perfectly clean, and the drug doesn’t carry a blanket safety endorsement for pregnancy. It’s typically reserved for cases where nausea and vomiting are severe enough that the benefits clearly outweigh theoretical risks.
People With Liver Disease
Ondansetron is processed by the liver, so people with significant liver impairment clear the drug more slowly. This means the medication stays in the body longer and reaches higher levels, increasing the likelihood of side effects, particularly QT prolongation. Providers typically reduce both the dose and frequency for patients with moderate to severe liver disease.