Zofran (ondansetron) is generally well tolerated, but it does cause side effects. The most common are headache (affecting up to 27% of users), constipation (up to 11%), and fatigue (up to 13%). Most of these are mild and fade on their own, but there are a few less common effects worth knowing about, especially if you take other medications or are pregnant.
The Most Common Side Effects
Headache is the side effect people notice most often. In clinical trials, more than one in four patients reported it. For most people it’s manageable with over-the-counter pain relievers and doesn’t require stopping the medication.
Digestive issues are also frequent. Constipation affects roughly 11% of users, while diarrhea is reported in up to 16%. This might seem contradictory, but Zofran works by blocking a specific signaling molecule in the gut, and that disruption can push digestion in either direction depending on your body. Fatigue or general malaise rounds out the list, showing up in about 13% of patients in trials.
These side effects typically don’t require medical attention and often improve as your body adjusts to the drug.
Constipation and How to Deal With It
Because Zofran slows gut motility as part of how it controls nausea, constipation can become a real nuisance, particularly if you’re taking it for several days or weeks (as many people with pregnancy-related nausea do). A daily stool softener is often needed while you’re on the medication. Magnesium citrate or docusate are commonly recommended options, and some people benefit from using both together. Staying well hydrated also helps counteract the slowdown.
Heart Rhythm Changes
Zofran can lengthen the QT interval, a measure of the heart’s electrical cycle, in a dose-dependent way. At standard doses, this effect is small and clinically insignificant for most people. At higher intravenous doses, the change becomes more meaningful. A study of the former highest IV dose (32 mg) showed it lengthened the QT interval by about 20 milliseconds on average, which prompted the FDA to lower the maximum single IV dose to 16 mg.
For the vast majority of people taking oral Zofran at normal doses, this isn’t a practical concern. The risk becomes relevant if you have a condition called long QT syndrome (a genetic heart rhythm disorder), if you take other medications that also affect the QT interval, or if you have significant electrolyte imbalances. Cases of a dangerous irregular rhythm called Torsade de Pointes have been reported after marketing, though they remain rare.
Serotonin Syndrome Risk
Zofran works by blocking a type of serotonin receptor, which means it interacts with the body’s serotonin system. If you’re also taking an antidepressant that raises serotonin levels, particularly SSRIs or SNRIs, the combination can occasionally trigger serotonin syndrome. This is uncommon but serious.
Symptoms develop relatively quickly and include confusion, agitation, rapid heart rate, excessive sweating, shivering, high blood pressure, fever, and exaggerated reflexes or tremor. In severe cases it can progress to loss of consciousness. If you take an antidepressant and are prescribed Zofran, the combination is still used regularly in practice, but you should be aware of these warning signs.
Allergic Reactions
Severe allergic reactions to Zofran are rare. They can include throat swelling, difficulty breathing, wheezing, a drop in blood pressure, and hives or swelling of the face and tongue. Because these reactions are reported voluntarily after the drug reaches the market, there’s no precise frequency estimate. The FDA includes them on the label as a known possibility, not as a common occurrence.
Side Effects in Children
Zofran is widely used in children, particularly to control vomiting from stomach bugs. It works well for that purpose, reducing the need for IV rehydration. However, diarrhea is a more notable concern in kids than in adults. Three out of five clinical trials found significantly more diarrhea episodes in children given Zofran compared to placebo. This matters because worsening diarrhea in a child who’s already losing fluids to a stomach illness can create a cycle of dehydration, even after the vomiting stops. The exact added risk hasn’t been precisely quantified yet.
Use During Pregnancy
Many people encounter Zofran for the first time during pregnancy, where it’s prescribed for severe morning sickness. The safety data here is nuanced. A large 2018 study found that first-trimester use was linked to a slightly increased risk of oral clefts (cleft lip or palate), but the absolute increase was very small: about 0.03%. That same study found no increased risk of heart defects.
A separate 2019 study did find a small increase in the risk of a specific heart defect called a ventricular septal defect (a hole between the heart’s lower chambers), with an absolute risk increase of roughly 0.3%. To put that in perspective, the “number needed to harm” was 263 to 385, meaning you’d need to treat that many pregnancies before one additional case would appear.
Another study looking specifically at intravenous Zofran found no increased risk of heart defects, oral clefts, or birth defects overall. The picture across all the research is that if a risk exists, it is small in absolute terms. For many women with severe nausea and vomiting, the benefits of being able to eat, drink, and function may outweigh that small potential risk.
Visual Disturbances With IV Use
When Zofran is given intravenously, particularly if pushed quickly, some patients experience transient visual disturbances including blurred vision. In rare cases, temporary blindness has been reported. These episodes typically resolve on their own, anywhere from a few minutes to about 28 hours. This side effect is specific to IV administration and not a concern with oral tablets or dissolving tablets. Dizziness has also been linked to rapid IV infusion.