As with other drugs, ondansetron can cause side effects, such as fatigue, headache, and constipation. If side effects of ondansetron become difficult to tolerate, talk with your doctor or pharmacist.

Ondansetron is a generic drug that’s prescribed to help prevent nausea and vomiting caused by certain cancer treatments or following surgery.

Ondansetron is not available in a brand-name version. It used to be available as the brand-name drug Zofran.

Ondansetron comes as an oral tablet and orally disintegrating tablet (ODT), which dissolves on your tongue. Ondansetron also comes as an oral solution and in an injectable form, but this article does not cover them.

Read on to learn about the potential common, mild, and serious side effects of ondansetron. For more information about ondansetron, including details about its uses, see this ondansetron overview.

Ondansetron can cause certain side effects, some of which are more common than others. These side effects may be temporary, lasting a few days to weeks. However, if the side effects persist, bother you, or become severe, be sure to talk with your doctor or pharmacist.

These are just a few of the more common side effects reported by people who took ondansetron tablet in clinical trials. These side effects can vary depending on the condition being treated.

More common side effects in people taking ondansetron for prevention of nausea and vomiting from chemotherapy or radiation include:

More common side effects in people taking ondansetron for prevention of nausea and vomiting following surgery include:

For more information about some of these side effects, see the “Ondansetron: Side effect specifics” section.

Mild side effects can occur with ondansetron use. This list does not include all possible mild side effects of the drug. For more information, you can refer to the drug’s prescribing information.

Mild side effects that have been reported with ondansetron include:

  • fatigue
  • constipation
  • diarrhea
  • malaise (feeling of illness)
  • anxiety
  • dizziness
  • fever
  • itchy skin
  • headache (for details, see the “Ondansetron: Side effect specifics” section)

These side effects may be temporary, lasting a few days to weeks. However, if the side effects continue, bother you, or become severe, be sure to talk with your doctor or pharmacist.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks and reviews side effects of the medication. Sharing your experience helps make medications safer for everyone by giving doctors and researchers more information about how the drug works in real life. If you develop a side effect while taking x drug and want to tell the FDA about it, visit MedWatch or call 800-FDA-1088.

Ondansetron may cause serious side effects, but this is not common. The following list may not include all possible serious side effects of the drug. For more information, you can refer to ondansetron’s prescribing information.

If you develop serious side effects while taking ondansetron, call your doctor right away. If the side effects seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Serious side effects that have been reported and their symptoms include:

  • extrapyramidal symptoms, which may include symptoms such as:
    • muscle contractions
    • involuntary or uncontrollable movements, such as tremors
    • intense urge to move
  • QT prolongation (an abnormal heart rhythm), which may include symptoms such as:
  • urinary retention, which may include symptoms such as:
    • urinating repeatedly in a short period
    • straining to urinate
    • reduced sensation to urinate
  • serotonin syndrome, which may include symptoms such as:
  • elevated liver enzymes, which could be a sign of liver damage
  • hypoxia (for details, see the “Ondansetron: Side effect specifics” section)
Allergic reaction

For some people, ondansetron can cause an allergic reaction, including anaphylaxis and bronchospasm (tightening of the airways that makes it difficult to breathe).

In general, symptoms of allergic reaction can be mild or serious.

Ways to manage

For mild allergic reaction symptoms, such as a mild rash, call your doctor right away. They may recommend treatments to help manage your symptoms. They’ll also let you know whether you should keep taking the medication.

For severe allergic reaction symptoms, such as swelling or trouble breathing, call 911 or your local emergency number right away. These symptoms require immediate medical care because they can become life threatening.

If you’ve had a serious allergic reaction to any form of ondansetron, your doctor may recommend taking a different medication instead.

Learn more about some of the side effects that ondansetron may cause. To find out how often side effects occurred in clinical trials, see the prescribing information for ondansetron.

Headache

Headache was a common side effect reported in studies of ondansetron. A headache may affect one or both sides of your head. It might come on suddenly or get steadily worse with time. Headache pain may be described as:

  • sharp
  • throbbing
  • dull
  • gripping

What you can do

If you have headaches during ondansetron treatment, consider the following home remedies to help relieve your symptoms:

If home remedies don’t help, consider over-the-counter pain relievers or talk with your doctor. They may recommend acetaminophen (Tylenol) or nonsteroidal anti-inflammatory drugs (NSAIDs). (Note: NSAIDs can interact with several types of medications, so check with your doctor first before taking an NSAID.)

In some cases, headache can be caused by a serious underlying condition. If you have headache along with other symptoms, talk with your doctor right away. They’ll check your symptoms and determine whether you may have a more serious condition. They’ll also discuss treatment options. Other symptoms to watch for include:

  • headache that worsens severely within minutes
  • changes in vision
  • facial drooping and/or numbness or weakness on one side of the body
  • changes in your speech

Hypoxia

Some people developed hypoxia during clinical trials of ondansetron for postoperative nausea but not chemotherapy-induced nausea. With hypoxia, oxygen levels in your body’s cells and tissues become low enough that they can’t function properly. Note: Hypoxia was also seen with a placebo (tablet with no active ingredient) but it was more common with ondansetron.

Since this side effect was seen only in trials for postoperative nausea, it’s possible that other medications administered before, during, or after surgery could have contributed to hypoxia.

Hypoxia can be mild to severe. Symptoms of mild or moderate hypoxia may include:

  • confusion
  • headache
  • shortness of breath
  • tiredness or low energy

Symptoms of severe hypoxia may include:

What you can do

Talk with your doctor right away if you experience symptoms of hypoxia, especially difficulty breathing, change in skin tone, fast heart rate, or reduced consciousness. Your doctor will order tests to determine whether you have hypoxia and a possible cause. They’ll also prescribe treatment, which may include oxygen therapy to increase your oxygen levels.

If your symptoms seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Ondansetron may cause several side effects. Here are some frequently asked questions about the drug’s side effects and their answers.

Does ondansetron make you sleepy?

It’s possible. Sleepiness was not reported as a side effect in ondansetron’s studies. However, drowsiness and sleepiness can be a symptom of sedation, a rare but serious side effect reported in studiesTrusted Source.

Ondansetron may cause sedation because it blocks serotonin receptors (attachment sites). Serotonin is a natural chemical messenger (neurotransmitter) that your intestines and brain make. Serotonin plays a role in several mental and physical functions, including mood and digestion.

If you experience unusual sleepiness or drowsiness after taking ondansetron, tell your doctor right away. They’ll help determine whether ondansetron or another factor is the cause. In some cases, they may adjust your dosage or have you stop taking ondansetron.

How do side effects from a 4-mg dose of ondansetron compare with those of higher strengths or dosages?

It’s not clear. The drug’s clinical trials didn’t include a direct comparison between 4 milligrams (mg) of ondansetron and other strengths or dosages. However, 4 mg is a low dose of the drug that’s typically recommended for children ages 4 to 11 years for the prevention of nausea and vomiting related to chemotherapy. The typical starting dose for adults is 8 mg.

The drug’s manufacturer does not provide information about dose-dependent side effects. These are side effects that become more likely or more severe with higher doses. If you experience bothersome or severe side effects, be sure to talk with your doctor. They may decide to adjust your dosage.

What are the toxic effects of ondansetron?

Toxic effects of ondansetron when taken as prescribed have not been reported. However, taking more ondansetron than prescribed or recommended can lead to harmful effects.

In addition to the serious side effects listed earlier in this article for ondansetron oral tablets, the following effects have been reported with overdose of an injectable form of ondansetron:

Do not take more ondansetron than your doctor prescribes. Call your doctor right away if you think you’ve taken too much ondansetron. You can also call 800-222-1222 to reach America’s Poison Centers or use its online resource.

Take note of the following important information before taking ondansetron tablets.

Before starting ondansetron, discuss your health history with your doctor. Ondansetron may not be right for you if you have certain medical conditions or other factors affecting your health. Be sure to talk with your doctor if any of the following apply to you:

Disclaimer: Medical News Today has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.