Ondansetron side effects and how to avoid them

In this article:

• Common side effects of ondansetron
• Serious side effects of ondansetron
• How long do ondansetron side effects last?
• How to avoid ondansetron side effects

In this article

• Common side effects of ondansetron
• Serious side effects of ondansetron
• How long do ondansetron side effects last?
• How to avoid ondansetron side effects

Ondansetron is a prescription drug used for the prevention of nausea and vomiting in people undergoing cancer chemotherapy, radiation therapy, or surgery. While Zofran is the brand name, it is no longer available in pharmacies in the U.S. Generic versions—ondansetron oral tablets and orally disintegrating tablets (ODT)—are still commonly prescribed, although many doctors and patients continue to refer to the medication as Zofran.

Ondansetron belongs to a class of antiemetics (anti-nausea drugs) called 5-HT3 receptor antagonists. These drugs block serotonin, a chemical in the brain that passes signals between nerves. By blocking serotonin, ondansetron shuts down the “chemoreceptor trigger zone,” the area of the brain responsible for the nausea reflex. Ondansetron is a widely used drug, but it may not be right for everyone. Side effects can be a problem, particularly for people with certain medical or physical conditions. 

Common side effects of ondansetron

The most common side effects of ondansetron include:

• Headache
• Drowsiness
• Constipation
• Tiredness
• Weakness
• Chills

Serious side effects of ondansetron

Serious side effects are less common, but may occur in some cases, including:

• Abnormal heart rhythms (arrhythmias)
• Serotonin syndrome
• Temporary blindness or other vision problems
• Hallucinations (seeing or hearing things that are not there)
• Drug-induced movement problems (extrapyramidal effects including twitching, slurred speech, and involuntary rolling of the eyes upward)
• Severe allergic reactions such as anaphylaxis

Ondansetron and irregular heartbeat

Ondansetron can affect the heart’s rhythm. In particular, ondansetron affects the QT interval, the time it takes for the bottom chambers of the heart to contract and then fully relax. Ondansetron can increase that time, a phenomenon called QT prolongation or long QT syndrome. If QT prolongation is too great, the bottom chambers lose their rhythm and start to beat irregularly, causing conditions such as torsades de pointes, a potentially life-threatening heart condition. When taking ondansetron, get immediate medical attention at any sign of QT prolongation, such as:

• Palpitations
• Shortness of breath
• Chest pain
• Lightheadedness
• Fainting

Healthcare providers do not know the exact incidence of QT prolongation in people taking ondansetron, but as a side effect, it is more likely in people who already have heart rhythm abnormalities, a family history of QT prolongation, slow heart rate, congestive heart failure, heart attack, or an electrolyte imbalance such as low magnesium, calcium, or potassium levels. It is also more likely when ondansetron is combined with other drugs that lengthen the QT interval, such as selective serotonin reuptake inhibitors (SSRIs), antipsychotics, antifungal drugs, and some types of antibiotics.

Ondansetron and serotonin syndrome

Serotonin syndrome is a drug-induced condition that can occur when serotonin levels in the brain get too high. Symptoms are often mild and last for only a few days, but the most severe cases can be life-threatening. In many cases, serotonin syndrome is brought on by taking more than one drug that affects serotonin levels. However, ondansetron and similar drugs—5-HT3 receptor antagonists such as granisetron—can cause serotonin syndrome when taken alone. The exact incidence is not known, but get immediate medical help at any sign of serotonin syndrome, such as:

• Agitation
• Hallucinations
• Rapid heartbeat
• High or low blood pressure
• Dizziness
• Sweating
• Flushing
• Fever
• Tremor
• Muscle rigidity
• Overactive reflexes
• Loss of coordination
• Nausea

How long do ondansetron side effects last?

Most common ondansetron side effects such as headache, drowsiness, or constipation go away when the drug is discontinued. Ondansetron has a half-life of three to four hours in healthy adults, but this may be increased to six to eight hours in older adults. This means it takes the body three to four hours to eliminate half a dose of ondansetron from the body, so the entire dose should be eliminated in 15 to 20 hours (if no more doses are taken). That’s when most side effects should fade.

Some more serious side effects may take longer to resolve. Some, like serotonin syndrome, severe allergic reactions, or heart rhythm abnormalities, may require emergency medical treatment to get better. Severe heart rhythm problems, however, may produce long-lasting or lifelong complications.

Ondansetron interactions, contraindications, and warnings

Physical and medical conditions may affect the safety or effectiveness of ondansetron, so some people may not be able to take ondansetron, and others may require dosing changes or closer monitoring for adverse reactions.

Interactions

As with any other prescription drug, certain drug interactions can cause problems when people are taking ondansetron. Before taking this medicine, make sure your health care provider and pharmacist know about all the medicines you take, including prescription drugs, OTC medicines, and vitamins or supplements. 

Some drug interactions with ondansetron include:

• The Parkinson’s medication Apokyn (apomorphine) 
• Azole antifungal drugs
• Certain antibiotics

Additionally, ondansetron may interact with drugs that affect serotonin levels, including:

• Antidepressants such as Prozac (fluoxetine), Zoloft (sertraline), and Paxil (paroxetine)
• Anxiety medications
• Antipsychotics
• Bipolar disorder medications
• Opioids
• Migraine drugs (triptans)
• Seizure medications such as carbamazepine, phenytoin, or rifampin
• Other anti-nausea drugs
• Drugs that treat Parkinson’s disease
• Certain over-the-counter supplements

Some interactions will require avoiding a drug, while others may require adjusting the dose. Talk to your healthcare provider about drug interactions and how they may affect you. 

Abuse and dependence

The U.S. Food and Drug Administration (FDA) does not classify ondansetron as a controlled substance, meaning it has no potential for abuse or physical dependence. Ondansetron is given either as a single dose or for no longer than a few days at a time during cancer treatment, radiation treatment, or surgery. Withdrawal symptoms are not experienced when ondansetron treatment is discontinued.

Overdose

An overdose of ondansetron can be a serious medical emergency, so get emergency medical attention if too much ondansetron has been taken. An ondansetron overdose may cause symptoms such as temporary vision loss, severe constipation, low blood pressure, lightheadedness, and fainting. In addition, serotonin syndrome has been reported in children who have overdosed on ondansetron.

Restrictions

When a drug is never given to people with certain medical conditions, those medical conditions are called contraindications. Ondansetron is contraindicated in:

• People who are allergic to ondansetron or any ingredient in the medicine
• People who have congenital long QT syndrome
• People who take the medicine apomorphine

Health care professionals will avoid ondansetron or use it cautiously in people with certain medical conditions such as:

• People with a family history of long QT syndrome because of the risk of serious heart rhythm abnormalities. 
• People with a history of heart rhythm abnormalities, heart attack, or heart failure
• Irregular heartbeats are also a risk if ondansetron is given to people with electrolyte imbalances, particularly low magnesium, low potassium, or low calcium. 
• Ondansetron is used cautiously and with extra monitoring in people at risk of gastrointestinal obstruction, including people who have undergone recent abdominal surgery.
• People with phenylketonuria, a hereditary condition that can produce a toxic buildup in the body of the amino acid phenylalanine. 

How to avoid ondansetron side effects

For the most part, ondansetron’s side effects are minor—usually a headache, drowsiness, and constipation. A few tips, however, can reduce both the incidence and severity of side effects.

1. Take ondansetron as directed

Take ondansetron tablets exactly as instructed. Do not skip a dose or stop taking the medicine early. The doctor may give special instructions if you vomit after taking ondansetron tablets, so follow these instructions carefully.

2. Tell the healthcare provider about all medical conditions

The first step in avoiding serious side effects is to make sure the prescribing healthcare professional knows about your medical conditions and medical history. For ondansetron, side effects are more likely in people with the following medical conditions:

• Heart disease
• Heart rhythm abnormalities
• Slow heartbeats
• Electrolyte imbalance
• Any history or family history of QT prolongation
• Any intestinal blockage
• Liver disease
• Any history of allergies to ondansetron, granisetron, palonosetron, or dolasetron

While there is no certainty about the risks, make sure to tell the healthcare provider about pregnancy, breastfeeding, or any plans to get pregnant or breastfeed.

3. Tell the healthcare provider about all medications

Drug interactions between ondansetron and other medications can also cause problems. The healthcare provider particularly needs to know if you are taking any of the following medications:

• Apokyn (apomorphine)
• Drugs that treat heart rhythm conditions
• Medicines that treat depression, anxiety, psychosis, bipolar disorder, or other mental health conditions 
• Migraine medications, especially triptans

Do not take other prescriptions or over-the-counter medicines until checking in with a doctor or other healthcare provider.

4. Be careful about driving and other risky activities

Ondansetron can cause drowsiness, dizziness, and other impairments. For most people, these effects will be minor. Still, it’s a good idea to be careful about driving, operating machinery, or participating in potentially hazardous activities when taking ondansetron.

5. Contact a healthcare provider if vomiting continues

If vomiting continues, contact a healthcare provider. An additional dose may be prescribed or the healthcare provider may switch to another medication.

6. Watch for signs of heart rhythm problems

Get medical help right away if there are any signs of heartbeat problems such as palpitations, lightheadedness, or chest pain.

7. Watch for signs of intestinal problems

Consult with a healthcare provider if abdominal pain or swelling is experienced. It may be a sign of a serious blockage.