Understanding the Medication: What Pill is F 91?

October 6, 2025 •

3 min read

Over 70-80% of patients receiving chemotherapy experience nausea and vomiting. The white, oval pill marked with 'F 91' is identified as Ondansetron 4 mg, a key medication used to prevent this distressing side effect.

Quick Summary

The 'F 91' pill is Ondansetron Hydrochloride 4 mg, an antiemetic medication. It's prescribed to prevent nausea and vomiting from chemotherapy, radiation, or surgery.

Key Points

Identification: The white, oval pill imprinted with 'F 91' is Ondansetron Hydrochloride 4 mg.
Primary Use: It is a prescription medication primarily used to prevent nausea and vomiting from cancer chemotherapy, radiation, and surgery.
Mechanism of Action: It works by selectively blocking serotonin at 5-HT3 receptors, which prevents the vomiting reflex from being triggered.
Common Side Effects: The most common side effects are headache, malaise/fatigue, constipation, and diarrhea.
Major Warning: A significant risk is QT prolongation, an abnormal heart rhythm. It should be used with caution, especially with other QT-prolonging drugs.
Critical Contraindication: Ondansetron must not be used with apomorphine due to the risk of severe hypotension.
Serotonin Syndrome Risk: Combining Ondansetron with other serotonergic drugs (like many antidepressants) can lead to a serious condition called Serotonin Syndrome.

Identifying the 'F 91' Pill

Identifying unknown medication is crucial for safety. The white to off-white, oval, film-coated tablet with 'F' on one side and '91' on the other is Ondansetron Hydrochloride 4 mg. This prescription drug is a 5-HT3 receptor antagonist and is not a controlled substance. Be careful not to confuse it with other medications like Doxycycline Hyclate, which also has an 'F 91' imprint but is a different drug.

How Ondansetron Works: The Pharmacology

Ondansetron prevents nausea and vomiting (emesis) by selectively blocking the serotonin 5-HT3 receptor. Serotonin, released during treatments like chemotherapy and radiation, stimulates these receptors on nerve terminals and in the brain's chemoreceptor trigger zone, initiating the vomiting reflex. By blocking these receptors, Ondansetron interrupts this process. It differs from some other antiemetics as it does not block dopamine receptors.

Approved Uses of Ondansetron (F 91)

Ondansetron is used to prevent nausea and vomiting in specific clinical situations. It is prescribed for:

Chemotherapy-Induced Nausea and Vomiting (CINV): Used before and after chemotherapy to help manage nausea and vomiting.
Radiation-Induced Nausea and Vomiting (RINV): Used for nausea resulting from radiotherapy, particularly when it affects the abdomen or is total body radiation.
Postoperative Nausea and Vomiting (PONV): Can be administered before or after surgical procedures to prevent nausea and vomiting.

Appropriate use and frequency depend on the patient's condition and the specific treatment they are receiving. Ondansetron comes in various forms like tablets and oral solutions.

Potential Side Effects and Safety Warnings

Ondansetron can cause side effects and has important safety warnings.

Common Side Effects

Frequent side effects include:

Headache
Malaise/Fatigue
Constipation
Diarrhea
Dizziness

Serious Warnings and Precautions

Serious conditions to monitor for include:

QT Interval Prolongation: This can lead to a serious heart arrhythmia. Avoid in patients with congenital long QT syndrome. ECG monitoring may be needed for some patients.
Serotonin Syndrome: This potentially life-threatening condition can occur when taken with other serotonergic drugs. Symptoms include mental changes and autonomic instability.
Myocardial Ischemia: Reduced blood flow to the heart has been reported. Monitor for chest pain.
Hypersensitivity Reactions: Allergic reactions like anaphylaxis can occur.

Drug Interactions and Contraindications

Ondansetron is contraindicated in patients with a known hypersensitivity and with concomitant use of apomorphine, which can cause severe hypotension.

Inform your doctor about all medications. Key interactions include:

Serotonergic Drugs: Increased risk of Serotonin Syndrome when taken with drugs like SSRIs and SNRIs.
Drugs Affecting Cytochrome P-450 Enzymes: Medications like phenytoin and carbamazepine can potentially affect how Ondansetron is metabolized, which could impact its effectiveness.
Tramadol: Ondansetron may lessen tramadol's pain relief.

Comparison with Other Nausea Treatments


Feature	Ondansetron (F 91)	Dimenhydrinate (Dramamine)	Scopolamine Patch
Mechanism	Blocks serotonin (5-HT3) receptors centrally and peripherally	Antihistamine; blocks H1 receptors; anticholinergic effects	Anticholinergic; blocks muscarinic receptors
Primary Use	Chemotherapy, radiation, and post-op nausea/vomiting	Motion sickness, vertigo	Motion sickness, postoperative nausea/vomiting
Common Side Effects	Headache, constipation, diarrhea	Drowsiness, dry mouth, dizziness	Dry mouth, blurred vision, drowsiness
Prescription Status	Prescription only	Over-the-counter (OTC)	Prescription only

Conclusion

The 'F 91' pill is Ondansetron 4 mg, a prescription medication for preventing nausea and vomiting from medical treatments by blocking serotonin receptors. It has potential side effects, including serious warnings about heart rhythm and interactions with other drugs. It should only be used under a doctor's supervision.

For more information, visit the DailyMed page for Ondansetron.

Frequently Asked Questions

A white, oval pill with 'F' on one side and '91' on the other is identified as Ondansetron Hydrochloride 4 mg, a prescription anti-nausea medication.

Ondansetron 4 mg is primarily used to prevent nausea and vomiting caused by cancer chemotherapy, radiation therapy, and to prevent these symptoms after surgery.

No, the pill F 91 (Ondansetron) is not a controlled substance and does not have narcotic properties.

The most common side effects of Ondansetron (F 91) are headache, a feeling of tiredness or discomfort (malaise/fatigue), constipation, and diarrhea.

You must exercise caution. Taking Ondansetron with certain antidepressants (like SSRIs and SNRIs) can increase the risk of a serious condition called Serotonin Syndrome. Always consult your doctor about all medications you are taking.

Ondansetron can cause changes in the heart's electrical activity, known as QT prolongation, which can lead to serious irregular heartbeats. The risk is higher in patients with pre-existing heart conditions or electrolyte imbalances.

When taken orally for preventing chemotherapy-induced nausea, Ondansetron is typically administered before the start of treatment as part of a regimen to help prevent symptoms.

Ondansetron is specifically indicated for nausea and vomiting from chemotherapy, radiation, and surgery. While sometimes used off-label, you should only take it as prescribed by a healthcare provider for your specific condition.