What are the worse side effects of famotidine?

October 12, 2025 •

4 min read

While considered safe for most users, serious side effects from famotidine are rare but possible, including seizures, heart problems, and severe allergic reactions. It is crucial for patients and caregivers to understand what the worse side effects of famotidine are and when to seek immediate medical attention.

Quick Summary

Serious famotidine side effects include severe allergic reactions, heart rhythm abnormalities, and neurological issues like confusion and seizures. Rare liver damage, blood disorders, and severe skin conditions can also occur. The risk is heightened for older adults and individuals with kidney or liver impairment.

Key Points

Allergic Reactions: Severe allergic reactions (anaphylaxis) are rare but life-threatening and require immediate medical attention for symptoms like hives, swelling, or breathing difficulties.
Central Nervous System (CNS) Effects: Serious neurological issues, including confusion, hallucinations, and seizures, can occur, especially in elderly patients and those with kidney problems.
Cardiac Complications: Famotidine can rarely cause a prolonged QT interval, which can lead to life-threatening heart rhythm problems like torsades de pointes.
Liver Damage: Although uncommon, liver-related side effects such as hepatitis and jaundice are possible and more likely in individuals with pre-existing liver disease.
Long-Term Vitamin B12 Deficiency: Extended use of famotidine can lead to low vitamin B12 levels, which can cause anemia and neurological problems if not addressed.
Rare Blood and Skin Reactions: Severe adverse effects can also include thrombocytopenia (low platelet count) and life-threatening skin reactions like Stevens-Johnson Syndrome.

Famotidine, an H2 receptor antagonist commonly sold under brand names like Pepcid, is widely used to treat conditions caused by excess stomach acid, such as heartburn and ulcers. For most people, this medication is well-tolerated and its side effects are mild, such as headaches or dizziness. However, in rare instances, more severe and potentially life-threatening adverse reactions can occur. Understanding these risks is crucial for anyone taking the medication, especially those in high-risk groups.

Severe and Life-Threatening Allergic Reactions

One of the most severe risks associated with famotidine is the potential for a severe allergic reaction, or anaphylaxis. While rare, this can develop rapidly and requires immediate emergency medical care. Symptoms of anaphylaxis can include:

Breathing problems or wheezing
Swelling of the face, throat, tongue, or lips
Hives, itching, or a severe skin rash
Throat tightness or difficulty swallowing
Dizziness, fainting, or a drop in blood pressure
A rapid, pounding, or irregular heartbeat Any sign of a severe allergic reaction should be treated as a medical emergency. Other serious, though less urgent, skin reactions like Toxic Epidermal Necrolysis (TEN) and Stevens-Johnson Syndrome (SJS) have also been reported rarely.

Neurological and Central Nervous System (CNS) Effects

Famotidine can cross the blood-brain barrier and, in rare cases, cause significant CNS side effects, particularly in vulnerable populations. These effects can range from mood changes to severe neurological events. Reported CNS side effects include:

Confusion, delirium, or disorientation
Hallucinations (seeing or hearing things that aren't there)
Agitation, anxiety, or depression
Lethargy, unusual tiredness, or weakness
Seizures The risk for these side effects is significantly higher in older adults and those with moderate to severe kidney impairment, as famotidine can accumulate in the body. The American Geriatrics Society advises older adults with kidney issues or a history of delirium to avoid this medication.

Cardiac and Heart Rhythm Problems

Cardiovascular side effects, while very rare, are among the most serious risks of famotidine. They primarily involve heart rhythm disturbances, which can be life-threatening.

Prolonged QT interval: This is a rare but serious heart rhythm problem that can lead to a dangerous, rapid, and irregular heartbeat. This risk is heightened in patients with moderate to severe kidney problems.
Torsades de pointes and cardiac arrest: In rare instances, prolonged QT syndrome can lead to these life-threatening complications.
Arrhythmia and palpitations: Other reported heart rhythm issues include general irregular heartbeats and palpitations. Patients with pre-existing heart conditions or kidney problems should be closely monitored by a healthcare provider while on famotidine, especially during long-term treatment.

Liver-Related Complications

Though infrequent, famotidine can cause liver problems, which have been reported in post-marketing experience. These include:

Hepatitis: Inflammation of the liver.
Jaundice: Yellowing of the skin or whites of the eyes, which can indicate liver damage.
Elevated liver enzymes: Abnormal liver enzyme levels, often detected through blood tests, can signal underlying liver issues. The risk of developing liver-related side effects is greater for individuals with existing liver conditions, such as cirrhosis. Symptoms like jaundice, dark urine, or upper right abdominal pain should be reported to a doctor immediately.

Rare Blood Disorders

In extremely rare instances, famotidine has been linked to blood disorders. One such condition is thrombocytopenia, a low platelet count, which can increase the risk of unusual bleeding or bruising. Patients should watch for symptoms like red or purple spots on the skin (petechiae or purpura), unexplained bleeding, or easy bruising.

Potential Long-Term Use Risks

Over-the-counter famotidine is intended for short-term use, and long-term use can carry additional risks. One of the most notable is vitamin B12 deficiency, which can result from prolonged suppression of stomach acid. Low vitamin B12 can lead to anemia and, if left untreated, cause permanent neurological problems. Regular monitoring and potential supplementation may be necessary for patients on long-term prescription famotidine.

Famotidine vs. Other H2 Blockers: A Side Effect Comparison

When considering H2 blockers, it's helpful to compare the side effect profiles. Famotidine is often preferred over older H2 blockers like cimetidine due to its better safety profile, particularly regarding drug interactions and hormonal side effects.


Feature	Famotidine (Pepcid)	Cimetidine (Tagamet)	Ranitidine (Zantac 360)*
Drug Interactions	Minimal, less effect on liver enzymes involved in drug metabolism (cytochrome P-450).	High potential for interactions, affecting many drugs.	Original formula withdrawn; newer versions contain famotidine.
Hormonal Side Effects	No known antiandrogenic effects.	Can cause antiandrogenic effects, such as gynecomastia (enlarged breasts in men).	Original formula did not cause these issues.
CNS Effects	Rare, but risk increases with kidney impairment and advanced age.	Can cause CNS effects, especially in high doses or with kidney issues.	Original formula had minimal CNS risk; modern versions are famotidine-based.
Cardiac Issues	Very rare, but includes prolonged QT interval, especially with kidney impairment.	Rare, similar to famotidine but less detail available on recent cardiac findings.	Original formula had minimal cardiac risk; modern versions are famotidine-based.

*Note: The original ranitidine was withdrawn due to contamination concerns. Current versions contain famotidine.

Conclusion

While famotidine is generally a safe and effective medication, it's important to be aware of the rare but serious side effects. These include severe allergic reactions, heart rhythm problems, and neurological disturbances like confusion and seizures. Patients with underlying kidney or liver disease, as well as older adults, are at a heightened risk for these severe reactions. Prompt communication with a healthcare provider is essential if any severe or unusual symptoms arise. As with any medication, the decision to use famotidine should be a careful consideration of its benefits versus its risks, under the guidance of a medical professional.

For more detailed prescribing information and a complete list of potential side effects, refer to the official package insert or visit the Drugs.com Famotidine Side Effects page.

Frequently Asked Questions

The first signs of a serious side effect can vary, but may include symptoms of a severe allergic reaction (such as swelling of the face or throat, hives, or trouble breathing), sudden confusion or agitation, fainting, seizures, or a fast or irregular heartbeat.

Yes, in rare cases, famotidine can affect the heart. It can cause heart rhythm problems, most notably a prolonged QT interval, which can increase the risk of a dangerous irregular heartbeat.

Older adults and individuals with pre-existing kidney or liver impairment are at the highest risk for developing severe side effects, including neurological and cardiac issues, because the drug can accumulate in their systems.

Yes, famotidine can cause neurological side effects like confusion, disorientation, and delirium, especially in older patients or those with poor kidney function. These effects are generally reversible upon stopping the medication.

Yes, long-term use, especially for over-the-counter famotidine, can lead to different side effects. Prolonged acid suppression can result in vitamin B12 deficiency, which may cause anemia and permanent neurological damage over time.

If you notice symptoms such as yellowing of your skin or eyes (jaundice), dark urine, unexplained fatigue, or abdominal pain, contact your doctor immediately. This could indicate a rare but serious liver complication.

Famotidine is generally considered safer than older H2 blockers like cimetidine, primarily because it has fewer clinically significant drug interactions. It also lacks the hormonal side effects associated with high-dose cimetidine.