Famotidine, commonly known by brand names like Pepcid, is an H2 blocker that works by decreasing the amount of acid produced in the stomach [1.4.1]. While it is a widely used and effective medication for treating ulcers, GERD, and other conditions involving excessive stomach acid, it's essential to be aware of its potential interactions with other drugs [1.4.2, 1.10.3]. These interactions can render other medications less effective or increase the risk of side effects.
The Primary Mechanism: Altered Stomach pH
Most of famotidine's significant drug interactions stem from its primary function: reducing gastric acid [1.4.2]. By increasing the stomach's pH (making it less acidic), famotidine can interfere with the absorption of other drugs that require an acidic environment to dissolve and be absorbed into the bloodstream properly [1.2.4, 1.4.5]. This can lead to lower-than-intended levels of the affected drug in the body, potentially causing treatment failure [1.5.3].
Drugs Requiring Acid for Absorption
A number of critical medications rely on gastric acid for proper absorption. Taking them with famotidine can significantly decrease their efficacy. It is generally recommended to avoid these combinations or consult a healthcare provider for management strategies, such as dose adjustments or timing separation [1.7.2].
- Certain Antifungals: Medications like ketoconazole and itraconazole need an acidic environment. Famotidine can make these antifungals less effective [1.2.2, 1.5.1].
- Certain HIV Medications: The effectiveness of HIV drugs such as atazanavir, rilpivirine, and delavirdine can be reduced when taken with famotidine, as they require stomach acid to be absorbed [1.2.4, 1.6.5]. Studies have shown that famotidine can reduce atazanavir concentrations [1.6.2, 1.6.3].
- Certain Cancer Medications: Targeted therapies for cancer, including dasatinib, erlotinib, and pazopanib, are pH-dependent for absorption [1.2.2, 1.7.3]. Co-administration with famotidine can lead to reduced absorption and make the cancer treatment less effective [1.7.1, 1.7.4].
- Certain Antibiotics: The antibiotic cefditoren's absorption and blood levels can be decreased by famotidine, potentially making it less effective against infections. This combination is generally not recommended [1.8.1, 1.8.2].
- Iron Supplements: Famotidine may also reduce the body's ability to absorb iron from supplements [1.2.5]. A doctor might recommend separating the doses by several hours to mitigate this interaction [1.3.3].
Other Interaction Mechanisms
Beyond changing stomach pH, famotidine can also interact with other drugs by affecting the enzymes that process them in the body. This is a less common mechanism for famotidine compared to older H2 blockers like cimetidine, but a few notable interactions exist.
Tizanidine (Zanaflex)
One of the most significant interactions involves the muscle relaxant tizanidine [1.2.4]. Famotidine can slow down the enzyme that clears tizanidine from the body [1.9.2]. This can lead to an accumulation of tizanidine, increasing the risk of its side effects, which include drowsiness, dizziness, low blood pressure, and a slowed heart rate [1.9.1, 1.9.2]. If this combination is necessary, a doctor may prescribe a lower dose of tizanidine and monitor the patient closely [1.9.2].
Comparison of Key Famotidine Interactions
| Interacting Drug(s) | Drug Class | Mechanism of Interaction | Potential Effect on Interacting Drug |
|---|---|---|---|
| Ketoconazole, Itraconazole | Azole Antifungals | Decreased absorption due to higher stomach pH [1.5.3] | Reduced effectiveness [1.2.5] |
| Atazanavir, Rilpivirine | HIV Antiretrovirals | Decreased absorption due to higher stomach pH [1.2.4] | Reduced effectiveness [1.2.4] |
| Dasatinib, Erlotinib | Cancer Kinase Inhibitors | Decreased absorption due to higher stomach pH [1.7.3] | Reduced effectiveness [1.7.4] |
| Cefditoren | Cephalosporin Antibiotic | Decreased absorption due to higher stomach pH [1.8.1] | Reduced effectiveness [1.8.1] |
| Tizanidine (Zanaflex) | Muscle Relaxant | Inhibition of enzyme (CYP1A2) that clears tizanidine [1.3.4] | Increased risk of tizanidine side effects [1.9.2] |
| Iron Supplements | Minerals | Decreased absorption due to higher stomach pH [1.3.3] | Reduced iron absorption [1.2.5] |
General Precautions and When to Consult a Professional
Before starting famotidine, it's crucial to inform your doctor and pharmacist of all prescription and nonprescription medications, vitamins, and herbal products you are taking [1.11.2]. Special caution is advised for elderly patients and those with kidney disease, as they may be more susceptible to side effects or have reduced drug clearance [1.11.3].
Do not take over-the-counter famotidine for more than two weeks without a doctor's advice [1.11.2]. You should also avoid other acid reducers while taking famotidine unless directed by a healthcare provider [1.3.1].
Conclusion
While famotidine is a beneficial medication for many, its ability to alter stomach acidity is the primary reason why certain drugs cannot be taken with it. Medications for HIV, fungal infections, and cancer are of particular concern due to reduced absorption and efficacy [1.2.4]. Additionally, its effect on enzymes can increase side effects from other drugs like tizanidine [1.9.1]. Always consult with a healthcare professional to ensure your medication regimen is safe and effective.
For more detailed information from a trusted source, you can visit the Mayo Clinic's page on Famotidine. [1.2.1]
