Famotidine Drug Interactions: What to Avoid and What to Tell Your Doctor

One of the reasons famotidine (Pepcid) is preferred over older H2 blockers like cimetidine (Tagamet) is that it has significantly fewer drug interactions. Cimetidine inhibits major CYP450 liver enzymes, affecting the metabolism of dozens of medications. Famotidine largely avoids this problem. However, famotidine does interact with some important drugs — mainly because it raises stomach pH (reduces acid), which affects how some medications are absorbed. Here's what you need to know.

How Famotidine Interacts With Other Drugs

Famotidine's drug interactions work through two main mechanisms:

Reduced stomach acid changes drug absorption. Many medications rely on the acidic environment of the stomach to dissolve and be absorbed properly. When famotidine reduces acid, it raises gastric pH — making the stomach less acidic. This can reduce the absorption (and therefore effectiveness) of drugs that need acid to dissolve.

CYP1A2 enzyme inhibition (mild). Famotidine mildly inhibits CYP1A2, a liver enzyme that metabolizes certain drugs. While this effect is minimal for most drugs, it can significantly affect tizanidine — a muscle relaxant that is a major CYP1A2 substrate.

Major Interactions: Drugs to Avoid With Famotidine

The following medications should not be used with famotidine, per the manufacturer's prescribing information:

Tizanidine (Zanaflex): Famotidine inhibits CYP1A2, which is responsible for breaking down tizanidine in the liver. When used together, tizanidine blood levels can rise significantly, increasing the risk of hypotension (low blood pressure), bradycardia (slow heart rate), and excessive drowsiness. Avoid this combination; if necessary, closely monitor blood pressure and heart rate.

Cefuroxime (Ceftin, Zinacef): This antibiotic requires stomach acid for absorption. Famotidine's acid reduction decreases cefuroxime absorption, potentially making the antibiotic less effective. Avoid concurrent use if possible.

Dasatinib (Sprycel): A cancer drug used for leukemia. Its absorption is highly pH-dependent — reduced stomach acid from famotidine can decrease dasatinib blood levels, reducing its cancer-fighting effectiveness. Avoid concurrent use.

Delavirdine (Rescriptor): An antiretroviral drug (HIV treatment). Requires acidic environment for absorption. Famotidine reduces delavirdine absorption — avoid concurrent use.

Neratinib (Nerlynx): A cancer drug used for HER2-positive breast cancer. H2 blockers significantly reduce neratinib absorption. Avoid concurrent use.

Pazopanib (Votrient): A targeted cancer therapy. Requires acidic pH for dissolution. Famotidine markedly reduces pazopanib exposure — avoid concurrent use.

Risedronate (Actonel, Atelvia): A bisphosphonate for osteoporosis. Famotidine decreases risedronate absorption. Avoid concurrent use; risedronate should be taken on an empty stomach with plain water.

Moderate Interactions: Use With Caution

These drugs may interact with famotidine and require monitoring or dose adjustment:

Atazanavir (Reyataz): HIV protease inhibitor. Famotidine reduces atazanavir absorption. If combination is necessary, famotidine dose should not exceed 20 mg twice daily, and specific timing relative to atazanavir is required. Consult your pharmacist or HIV specialist.

Itraconazole (Sporanox) and Ketoconazole: Antifungal medications that require an acidic environment to dissolve. Famotidine reduces their absorption, potentially lowering antifungal drug levels and reducing effectiveness. Alternative antifungals like fluconazole (which does not depend on stomach acid) may be preferred.

Levoketoconazole (Recorlev): Similar to ketoconazole — absorption reduced by famotidine.

Sparsentan: Concurrent use with H2 blockers may reduce sparsentan absorption — use with caution.

Fezolinetant: Potential interaction; monitor if using concurrently.

What Famotidine Does NOT Interact With (Contrary to What You Might Think)

Famotidine is often confused with cimetidine, which has extensive drug interactions. Unlike cimetidine, famotidine does NOT significantly interact with:

Warfarin — no clinically significant effect on warfarin levels or INR

Phenytoin (Dilantin) — no significant CYP2C9 inhibition

Theophylline — no significant interaction

Diazepam (Valium) — no significant interaction

This makes famotidine significantly safer than cimetidine for patients on multiple medications.

Food and Alcohol Interactions

Famotidine can be taken with or without food. There are no significant food interactions. Alcohol is not known to interact directly with famotidine, but alcohol can worsen heartburn, GERD, and ulcer symptoms — so avoiding or limiting alcohol is advisable when being treated for these conditions. Alcohol and famotidine may both cause dizziness, so the combination could intensify this effect.

What to Tell Your Doctor Before Taking Famotidine

Always tell your doctor or pharmacist if you take any of the following before starting famotidine:

Any cancer drugs (especially targeted therapies like dasatinib, neratinib, or pazopanib)

HIV antiretroviral therapy (especially atazanavir or delavirdine)

Tizanidine (muscle relaxant)

Antifungals like itraconazole or ketoconazole

Bisphosphonates for osteoporosis (e.g., risedronate, alendronate)

Any antibiotics, especially cefuroxime

Kidney disease — famotidine requires dose reduction with reduced kidney function

Also review our guide on Famotidine Side Effects: What to Expect and When to Call Your Doctor to understand what to watch for during treatment.