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

Methotrexate has a long list of potential drug interactions, some of which are serious or life-threatening. Understanding these interactions is essential — especially because some of the most dangerous culprits are common, over-the-counter medications. Here's what every patient on methotrexate must know.

Why Drug Interactions With Methotrexate Are Particularly Dangerous

Methotrexate has a very narrow therapeutic window — meaning the gap between a beneficial dose and a toxic dose is small. Many interactions work by either increasing methotrexate blood levels (raising toxicity risk) or adding to its toxic effects on the liver, kidneys, or bone marrow. Even common OTC medications can push this balance into dangerous territory.

Major Drug Interactions — Avoid or Use With Extreme Caution

1. NSAIDs (Ibuprofen, Naproxen, and Others)

This is one of the most clinically important interactions. NSAIDs — including ibuprofen (Advil, Motrin), naproxen (Aleve), and high-dose aspirin — reduce kidney blood flow, which slows methotrexate elimination. The result is elevated and prolonged methotrexate blood levels, increasing the risk of severe hematologic and GI toxicity.

Deaths have been reported from this combination. Rheumatologists may allow low-dose aspirin (81 mg for cardiovascular protection) but generally advise against regular NSAID use with methotrexate. Always ask your provider before taking any NSAID, including OTC versions.

2. Trimethoprim/Sulfamethoxazole (Bactrim, Septra)

Trimethoprim is itself an antifolate drug. When combined with methotrexate, it produces additive folate-blocking effects that can rapidly cause severe bone marrow suppression, organ toxicity, and mucositis — even at methotrexate doses as low as 5–25 mg/week. This interaction can be life-threatening and has been reported to occur within 2 months of starting the combination. If you need an antibiotic, tell your doctor you are on methotrexate — alternative antibiotics exist.

3. Proton Pump Inhibitors (Omeprazole, Pantoprazole, Esomeprazole)

PPIs — medications commonly used for acid reflux and heartburn — can reduce methotrexate clearance, elevating plasma methotrexate concentrations. This is especially relevant at high chemotherapy doses. At low RA doses, the interaction is generally considered moderate risk, but it should be discussed with your prescriber if you're on both medications. Ranitidine (H2 blocker) appears to have less interaction.

4. Penicillin Antibiotics

Penicillin-class antibiotics (amoxicillin, ampicillin, etc.) can reduce the renal clearance of methotrexate, leading to higher methotrexate levels and increased toxicity risk. Always inform any prescriber or urgent care doctor about your methotrexate use before receiving a new antibiotic prescription.

5. Alcohol

Alcohol and methotrexate are both hepatotoxic. Their combination significantly accelerates liver damage, including fibrosis and cirrhosis. Most guidelines recommend complete abstinence from alcohol during methotrexate therapy, or at most 1 standard drink per week for very low-risk patients — discuss your personal threshold with your provider.

6. Live Vaccines

Methotrexate suppresses the immune system, making live vaccines dangerous. Do not receive MMR (measles, mumps, rubella), varicella (chickenpox), live flu vaccines, or live shingles vaccine (Zostavax) while on methotrexate. The inactivated shingles vaccine (Shingrix) and inactivated flu vaccines are generally safe, though methotrexate may reduce their efficacy.

Moderate Drug Interactions — Monitor Carefully

Leflunomide (Arava): Additive hepatotoxicity; combination used in RA under specialist care but requires frequent LFT monitoring

Theophylline: Methotrexate decreases theophylline clearance; can increase theophylline levels — monitor and adjust dose accordingly

Probenecid: Inhibits methotrexate renal excretion, raising plasma levels — avoid if possible

Valproate (valproic acid/Depakote): Can increase methotrexate plasma concentrations; monitor for signs of toxicity

Cyclosporine: Both have nephrotoxic potential; combination increases toxicity risk

Supplements and Foods That May Interact

High-dose folic acid (>5 mg/day): May reduce methotrexate efficacy for cancer; standard 1 mg/day supplementation for arthritis is appropriate and recommended

Herbal supplements: St. John's Wort, echinacea, and other immune-modulating supplements may interfere with methotrexate's immunosuppressive effects; discuss all supplements with your prescriber

What to Tell Every Doctor and Prescriber

Because methotrexate interactions can be dangerous, make sure every provider who prescribes or recommends a medication for you knows you are on methotrexate. This includes:

Your primary care doctor, urgent care providers, and emergency room staff

Your pharmacist — they can catch interactions before dispensing

Any dental professional who may prescribe antibiotics

Any telehealth provider who may not have access to your full medication history

For a full picture of what to watch out for while on methotrexate, read our post on methotrexate side effects. If you're having trouble locating methotrexate at your pharmacy, medfinder can help you find it near you.