Several common medications can dangerously increase methotrexate levels in your body or amplify its side effects. The most important ones to avoid or carefully manage include NSAIDs like ibuprofen, certain antibiotics, proton pump inhibitors, and live vaccines. Because methotrexate is cleared through the kidneys and suppresses part of your immune system, anything that slows that clearance or adds similar stress to your body can tip you into toxicity.
NSAIDs: Ibuprofen, Aspirin, and Similar Painkillers
Non-steroidal anti-inflammatory drugs are the most well-known interaction with methotrexate, and it’s an easy one to stumble into since many NSAIDs are available over the counter. Ibuprofen, naproxen, aspirin, ketoprofen, indomethacin, and piroxicam all compete with methotrexate for the same transport system in your kidneys. Specifically, they block proteins called organic anion transporters that your kidneys use to flush methotrexate out of your blood. When those transporters are occupied by an NSAID, methotrexate builds up instead of being excreted.
NSAIDs can also knock methotrexate loose from the proteins it normally binds to in your bloodstream, increasing the amount of “free” methotrexate circulating and doing damage. The combination of slower clearance and higher free levels is what makes this interaction potentially life-threatening. If you need pain relief while on methotrexate, talk to your prescriber about safer alternatives. Acetaminophen (Tylenol) is generally considered a reasonable option, though you should stay under 4 grams per day to protect your liver, which is already under some strain from methotrexate.
Trimethoprim and Co-trimoxazole Antibiotics
Trimethoprim, and the combination antibiotic co-trimoxazole (trimethoprim plus sulfamethoxazole), is one of the most dangerous pairings with methotrexate. Both drugs attack the same target: folate metabolism. Methotrexate works by blocking your body’s use of folic acid, and trimethoprim does something very similar. When you take both, the combined antifolate effect can shut down your bone marrow’s ability to produce blood cells, a condition called bone marrow suppression. Some reported cases have been fatal.
The sulfamethoxazole component adds a second problem. It can displace methotrexate from protein binding sites and compete with it for kidney clearance, raising free methotrexate levels in the same way NSAIDs do. Whether very low prophylactic doses of co-trimoxazole carry the same risk is still debated, but the safest approach is to use a different antibiotic entirely. If you’re prescribed an antibiotic for a urinary tract or respiratory infection, make sure your prescriber knows you take methotrexate so they can choose one that doesn’t interfere.
Proton Pump Inhibitors
Acid-reducing medications like omeprazole (Prilosec) and lansoprazole (Prevacid) can slow your body’s ability to clear methotrexate. One study found that taking omeprazole or lansoprazole reduced methotrexate clearance by 27%, and clearance of its metabolite dropped by 39%. That’s a meaningful increase in how long the drug stays active in your system, which raises the risk of toxicity, particularly mouth sores, nausea, and blood count drops.
If you regularly take a proton pump inhibitor for heartburn or acid reflux, your doctor may want to switch you to a different type of antacid or adjust your methotrexate monitoring schedule. This is especially important if you’re on higher doses of methotrexate.
Penicillin and Certain Other Antibiotics
Penicillin G competes with methotrexate for the same kidney transport proteins (OAT1, OAT3, and OAT4) that NSAIDs block. The result is the same: methotrexate stays in your body longer and reaches higher levels. This interaction has been described as potentially life-threatening in pharmacology research. Not every penicillin-type antibiotic carries equal risk, but if you need antibiotics while on methotrexate, it’s worth flagging so your prescriber can choose carefully and monitor your blood work.
Diuretics and Epilepsy Medications
Water pills (diuretics) like bendroflumethiazide and indapamide can reduce kidney function enough to slow methotrexate clearance. Since your kidneys are the main exit route for methotrexate, anything that changes how they filter blood can cause the drug to accumulate.
Epilepsy medications such as phenytoin and levetiracetam also interact with methotrexate, though through different pathways. Methotrexate can alter how these drugs are absorbed or metabolized, potentially making seizure control less predictable. If you take anticonvulsants, your neurologist and rheumatologist (or oncologist) need to coordinate dosing and monitoring.
Theophylline
Theophylline, used to open the airways in asthma and chronic lung disease, can interact with methotrexate. Methotrexate may reduce how quickly your body clears theophylline, raising theophylline levels and increasing the risk of side effects like nausea, rapid heartbeat, and tremors. If you use theophylline, your levels may need to be checked more frequently after starting methotrexate.
Live Vaccines
Because methotrexate suppresses your immune system, live vaccines pose a real risk. A live vaccine contains a weakened but active version of a virus or bacterium, and a suppressed immune system may not be able to keep it in check, potentially causing the very infection the vaccine is meant to prevent.
The CDC notes that severely immunocompromised people generally should not receive live vaccines. However, for people on low-dose methotrexate (0.4 mg/kg per week or less) for conditions like rheumatoid arthritis or psoriasis, some live vaccines like the shingles vaccine may still be considered safe. The nasal spray flu vaccine, which is a live vaccine commonly offered to children, should be avoided entirely. Injectable (inactivated) vaccines like the regular flu shot are fine.
Alcohol
Alcohol isn’t a drug interaction in the traditional sense, but it matters because methotrexate is processed through the liver and can cause liver damage on its own. Adding alcohol increases that strain. The NHS advises that moderate drinking is usually acceptable if you’re on a low weekly dose of 25 mg or less for an inflammatory condition. If you’re on higher doses, particularly for cancer treatment, you should get specific guidance from your prescriber about whether any alcohol is safe.
Folic Acid Supplements and Timing
This one is a bit counterintuitive. Methotrexate works by blocking folate, and doctors routinely prescribe folic acid alongside it to reduce side effects like mouth sores, nausea, and drops in blood cell counts. The standard recommendation is 5 mg of folic acid once a week, taken on a different day than your methotrexate dose. That dose can be increased to 10 mg weekly if you’re still experiencing side effects. Taking it roughly 48 hours before your methotrexate dose may offer extra protection against stomach-related side effects.
The key rule: do not take folic acid on the same day as methotrexate, because it can directly interfere with how the drug works. And if your doctor has already prescribed folic acid, don’t stack additional multivitamins or supplements containing folate on top of it. Too much folic acid can blunt methotrexate’s therapeutic effect, while too little leaves you vulnerable to its toxicity. Let your prescriber control the dose.
Herbal Supplements and Over-the-Counter Products
There is not enough safety data to confidently say which herbal supplements are safe with methotrexate. Unlike prescription and pharmacy medications, supplements are not tested for drug interactions in the same rigorous way. St. John’s wort, echinacea, and other immune-modulating herbs are of particular concern because they could either amplify or counteract methotrexate’s effects on the immune system. If you take any supplements, bring the full list to your prescriber so they can flag anything potentially problematic.
Cold and flu remedies deserve special attention because many contain hidden NSAIDs. A combination cold medicine with ibuprofen carries the same kidney-clearance risk described above, and it’s easy to overlook when you’re just grabbing something off the shelf for a sore throat. Always check the active ingredients list for ibuprofen, naproxen, or aspirin before taking any over-the-counter product.