Do antibiotics work if your on methotrexate?

Do antibiotics still work if you’re taking methotrexate?

In most cases, yes. Antibiotics generally still work because methotrexate is not a medication that blocks antibiotic action. The bigger issue is whether a specific antibiotic interacts with methotrexate or increases the risk of side effects.

Which antibiotic combinations are most likely to be a problem?

Some antibiotics can raise methotrexate levels in the body or increase toxicity, especially for people taking methotrexate at higher doses (often used for cancer) or even at lower doses in certain situations (like kidney problems, dehydration, or older age).

A common high-risk example is trimethoprim-sulfamethoxazole (TMP-SMX, Bactrim/Septra). This combo can increase methotrexate-related toxicity because it can affect how drugs are processed and can add to bone-marrow suppression risk.

What side effects should you watch for?

If an antibiotic raises methotrexate levels, the main concerns are often:
- Mouth sores (stomatitis), sore throat, unusual ulcers
- Severe fatigue or weakness from low blood counts
- Fever or signs of infection that don’t improve
- Easy bruising or bleeding
- Unusual nausea/vomiting, diarrhea
- Shortness of breath or persistent cough (if symptoms worsen)

If you develop these while starting an antibiotic, you should contact your prescriber promptly.

What about methotrexate + infections: will the dose need to change?

Clinicians sometimes adjust methotrexate during infections depending on:
- How severe the infection is
- Whether the antibiotic is one that interacts with methotrexate
- Your blood counts and kidney function
- Whether the infection is viral vs bacterial

Your prescriber decides this; stopping methotrexate on your own can be risky, especially if you take it for inflammatory disease.

Does it matter if methotrexate is weekly vs daily?

Yes. Methotrexate for conditions like rheumatoid arthritis/psoriasis is typically taken once weekly, while cancer regimens can differ. Interaction risk and toxicity concern can be higher with higher-dose schedules and in people with kidney impairment.

What should you do before starting antibiotics?

Before starting any antibiotic, it helps to:
- Tell the prescribing clinician you take methotrexate (and the dose, if you know it)
- Ask if there are any interaction concerns with that specific antibiotic
- Make sure your other meds are listed (especially NSAIDs like ibuprofen/naproxen, sulfa drugs, and other medicines that can affect blood counts)

If you’re able to share the name of the antibiotic and your methotrexate dose (and whether you have kidney disease), I can tell you which interaction issues are most relevant.