What antibiotics can be taken with methotrexate (and what changes)?
In most cases, antibiotics can be taken while you’re on methotrexate, but safety depends on which antibiotic and how your body clears methotrexate.
Methotrexate can build up to higher levels if kidney function is reduced, if dehydration occurs, or if certain drug interactions slow its clearance. Some antibiotics also affect gut bacteria and folate balance, which can increase the risk of methotrexate side effects (like mouth sores, low blood counts, or liver irritation).
Because the exact interaction depends on the specific antibiotic and your methotrexate dose, the safest approach is to confirm with your prescriber or pharmacist before starting.
Which antibiotics are most concerning with methotrexate?
The best-known high-risk interaction is with antibiotics that contain (or strongly resemble) trimethoprim, because they can affect folate metabolism alongside methotrexate. This raises the risk of blood count suppression and other toxicity.
You should ask specifically about medicines in the “trimethoprim” family, including common combinations like trimethoprim-sulfamethoxazole (often called TMP-SMX or co-trimoxazole).
What side effects mean you should stop and get medical advice?
If you’re taking methotrexate and start an antibiotic, contact your clinician promptly (or seek urgent care) if you notice signs that could suggest methotrexate toxicity, such as:
- Mouth sores or painful swallowing
- Unusual bruising or bleeding, or signs of low white blood cells (fever, recurrent infections)
- Severe nausea/vomiting or worsening fatigue
- Yellowing of the skin/eyes, dark urine (possible liver issues)
- Shortness of breath or widespread rash
What should you tell your clinician or pharmacist before you start antibiotics?
Make sure your prescriber/pharmacist knows:
- Your methotrexate dose and whether it’s weekly (most common for inflammatory conditions)
- Your kidney function history (or if you have kidney disease)
- Any folic acid or folinic acid (leucovorin) you take with methotrexate
- Other meds you take that can affect kidneys or blood counts
This matters because the interaction risk is higher if methotrexate levels are already likely to rise.
Do folic acid supplements lower the risk?
Many people taking methotrexate for inflammatory disease take folic acid to reduce side effects. Whether that changes the risk with a particular antibiotic depends on your regimen and the antibiotic choice. Don’t change folic acid dosing on your own—ask your clinician.
Practical answer: what you should do today
- Tell the antibiotic prescriber you take methotrexate and ask if there are any interaction concerns with the specific antibiotic.
- If the antibiotic is one of the trimethoprim-containing options, confirm before taking it.
- If you already started the antibiotic, don’t stop methotrexate without medical advice; contact the prescribing clinician or pharmacist to review your combination.
If you tell me (1) the antibiotic name and dose, (2) your methotrexate dose and how often you take it, and (3) whether you have kidney problems, I can help you gauge the likelihood of interaction and what to ask your pharmacist.