Which antibiotics are generally considered OK with methotrexate?
The main concern with methotrexate is that some antibiotics can raise methotrexate levels or increase toxicity (such as mouth sores, diarrhea, low blood counts, or liver strain). The antibiotics listed below are commonly used with methotrexate when clinically needed, because they are not known for the same interaction risk as certain alternatives (especially “trimethoprim” drugs like co-trimoxazole).
Commonly used options that are generally considered compatible include:
- Penicillins (for example, amoxicillin, ampicillin)
- Cephalosporins (for example, cephalexin, cefixime, ceftriaxone)
- Macrolides (for example, azithromycin, clarithromycin)
- Clindamycin
Even with these, clinicians often still check kidney function and blood counts if the infection is serious, if methotrexate is taken at higher doses (for cancer regimens), or if you already have reduced kidney function.
Which antibiotics are most likely to interact (and are usually avoided or used with extra caution)?
The antibiotics most often flagged for interactions with methotrexate are those that can increase methotrexate exposure or affect folate pathways, particularly:
- Trimethoprim-sulfamethoxazole (co-trimoxazole, “Bactrim/Septra”)
- High-dose trimethoprim products
Because of this risk, many prescribers try to avoid these when a safer antibiotic will work. If co-trimoxazole is necessary, the prescriber may adjust the plan and monitor closely.
Does the interaction depend on your methotrexate dose and kidney function?
Yes. The higher the methotrexate dose and the worse the kidney function, the greater the risk that an interacting antibiotic could push methotrexate levels too high. This matters for:
- Methotrexate used for rheumatologic/skin diseases (often weekly, lower doses)
- Methotrexate used for cancer (often higher doses, different safety monitoring)
If you have kidney disease, dehydration, or are older, your prescriber may choose an antibiotic more carefully and monitor labs more often.
What should you watch for after starting an antibiotic with methotrexate?
Contact a clinician promptly if you develop signs of methotrexate toxicity, such as:
- Mouth sores, sore throat, or unusual ulcers
- Severe or persistent diarrhea or vomiting
- Easy bruising or unusual infections (possible low blood counts)
- Unusual fatigue or yellowing of the skin/eyes (liver issues)
- Shortness of breath or rash (could be an allergic reaction too)
What’s the safest next step for your specific case?
Tell the prescriber/pharmacist:
- The exact methotrexate dose (mg) and how you take it (usually weekly)
- Your kidney function status (if known)
- The infection being treated and which antibiotic was proposed (name and dose)
- Any other interacting medicines you take (for example, NSAIDs at high or frequent doses, other folate-related drugs, or blood thinners)
If you share the antibiotic name and your methotrexate dose (and whether it’s weekly for arthritis/psoriasis vs cancer dosing), I can give a more specific compatibility answer.