Treated for cystitis — so why the mouth ulcers, sore throat and bruises?
Methotrexate and trimethoprim both block folate. Together they can shut down the bone marrow.
Select any card for the reasoning, what to watch for and an exam tip.
The puzzle
A 70-year-old woman takes methotrexate once a week for rheumatoid arthritis. An out-of-hours doctor prescribes trimethoprim for cystitis. Ten days later she has mouth ulcers, a sore throat, a fever and bruises she can’t explain.
The answer, step by step
Who is most at risk
- Older people.
- Impaired kidney function, including dehydration.
- Anyone on methotrexate given trimethoprim or co-trimoxazole.
- Patients who take methotrexate more often than once a week by mistake.
What to do
Key takeaway
Trimethoprim with methotrexate risks severe, sometimes fatal, bone marrow suppression: the BNF says avoid. Choose another antibiotic, and teach everyone on methotrexate to report a sore throat, bruising or mouth ulcers at once.