Trimethoprim (UTI)

How it works

It inhibits bacterial dihydrofolate reductase, blocking folate synthesis and thereby bacterial DNA and protein production.

Prescribing in practice

• Avoid in the first trimester of pregnancy because its folate-antagonist action carries a risk of neural tube defects; choose an alternative and give folate where exposure occurs.
• It can cause hyperkalaemia and raise serum creatinine, with particular caution alongside ACE inhibitors, angiotensin receptor blockers, potassium-sparing agents and in renal impairment.
• Concurrent use with methotrexate increases the risk of folate-related bone marrow toxicity.

Monitoring

For short courses little monitoring is needed, but check renal function and potassium with prolonged use, in the elderly or with interacting drugs.

Counselling the patient

• Complete the prescribed course and seek review if symptoms persist or worsen.
• Tell your prescriber if you are or might be pregnant.

Evidence & guidelines

NICE urinary tract infection guidance lists trimethoprim as an empirical option for lower UTI, with choice guided by local resistance patterns.