Trimethoprim (UTI in Pregnancy — Caution)

How it works

It inhibits bacterial dihydrofolate reductase, blocking folate synthesis and so impairing bacterial DNA production.

Prescribing in practice

• Avoid in the first trimester of pregnancy — as an antifolate it is associated with an increased risk of neural tube defects.
• It can cause hyperkalaemia and a reversible rise in serum creatinine; use caution with ACE inhibitors, angiotensin receptor blockers and potassium-sparing drugs.
• It must not be combined with methotrexate, as the additive antifolate effect can precipitate severe bone-marrow suppression.

Monitoring

For short courses in healthy patients little monitoring is needed, but check renal function and potassium in those at risk (renal impairment, older patients, or on interacting drugs such as ACE inhibitors).

Counselling the patient

• Tell your prescriber if you are or might be pregnant, as it is avoided in early pregnancy.
• Complete the prescribed course and report a rash, sore throat or unusual bruising.
• Mention any methotrexate or blood-pressure or potassium medicines you take, as these interact.

Evidence & guidelines

An established option for urinary tract infection where local resistance patterns allow; UK guidance directs first-trimester avoidance and folate-supplement consideration, and choice should reflect local sensitivity data.