An antibiotic for urinary infections · Written by the Lyng Pharmacy team, based on NHS and BNF guidance · Updated August 2026
Also sold asMonotrim
Type of medicineAntibiotic (urinary)
Available asTablets, oral suspension
How you get itPrescription only, including via Pharmacy First
Trimethoprim is a second-line option for urinary tract infections, used where nitrofurantoin is unsuitable or local resistance patterns favour it. It is straightforward to take, with two specific cautions worth knowing about.
What it is used for
Uncomplicated urinary tract infections, and occasionally other infections. It can be supplied under the Pharmacy First service for women aged 16 to 64 with an uncomplicated UTI where appropriate.
Resistance to trimethoprim is common in some areas, which is why nitrofurantoin is usually first choice.
How to take it
Usually twice daily for three days for an uncomplicated UTI in women, or as prescribed. Take it with or after food to reduce stomach upset, with a full glass of water.
Finish the course even if symptoms settle after a day or two. Drink plenty of fluids throughout.
Pregnancy and other cautions
Trimethoprim is generally avoided in the first trimester of pregnancy, as it interferes with folate. Tell your prescriber if you are pregnant, might be, or are trying to conceive — an alternative will be used.
Also tell your prescriber if you:
Take methotrexate — the combination is generally avoided
Take warfarin, phenytoin, ciclosporin, or medicines that raise potassium including ACE inhibitors and sartans
Have kidney problems, a folate deficiency, or a blood disorder
Side effects
Nausea, vomiting, diarrhoea, a sore mouth and itching are the common ones.
Get medical advice for:
A widespread rash, blistering or peeling skin, or swelling of the face or throat
Unexplained bruising, bleeding, sore throat or fever — possible blood problems
Muscle weakness or an irregular heartbeat — possible high potassium
When a UTI needs more than antibiotics
Contact your GP or NHS 111 if you develop back or side pain, a high temperature, shivering, vomiting, or blood in the urine, or if symptoms are not improving after 48 hours. Those can indicate a kidney infection.
Always read the label. This page is general information, not advice for your situation. Doses, warnings and interactions differ between products and between people. Check the leaflet inside your medicine, and ask our pharmacist if anything is unclear — that conversation is free and takes two minutes.
Questions about Trimethoprim?
1 Lyng Lane, West Bromwich B70 7RW · Mon–Fri 8:30am–6pm, Sat 9am–1pm · 0121 500 5756
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