Trimethoprim: uses for UTIs, side effects and cautions

Trimethoprim is best known as a urinary antibiotic. The NHS lists treating and preventing urinary tract infections and cystitis as its main purpose, with occasional use for chest infections and acne. It comes as tablets and as a liquid for children or people who cannot swallow tablets.

The NHS explains the mechanism clearly: it kills bacteria by stopping them making a substance called folic acid. That also explains the cautions about folate further down the page.

Quoted NHS amounts are 200 mg twice a day to treat a UTI and 100 mg once a day, at night, to prevent recurrences. Tablets come in 100 mg and 200 mg strengths and the liquid contains 50 mg in 5 ml. Your label is what counts.

Course length depends on the problem. The NHS describes a short course of about 3 days for a straightforward UTI, longer for severe or complicated infection, around 2 weeks for prostatitis (sometimes longer), and at least 3 months for acne. Finish the course even if you feel better, and speak to your doctor if you have not improved within 2 days or you are getting worse.

• You can take it with or without food.

• Missed dose: take it as soon as you remember unless the next is nearly due; never double up.

• One extra dose is unlikely to do serious harm but may cause nausea or diarrhoea; call NHS 111 after two or more extra doses or if you are worried.

Most adults and children can take trimethoprim. The NHS lists allergy to it or other medicines, liver or kidney problems, iron-deficiency anaemia, vitamin B12 or folate deficiency anaemia, porphyria and other blood disorders as reasons to check first.

Because the drug interferes with folic acid, anyone with a folate problem should tell the prescriber.

Trimethoprim can be used in pregnancy but is not usually recommended in the first 12 weeks because of the effect on folic acid. When it is necessary, doctors typically add folic acid, and the NHS mentions 5 mg a day, during early pregnancy and while trying to conceive.

A small amount passes into breast milk and is unlikely to affect a baby during short-term use. There is no evidence it harms fertility, but anyone taking it daily while trying to conceive should discuss options.

More than 1 in 100 people get itching or a rash, nausea or vomiting, diarrhoea, thrush or headaches. A pharmacist can suggest an emollient or antihistamine for itching; for thrush, keep the area dry, wear cotton underwear and avoid sex until it has cleared.

• Call NHS 111 or a doctor for bloody or severe diarrhoea, sudden severe stomach pain with sickness, yellow skin or eyes, unexplained bruising or bleeding, sore throat, mouth ulcers, fever or tiredness, or severe skin reactions.

• Call 999 for headache with high temperature, stiff neck and sensitivity to light (possible meningitis), muscle weakness, an abnormal heartbeat or chest pain, or signs of anaphylaxis.

The NHS names rifampicin, warfarin and acenocoumarol, digoxin, phenytoin, repaglinide, pioglitazone and methotrexate as medicines to review. An oral typhoid vaccine may work less well, so mention it if you have had or plan one.

The NHS also lists habits that can help prevent UTIs: staying well hydrated, good hygiene, cotton underwear, emptying the bladder fully when needed and avoiding irritating products near the genitals. It does not affect driving, contraception or alcohol.