Trimethoprim

Key facts

• Trimethoprim is an antibacterial that blocks bacteria from making folic acid, a vitamin they need to multiply; it's most often used for urinary tract infections.
• It's usually taken once or twice a day for three to seven days; longer, lower-dose courses are used to prevent recurrent infections.
• Avoid trimethoprim in early pregnancy: it's a folate antagonist and can raise the risk of neural tube defects, so alternative antibiotics are preferred instead.
• It can raise blood potassium, especially with ACE inhibitors, ARBs or potassium-sparing diuretics; seek urgent care for muscle weakness or an irregular heartbeat.

What Trimethoprim treats

Trimethoprim treats uncomplicated urinary tract infections, and is sometimes used as a long-term, low-dose preventive for recurrent bladder infections. It's also combined with sulfamethoxazole (co-trimoxazole) for a wider range of infections, though that combination carries its own separate risks.

How Trimethoprim works

Bacteria must make their own folic acid to build DNA and multiply; they can't absorb it from their surroundings the way human cells can. Trimethoprim blocks the bacterial enzyme that makes folic acid, starving the bacteria of a vitamin they need to grow.

Before you take it

• Avoid trimethoprim during the first trimester of pregnancy, and take a folic acid supplement if it's used later in pregnancy, it interferes with folate metabolism, which matters for early fetal development.
• Tell your prescriber about kidney disease, or if you take ACE inhibitors, ARBs, spironolactone or other medicines that raise potassium; trimethoprim can push potassium to dangerous levels.
• Tell your prescriber about methotrexate use; combining it with trimethoprim increases the risk of folate deficiency and blood toxicity.
• Avoid it if you have folate-deficiency anaemia, or a known sulfonamide allergy if it's combined with sulfamethoxazole.

Side effects

Nausea, loss of appetite and a mild rash are common.

Stop and seek urgent medical care for any of these:

• Muscle weakness, an irregular heartbeat, or confusion, possible signs of high potassium.
• Unusual bruising, bleeding, a sore throat or fever, possible signs of low blood cell counts.
• Severe rash or blistering skin.
• Reduced urine output or swelling of the legs.

Safety essentials

• Trimethoprim blocks folate production, so it's avoided in early pregnancy because of the risk of neural tube defects; tell your prescriber immediately if you might be pregnant.
• It can raise blood potassium to dangerous levels, particularly alongside ACE inhibitors, ARBs or potassium-sparing diuretics; your prescriber may check your blood levels on these combinations.
• Finish the full course as directed, even once you feel better, and don't reuse leftover trimethoprim, this drives antibiotic resistance.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.