A urinary (UTI) antibiotic
Trimethoprim
A common antibiotic for urinary tract infections — avoided in early pregnancy, with several important interactions and effects on potassium and kidney blood tests.
What is Trimethoprim?
Trimethoprim is a commonly used antibiotic and one of the standard first-choice options for an uncomplicated lower urinary tract infection (UTI). It works by interfering with the way bacteria make the folate they need to grow.
Education and reference only. This is a plain-language guide to Trimethoprim — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Trimethoprim is a commonly used antibiotic for urinary tract infections (UTIs), and is one of the standard first-choice options for an uncomplicated lower UTI alongside nitrofurantoin. It is taken as a short course. It is also occasionally used for other infections and, in some settings, for prevention of recurrent UTIs. It works against bacteria, not viruses.
How it works
Trimethoprim blocks a step that bacteria need to make folate, a building block they require to produce DNA and multiply. Without it, the bacteria cannot grow and the body can clear the infection. Because human cells get folate a different way, the medicine targets the bacteria — but its effect on the folate pathway is also the reason it is avoided in early pregnancy and can interact with other folate-affecting medicines.
Company & origin
Originated / developed by: Burroughs Wellcome (now GSK).
Trimethoprim was developed from the folate-antagonist research of George Hitchings and colleagues at Burroughs Wellcome in the United States during the 1950s and 1960s, with the compound identified around 1962. It came into clinical use later in the 1960s; Burroughs Wellcome is now part of GSK.
Practical use
How to take Trimethoprim
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it as prescribed and complete the full course, even if symptoms improve quickly.
- Can be taken with or without food, spaced evenly through the day.
- If you miss a dose, take it as soon as you remember unless the next is nearly due; do not double up.
- Tell your prescriber if you are pregnant or trying to conceive, as it is usually avoided in early pregnancy.
- Mention any kidney problems or medicines that affect potassium, as these may need monitoring.
Weighing it up
Advantages & disadvantages of Trimethoprim
Advantages
- An effective and well-established first-choice for uncomplicated UTIs.
- Usually a short, convenient course taken by mouth.
- Generally well tolerated by most people.
- Inexpensive and widely available.
Disadvantages
- Best avoided in early pregnancy because it can interfere with folate.
- Resistance among UTI bacteria has been rising, so it does not always work.
- Can raise potassium levels and needs caution with kidney problems or certain other medicines.
- May cause rash, nausea or, rarely, more serious skin or blood reactions.
Practical use
Good to know
Trimethoprim is effective and well established, but a few cautions matter. It is avoided in pregnancy, especially the first trimester, because by interfering with folate it can affect a baby's early development. It has several important interactions — notably with methotrexate (also a folate-affecting drug), warfarin, and medicines that raise potassium such as ACE inhibitors, ARBs and potassium-sparing diuretics — and it can itself nudge up blood potassium and the creatinine reading used to estimate kidney function. It is taken exactly as prescribed and finished as advised, in keeping with using antibiotics responsibly. Local resistance patterns mean it is not always the first choice, and the prescriber will pick the best option for your situation.
Who should not take it / use with caution
- Women who are pregnant or planning pregnancy, particularly in the first trimester, because it interferes with folate, which is important for a baby's early development.
- People taking methotrexate, where the combination can dangerously increase methotrexate's effects.
- Used with caution in significant kidney impairment, in people with a high blood potassium level, and in those with certain blood disorders.
Monitoring
- Whether the UTI symptoms are settling
- Blood potassium and kidney function (creatinine), especially with interacting medicines or in older people
- Blood counts with long-term or repeated use
Side effects
- Feeling sick, stomach upset, loose stools or a sore mouth — common and usually mild.
- An itchy rash, which can occasionally signal allergy.
- A rise in blood potassium and in the creatinine reading (watched with blood tests, especially in older people or those on interacting medicines); rarely, blood-count problems with long-term use, or a serious allergic reaction (call 999 for swelling of the face or throat, wheeze or collapse).
Key interactions
- Methotrexate — the combination can sharply increase methotrexate toxicity and is generally avoided.
- It enhances the effect of warfarin (more bleeding risk) and can raise blood potassium when combined with ACE inhibitors, ARBs, potassium-sparing diuretics or potassium supplements.
- It can increase the effect of certain diabetes tablets and other medicines; always tell your pharmacist what else you take.
Available as: Tablets and an oral suspension (liquid) for those who cannot swallow tablets.
Answers
Trimethoprim: frequently asked questions
Can I take trimethoprim in pregnancy?
It is avoided in pregnancy, especially in the first trimester, because it interferes with folate — a nutrient that is very important early in a baby's development. If you are pregnant or could be, tell your prescriber so a safer antibiotic can be chosen for your water infection.
Why does trimethoprim affect my blood tests?
Trimethoprim can nudge up the blood potassium level and the creatinine reading used to estimate kidney function. This is usually mild and reversible, but it is why your prescriber may check blood tests, particularly if you are older or take medicines such as ACE inhibitors or ARBs that also affect potassium.
I take methotrexate — is trimethoprim safe for me?
Generally no. Both trimethoprim and methotrexate affect the folate pathway, and together they can dangerously increase methotrexate's effects on the blood and other tissues. Always tell your prescriber and pharmacist that you take methotrexate so a different antibiotic is chosen.
Is trimethoprim or nitrofurantoin better for a UTI?
Both are recommended first-choice options for an uncomplicated lower UTI, and the better one depends on your kidney function, pregnancy status, other medicines and local resistance patterns. Your prescriber weighs these up — for example, trimethoprim is avoided in early pregnancy, while nitrofurantoin needs reasonable kidney function.
Does trimethoprim come as a brand?
In the UK trimethoprim is usually supplied as the generic medicine without a common brand name. Generic trimethoprim contains the standard active ingredient and works the same way regardless of the manufacturer.
The wider class
About UTI antibiotics
Trimethoprim belongs to the uti antibiotics class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF: Trimethoprim.
- electronic Medicines Compendium (SmPC): Trimethoprim.
- NICE CKS: Trimethoprim.
- NICE: Urinary tract infection (lower) — antimicrobial prescribing.
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