What Is Trimethoprim? Uses, How It Works and How to Get It in the UK

What Is Trimethoprim?

Trimethoprim is a commonly prescribed antibiotic in the UK, used primarily to treat urinary tract infections (UTIs). It belongs to a class of antibiotics known as dihydrofolate reductase inhibitors — it works by disrupting the bacteria’s ability to produce folic acid, a nutrient essential for their growth and survival. By blocking this process, trimethoprim stops bacteria multiplying and allows the body’s immune system to clear the infection.

It has been used in the UK for decades and was historically the first-choice UTI antibiotic. Rising resistance has since moved it to a second-line role behind nitrofurantoin, but it remains widely prescribed and effective for most uncomplicated infections.

What Is Trimethoprim Used For?

Trimethoprim is licensed and used across several infection types, though UTIs are by far the most common reason it is prescribed:

• UTIs and cystitis — bladder infections in women; the main use. NICE second-line after nitrofurantoin
• Recurrent UTIs — sometimes at a low nightly dose for prevention, under regular review
• Chest infections — certain respiratory bacterial infections (less common)
• Acne — used long-term at lower doses for inflammatory acne
• Ear, nose and throat infections — occasional use for bacterial ENT infections
• Traveller’s diarrhoea — some bacterial causes respond to trimethoprim

Trimethoprim does not work against viral infections — including colds, flu or COVID-19. It should only be used when a bacterial infection has been confirmed or strongly suspected by a healthcare professional. For how it compares with the first-line antibiotic, see: nitrofurantoin vs trimethoprim compared.

How Does Trimethoprim Work?

Trimethoprim inhibits the enzyme dihydrofolate reductase, which bacteria need to convert dihydrofolic acid into tetrahydrofolic acid — the active form of folate required for DNA synthesis. Without this process, bacteria cannot replicate their genetic material and are unable to multiply. The body’s immune cells then clear the static bacterial population.

Human cells also need folate, but use a different pathway not affected by trimethoprim at therapeutic doses — which is why the drug selectively targets bacteria. In vulnerable groups (pregnant women, those with folate deficiency), this selectivity is less complete, which informs the prescribing precautions below.

Trimethoprim Dosage (In Brief)

For a urinary tract infection, the standard adult dose is 200mg twice daily, usually for a 7-day course. Tablets are swallowed whole with water and can be taken with or without food. Always complete the full course, even once you feel better.

Can You Drink Alcohol With Trimethoprim?

Yes — there is no dangerous interaction between trimethoprim and alcohol. Unlike metronidazole, trimethoprim does not cause a reaction with alcohol, and drinking in moderation will not stop it working.

That said, heavy drinking can weaken your immune response and slow your recovery from any infection, and alcohol can irritate the bladder and worsen dehydration during a UTI. It is sensible to keep alcohol to a minimum until you have finished the course and your symptoms have settled.

Important Considerations Before Taking Trimethoprim

Tell your prescriber before starting trimethoprim if any of the following apply:

• Any known allergies to trimethoprim or other antibiotics
• Liver or kidney problems
• Anaemia or low folic acid levels
• Porphyria or blood disorders
• Pregnancy or breastfeeding (first trimester — avoid; second/third — discuss)
• All other medications, including over-the-counter and herbal supplements (trimethoprim interacts with warfarin, methotrexate, and potassium-raising medicines such as ACE inhibitors and spironolactone)

For the full list of side effects and warning signs, see: Trimethoprim side effects explained.

Trimethoprim and Antibiotic Stewardship: Using Antibiotics Responsibly

Antibiotic resistance is one of the most significant public health challenges of our time. Trimethoprim resistance has been rising in UK uropathogens — a direct result of decades of widespread prescribing. Using trimethoprim appropriately matters both for your own treatment success and for preserving the antibiotic’s effectiveness for future patients.

• Only take trimethoprim when prescribed for a confirmed or strongly suspected bacterial infection
• Complete the full prescribed course — stopping early does not eliminate all bacteria and selects for resistant strains
• Never save leftover trimethoprim tablets for a future infection — the next infection may be a different organism or resistance pattern
• Never share your antibiotic prescription with someone else
• Report any side effects via the MHRA Yellow Card scheme

Side Effects of Trimethoprim

Common side effects (nausea, vomiting, diarrhoea, rash, headache) are usually mild and temporary. For a complete guide to trimethoprim side effects — including serious reactions, how long they last, risks in the elderly and what to do — see: Trimethoprim Side Effects UK: Common, Serious and Long-term Risks Explained.

Getting Trimethoprim from Access Doctor

Access Doctor provides a convenient alternative to a GP appointment for qualifying UTI treatment. Complete a GPhC-regulated online consultation and our prescribers will assess whether trimethoprim is appropriate for you. Once approved, it is delivered in discreet packaging, with next-day delivery available. For how the online prescribing process works, see: UTI Antibiotics Online UK for Women.

For a comprehensive overview of cystitis and UTI — causes, symptoms, diagnosis and all treatment options — see our complete cystitis and UTI guide.