Trimethoprim for UTI: Dosage, Course Length and What to Expect
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For a urinary tract infection, the standard adult dose of trimethoprim is 200mg twice a day. Uncomplicated UTIs in women are often treated for 3 days; men and complicated infections are usually treated for 7 days — a 7-day course is 14 tablets. Trimethoprim is prescription-only in the UK and should be taken as a full course, even once you feel better.
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Trimethoprim comes as a single tablet strength for UTIs: 200mg. The standard adult dose is one 200mg tablet twice a day — morning and evening, roughly 12 hours apart. It is NICE second-line for uncomplicated UTIs, used where nitrofurantoin (first-line) is unsuitable or where local resistance is low.
| Situation | Dose | Course |
|---|---|---|
| Uncomplicated UTI (women) | 200mg twice daily | 3 days (6 tablets) |
| Men / complicated or longer treatment | 200mg twice daily | 7 days (14 tablets) |
| Recurrent-UTI prevention (prophylaxis) | 100mg at night | Long-term, under review |
Course Length: 3 Days, 7 Days and 14 Tablets
The number of tablets you are given reflects your course length — the dose itself does not change.
Why have I been given 14 tablets?
A pack of 14 trimethoprim tablets is the standard 7-day course: one 200mg tablet twice a day for seven days. This is the usual course for men, and for complicated or longer treatment. A shorter 3-day course for an uncomplicated UTI in a woman is 6 tablets. If your pack size doesn’t match what you expected, check the label and your prescriber’s instructions — and always complete the full course, even if symptoms clear after a couple of days.
How to Take Trimethoprim Tablets
- Timing: one dose in the morning and one in the evening, roughly 12 hours apart
- With or without food: either is fine; take with food if your stomach is sensitive
- Swallow whole: do not chew or crush — take with a full glass of water
- Missed dose: take it as soon as you remember, unless your next dose is almost due — then skip it. Never double up
- Complete the course: finish every tablet even if you feel better, to fully clear the infection and reduce resistance
How Long Does Trimethoprim Take to Work?
You should start to feel better within a few days of beginning treatment. If symptoms have not improved after 3 days, or if they worsen at any time, contact your prescriber promptly — you may need a urine culture to check for resistant bacteria, or an alternative antibiotic.
Not improving after 48–72 hours? Don’t just keep going or take a second course of the same antibiotic — contact your prescriber. Persisting symptoms can mean the bacteria are resistant to trimethoprim and a different antibiotic is needed.
Which UTIs Does Trimethoprim Treat?
Cystitis (Bladder Infection)
An infection of the bladder causing frequent urination, urgency, and burning. The most common UTI — treated with a 3 or 7-day course of trimethoprim.
Urethritis
Inflammation of the urethra causing pain and possibly discharge alongside urinary symptoms.
Kidney Infection
A more serious infection causing back pain, fever, and nausea. Seek urgent in-person medical care rather than an online consultation.
Recurrent UTIs (Preventive Use)
Trimethoprim may be prescribed long-term at a low nightly dose to prevent recurrent UTIs. Discuss with your GP if you have 3+ UTIs per year.
For what trimethoprim is and how it works across all its uses, see: What is trimethoprim? Uses and how it works.
Trimethoprim for Recurrent UTIs: Preventive Use and NICE Guidance
For women with recurrent UTIs (three or more per year), NICE supports prophylactic antibiotic use after non-antibiotic options have been considered. Trimethoprim can be used as long-term prophylaxis at low doses (trimethoprim 100mg at night) where nitrofurantoin prophylaxis is not suitable or has failed.
- Typically prescribed once daily at night for 6–12 months
- Requires ongoing prescriber review to assess effectiveness and monitor for side effects
- Considered alongside non-pharmacological measures (post-coital voiding, hydration, vaginal oestrogen in post-menopausal women)
- Increasing resistance may limit long-term effectiveness
Recurrent UTI management requires an ongoing GP or specialist relationship and is not appropriate for online-only prescribing. Access Doctor can treat acute episodes; discuss long-term prevention with your GP.
Side Effects of Trimethoprim Tablets
Trimethoprim is generally well tolerated. Common side effects include nausea, vomiting, diarrhoea, loss of appetite, skin rash, and itching — usually mild and settling after the course. For serious reactions, how long side effects last, and risks in the elderly, see the full guide: Trimethoprim Side Effects UK: Common, Serious and Long-term Risks Explained.
Precautions and Interactions
| Medication | Interaction |
|---|---|
| Methotrexate | Increased toxicity risk — inform prescriber |
| Pyrimethamine | Combined folate inhibition — potentially serious |
| Warfarin | Enhanced anticoagulant effect — INR monitoring may be needed |
| ACE inhibitors / spironolactone | Raised potassium risk, especially in older patients |
| Other antibiotics | Discuss all concurrent medications with your prescriber |
Buy Trimethoprim Tablets Online
Trimethoprim is prescription-only. Access Doctor offers a GPhC-regulated online consultation — a qualified prescriber reviews your symptoms and medical history and, if appropriate, issues a prescription fulfilled by our registered pharmacy. For more on online access routes, see: UTI Antibiotics Online UK for Women.
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MHRA-approved first-line antibiotic for cystitis & UTIs. Prescription via GPhC-regulated online consultation.
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Treat Cystitis Online →For a comprehensive overview of cystitis and UTI — causes, symptoms, diagnosis and all treatment options — see our complete cystitis and UTI guide.
Frequently Asked Questions
Why have I been given 14 tablets of trimethoprim?
A pack of 14 trimethoprim tablets is the standard 7-day course: one 200mg tablet twice a day (morning and evening) for seven days equals 14 tablets. This is the usual course length for men and for complicated or longer UTI treatment; uncomplicated UTIs in women are often a shorter 3-day course. Always finish the full course your prescriber has given you, even if you feel better sooner.
How long does trimethoprim take to work?
You should start to feel better within a few days of beginning treatment. If symptoms have not improved after 3 days, or if they worsen at any time, contact your prescriber promptly — you may need a urine culture to check for resistant bacteria. Always complete the full course even if you feel better before finishing all tablets.
Can I take trimethoprim and nitrofurantoin at the same time?
While both can technically be prescribed, no prescriber would normally recommend both simultaneously for a straightforward UTI. Nitrofurantoin is prescribed first. If the infection proves resistant to nitrofurantoin, trimethoprim or a different antibiotic is then prescribed separately — ideally guided by a urine culture. Never take a second antibiotic alongside a current course without prescriber guidance.
Does trimethoprim affect the contraceptive pill?
Current NHS and NICE guidance does not recommend using additional contraception while taking trimethoprim. Unlike some antibiotics (notably rifampicin), trimethoprim does not reduce the effectiveness of the combined pill or progestogen-only pill through enzyme induction. However, if you experience significant vomiting or diarrhoea as a side effect, this may affect pill absorption — follow your pill’s patient information leaflet guidance on missed doses.
Is trimethoprim safe during pregnancy?
Trimethoprim should be avoided in the first trimester as it interferes with folate metabolism, which is critical for foetal neural tube development. It may be used in the second and third trimesters when clinically necessary and under medical supervision. Pregnant women with any UTI symptoms should always contact their GP or midwife for in-person assessment rather than using an online consultation service.
How do I buy trimethoprim tablets online?
Trimethoprim is prescription-only in the UK. Access Doctor’s GPhC-regulated online consultation allows a qualified prescriber to assess your suitability and issue a prescription for next-day delivery in discreet packaging. The consultation takes under five minutes and no GP appointment is needed.
References
- NICE. Urinary tract infection (lower) — women. Clinical Knowledge Summaries. cks.nice.org.uk
- NHS. Trimethoprim. nhs.uk/medicines/trimethoprim
- NICE. British National Formulary: Trimethoprim. bnf.nice.org.uk
- UKHSA. ESPAUR report 2023. gov.uk
- Electronic Medicines Compendium. Trimethoprim 200mg tablets — SmPC. medicines.org.uk/emc
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for diagnosis and treatment. Nitrofurantoin and trimethoprim are prescription-only medicines — a medical consultation is required before they can be dispensed. If you have a fever, loin/back pain, or are pregnant, seek urgent in-person medical care. In a medical emergency, call 999.