Trimethoprim for UTI: Uses, 200mg Dose and How Long It Takes to Work
A plain-English UK guide to trimethoprim tablets: what they treat, how to take them, how quickly they work, who shouldn’t take them, and how they compare with nitrofurantoin.
Part of our cystitis guide: symptoms, causes and treatment.
Get urgent help first. Call 999 or go to A&E if you feel very unwell, confused or very drowsy, or have a very fast heartbeat or breathing, or cold, blotchy skin: these can be signs of sepsis. Call 999 too for swelling of the face, lips or throat, or difficulty breathing after taking a medicine. Contact your GP or NHS 111 (NHS 24 on 111 in Scotland) the same day if you have pain in your back or side, a high temperature, shivering or vomiting, which can be signs of a kidney infection, or if you can see blood in your urine. Online treatment isn’t suitable in these situations.
The short version: trimethoprim is one of the two first-choice antibiotics for cystitis in women in UK guidance (NICE NG109), alongside nitrofurantoin. The usual dose is one 200mg tablet twice a day for 3 days, and most women start to feel better within a day or two.
Get Cystitis Treatment Online
Access Doctor is a GPhC-registered pharmacy (#9011198). If you’re a woman aged 18 to 64 with cystitis symptoms, a pharmacist independent prescriber reviews your online consultation and prescribes an antibiotic if it’s suitable for you. Dispensed from our UK pharmacy. Approved before 3pm, Monday to Friday? Dispatched the same day, by Royal Mail Tracked 48, Tracked 24 or Special Delivery guaranteed by 1pm next working day: you choose.
Start Cystitis Consultation →Who our online service is for. Access Doctor treats cystitis online for women aged 18 to 64 with typical symptoms who aren’t pregnant or breastfeeding. Men, anyone under 18, women aged 65 and over, and anyone who is pregnant or breastfeeding should see a GP, because a urine test and a different plan are usually needed. In England, many pharmacies treat uncomplicated cystitis in women aged 16 to 64 for free through NHS Pharmacy First.
What is trimethoprim?
Trimethoprim is a prescription-only antibiotic, and in the UK it’s used mostly for urine infections. It works by blocking an enzyme called dihydrofolate reductase, which bacteria need to make folate. Without folate, they can’t make new DNA or multiply, and your body can clear the infection.
Here it usually comes as 200mg tablets, and sometimes as 100mg tablets (used for prevention) or a liquid. Monotrim is an older brand name for the same medicine.
What trimethoprim is used for
Cystitis (bladder infection)
Its main use. Burning when you pee, needing to go often or urgently, and pain low in your tummy. Usually a 3-day course.
Preventing repeated UTIs
A low dose, usually 100mg at night, prescribed and reviewed by a GP for women with frequent infections.
Not for kidney infections without a test
Back or side pain with a high temperature or shivering needs same-day, in-person care. Trimethoprim is only used for kidney infections when a urine culture shows it will work.
Other infections
Doctors occasionally use it for other infections, but in UK practice it’s mainly a urinary antibiotic.
When trimethoprim is the right choice
NICE points to two first-choice antibiotics for cystitis in women who aren’t pregnant: nitrofurantoin, if your kidneys work well enough (eGFR 45 or more), and trimethoprim, if the risk of resistance is low. Resistance to trimethoprim is more common than resistance to nitrofurantoin, so the prescriber weighs up how likely it is to work for you.
- You haven’t taken trimethoprim in the last 3 months. Recent use makes resistance much more likely, so it’s usually avoided.
- Nitrofurantoin isn’t suitable, for example because your kidneys don’t work well, you have G6PD deficiency, or you’ve reacted to nitrofurantoin before.
- A urine culture shows the bacteria are sensitive to it. A targeted antibiotic is always the most reliable choice.
- You’re not taking medicines that interact with it, such as methotrexate or medicines that raise potassium.
Check it’s cystitis first. Burning and needing to pee often can also be caused by thrush, an STI, vaginal dryness after the menopause or bladder pain syndrome. If you have unusual discharge or itching, or you’ve had a new sexual partner, an antibiotic for cystitis may not help.
How to take trimethoprim
1
The usual dose
One 200mg tablet twice a day, about 12 hours apart, usually for 3 days. If your prescriber gives you a 7-day course, take it for the full 7 days.
2
With or without food
Either is fine. Taking it with food can help if it upsets your stomach. Swallow each tablet with a glass of water.
3
If you miss a dose
Take it when you remember, unless it’s nearly time for the next one. Never take two doses at once to make up.
4
Finish the course
Keep taking it even if you feel better after a day or two, so the infection doesn’t come back. Don’t keep leftovers.
Paracetamol, or ibuprofen if it’s suitable for you, can ease the pain while the antibiotic works. Drink enough fluid so you’re not dehydrated: pale-yellow urine is a good guide.
6 or 14 tablets? Our trimethoprim 200mg tablets come as 6 tablets (a 3-day course) or 14 tablets (a 7-day course). The 3-day course is the usual treatment for cystitis in women, and the prescriber supplies the 14-tablet pack only if they decide a longer course is needed.
How long trimethoprim takes to work
Trimethoprim starts working against the bacteria within hours of the first dose, but it takes a little longer for symptoms to ease:
| When | What to expect |
|---|---|
| Day 1 | Symptoms may not change much yet. Keep taking it, and use painkillers if you need them. |
| Days 1 to 2 | Many women notice burning and urgency starting to ease. |
| By 48 hours | Symptoms should be improving. If they aren’t, or they’re worse, contact your GP or NHS 111: the bacteria may be resistant and you may need a urine test and a different antibiotic. |
| End of the course | Most symptoms have gone. Mild discomfort can take a few more days to settle completely. |
The 48-hour check: no improvement at all after 48 hours usually means resistance, not that you need to wait longer. Don’t start another antibiotic you have at home: get advice, as a urine culture shows which antibiotic will work.
Trimethoprim or nitrofurantoin?
Both are first-choice antibiotics for cystitis in UK guidance, and both work well when the bacteria are sensitive. They work in different ways, which is why one can succeed where the other fails. Nitrofurantoin attacks several targets inside bacteria at once, so resistance to it has stayed uncommon. Trimethoprim acts on a single target, so bacteria can become resistant more easily.
| Trimethoprim | Nitrofurantoin | |
|---|---|---|
| NICE NG109 | First choice if resistance risk is low | First choice if eGFR is 45 or more |
| How it works | Blocks folate production (one target) | Damages bacterial DNA and proteins (several targets) |
| Usual dose | 200mg twice a day | 100mg modified-release twice a day |
| Usual course | 3 days | 3 days |
| Food | With or without food | With food |
| Resistance | More common, especially after recent use | Uncommon |
| Main reasons to avoid | Used in the last 3 months; early pregnancy; methotrexate; medicines that raise potassium | Reduced kidney function; G6PD deficiency; cystitis sachets such as CanesOasis |
| Worth knowing | Can raise potassium on blood tests | Turns urine dark yellow or brown (harmless) |
The prescriber decides which suits you, based on your kidney health, recent antibiotics, other medicines and allergies. Our guide to UTI antibiotics in the UK compares all the options in more detail, and our nitrofurantoin guide covers the other first-choice antibiotic.
Who shouldn’t take trimethoprim
- Pregnancy: trimethoprim must not be used in the first 3 months of pregnancy, because it affects folate, which a baby needs as it develops. Urine infections in pregnancy always need a urine test, so see your GP or midwife the same day.
- Breastfeeding: trimethoprim passes into breast milk. See your GP or a pharmacist, who can advise what’s suitable.
- Severe liver or kidney disease, or a blood disorder: trimethoprim isn’t suitable without specialist advice.
- Low folate, porphyria or a previous reaction to trimethoprim: tell the prescriber, as another antibiotic is usually better.
Interactions, alcohol and contraception
| Medicine | Why it matters |
|---|---|
| Methotrexate | Both lower folate, which can make methotrexate much more toxic |
| ACE inhibitors, ARBs, spironolactone, potassium supplements, salt substitutes | Higher risk of high potassium, especially in older people or with kidney problems |
| Warfarin | Can increase its effect and the risk of bleeding; your INR may need checking |
| Phenytoin, digoxin, ciclosporin, azathioprine, repaglinide, pyrimethamine | Levels or side effects can change, so the prescriber needs to know |
Alcohol: alcohol doesn’t stop trimethoprim working, but it can irritate the bladder and make sickness worse, so keep it to a minimum until you’ve finished the course.
Contraception: trimethoprim doesn’t reduce the effect of the combined pill, the mini pill or other hormonal contraception. If you’re sick or have severe diarrhoea, the pill may not be absorbed properly, so follow the missed-pill advice in your pill’s leaflet.
Side effects
Most people get through a short course without any trouble. The common side effects are feeling sick, diarrhoea, headache, itching or a mild rash, and thrush. Blood tests can also show raised potassium, which matters most if you have kidney problems or take medicines that raise it. Serious reactions, such as a severe allergic reaction, a blistering rash or blood problems, are rare.
Our trimethoprim side effects guide explains each of these, how long they last and what to do. If you get thrush, see our guide to thrush after antibiotics.
Repeated UTIs and preventive trimethoprim
NICE defines recurrent UTIs as 2 or more in 6 months, or 3 or more in 12 months. If that’s you, it’s worth seeing your GP rather than ordering treatment again online. They can send a urine sample for culture, look for a cause, and talk about prevention.
- Self-care first: drinking enough fluid, and peeing when you need to and after sex. Women after the menopause may be offered vaginal oestrogen.
- Preventive antibiotics: if these aren’t enough, a GP may prescribe a low dose of trimethoprim (usually 100mg at night) or nitrofurantoin, either every day or as a single dose after a trigger such as sex.
- Regular review: preventive antibiotics are reviewed, usually within 6 months, because resistance and side effects can build up.
Getting trimethoprim in the UK
Trimethoprim is prescription-only, but you don’t always need a GP appointment. In England, many pharmacies treat uncomplicated cystitis in women aged 16 to 64 for free through NHS Pharmacy First. Or, if you’re a woman aged 18 to 64 and you’d prefer to order online, our cystitis treatment online service lets a GPhC-registered pharmacist independent prescriber review your consultation and choose the right antibiotic for you.
Cystitis treatment from Access Doctor
Both first-choice antibiotics are available after an online consultation. The prescriber decides which one suits you, and the course length.
Antibiotic · Rx · first choice (NICE) if resistance risk is low
Trimethoprim 200mg tablets
One tablet twice a day, usually for 3 days. Not if you’ve taken it in the last 3 months. From £12.99.
View product →Antibiotic · Rx · first choice (NICE)
Nitrofurantoin 100mg MR capsules
One capsule twice a day with food, usually for 3 days. Not suitable if your kidneys don’t work well. From £12.99.
View product →Safety, confidentiality and free NHS options
Nitrofurantoin and trimethoprim are prescription-only medicines in the UK. Before anything is prescribed, a GPhC-registered pharmacist independent prescriber reviews your consultation, including your kidney health, the other medicines you take and any antibiotics you’ve had recently. We only prescribe an antibiotic when it’s appropriate: using antibiotics when they aren’t needed makes them work less well for everyone.
- Read the leaflet in the pack before you start.
- Don’t share antibiotics or keep leftovers for another time. Return unused tablets or capsules to a pharmacy.
- Report side effects to a pharmacist or doctor. You can also report them to the MHRA through the Yellow Card scheme.
Your consultation is confidential, and we only contact your GP if you agree. Access Doctor is a private service, and there are free NHS options:
- England: NHS Pharmacy First at a local pharmacy (women aged 16 to 64), your GP, or NHS 111.
- Scotland: Pharmacy First at a community pharmacy, or NHS 24 on 111.
- Wales: many community pharmacies can treat uncomplicated urine infections; ask your local pharmacy, or call NHS 111 Wales.
- Northern Ireland: your GP, or a community pharmacy for advice.
If you’ve had 2 urine infections in 6 months, or 3 in a year, see your GP rather than ordering again online. A urine culture can show which antibiotic will work, and your GP can look for a cause and talk about prevention.
Start Your Cystitis Consultation
Nitrofurantoin or trimethoprim, chosen by a GPhC-registered pharmacist independent prescriber in line with NICE guidance. Approved before 3pm, Monday to Friday? Dispatched the same day, by Royal Mail Tracked 48, Tracked 24 or Special Delivery guaranteed by 1pm next working day: you choose.
Start Cystitis Consultation →For self-care alongside antibiotics, and what the evidence says about cranberry and sachets, see how to treat cystitis. For symptoms and causes, see our cystitis guide.
Frequently Asked Questions
What is trimethoprim used for?
Trimethoprim is an antibiotic used mainly to treat cystitis, a lower urinary tract infection. A GP may also prescribe a low dose to prevent repeated UTIs. It’s only used for kidney infections when a urine culture shows it will work, and back pain with a fever needs same-day, in-person care.
How long does trimethoprim take to work?
Many women start to feel better within a day or two. Keep taking it for the whole course, even if you feel better sooner. If your symptoms haven’t started to improve within 48 hours, or they get worse, contact your GP or NHS 111, as you may need a urine test and a different antibiotic.
How should I take trimethoprim 200mg?
Take one 200mg tablet twice a day, about 12 hours apart, with a glass of water. You can take it with or without food. The usual course for cystitis is 3 days. Your prescriber may give a 7-day course if it’s needed, and you should finish whichever course you’re given.
Which is better for a UTI, trimethoprim or nitrofurantoin?
Neither is better for everyone. Both are first-choice antibiotics for cystitis in UK guidance. Nitrofurantoin is often preferred because resistance to it is less common, but it isn’t suitable if your kidneys don’t work well. Trimethoprim is a good choice if you haven’t taken it in the last 3 months. The prescriber decides based on your health, recent antibiotics and other medicines.
When would a prescriber choose trimethoprim over nitrofurantoin?
Usually when nitrofurantoin isn’t suitable: for example if your kidney function is reduced (an eGFR below 45), you have G6PD deficiency, or you’ve reacted to nitrofurantoin before. Trimethoprim is also chosen when a urine culture shows the bacteria are sensitive to it.
Can I switch from trimethoprim to nitrofurantoin if it isn’t working?
Not on your own. If your symptoms haven’t improved within 48 hours, contact your prescriber, GP or NHS 111 rather than switching or stopping. No improvement usually means the bacteria are resistant, and a urine culture shows which antibiotic will work.
Can I take trimethoprim and nitrofurantoin at the same time?
No. They aren’t taken together for cystitis. If the first antibiotic doesn’t work, a prescriber may switch you to another one, ideally guided by a urine culture. Never add a second antibiotic to a course without a prescriber’s advice.
Does trimethoprim affect the contraceptive pill?
No. Trimethoprim doesn’t reduce the effect of the combined pill, the mini pill or other hormonal contraception. If you’re sick or have severe diarrhoea, the pill may not be absorbed properly. In that case, follow the missed-pill advice in your pill’s leaflet.
Can I drink alcohol with trimethoprim?
Alcohol doesn’t stop trimethoprim working, but it can irritate the bladder, make cystitis feel worse and add to feeling sick. It’s best to avoid it, or keep it to a minimum, until you’ve finished the course.
Do trimethoprim and nitrofurantoin have the same course length?
Yes, for cystitis in women who aren’t pregnant: NICE recommends 3 days of either. What differs is how you take them: nitrofurantoin is one 100mg modified-release capsule twice a day with food, and trimethoprim is one 200mg tablet twice a day, with or without food. Pregnant women and men usually need 7 days.
Can I take trimethoprim for a kidney infection?
Only if a urine culture shows the bacteria are sensitive to it, and only after an in-person assessment. A kidney infection causes a high temperature, shivering, back or side pain or vomiting, and needs same-day help from your GP or NHS 111. Our online service treats cystitis only.
Can I buy trimethoprim without a prescription?
No. Trimethoprim is prescription-only in the UK. In England, many pharmacies can supply it for uncomplicated cystitis in women aged 16 to 64 through NHS Pharmacy First, or you can see your GP or use a regulated online service, where a prescriber checks it’s suitable for you.
Can I take trimethoprim if I’m pregnant or breastfeeding?
Not through our online service. Trimethoprim must not be used in the first 3 months of pregnancy, and urine infections in pregnancy need a urine test and a specific treatment plan, so contact your GP or midwife the same day. If you’re breastfeeding, see your GP or a pharmacist.
References
- NICE. Urinary tract infection (lower): antimicrobial prescribing (NG109). nice.org.uk
- NICE Clinical Knowledge Summaries. Urinary tract infection (lower) – women. cks.nice.org.uk
- Electronic Medicines Compendium. Trimethoprim: Summary of Product Characteristics (50mg/5ml oral suspension; same active ingredient as the 200mg tablets). medicines.org.uk
- NICE. Urinary tract infection (recurrent): antimicrobial prescribing (NG112). nice.org.uk
- NHS. Trimethoprim. nhs.uk
- UK Health Security Agency. English Surveillance Programme for Antimicrobial Utilisation and Resistance (ESPAUR) report. gov.uk
- NHS England. Pharmacy First (uncomplicated UTI in women aged 16 to 64). england.nhs.uk
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. In a medical emergency, call 999.