Trimethoprim Side Effects UK: Common, Serious and Long-term Risks Explained
What to actually expect from a course of trimethoprim: the common side effects, the rare serious ones, alcohol, thrush, older people, and when to get help.
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 answer: the most common trimethoprim side effects are feeling sick, diarrhoea, headache, an itchy rash and thrush. They’re usually mild and settle once the course ends. Serious reactions are rare: stop taking it and get help straight away for swelling of the face or throat, a blistering rash, unusual bruising or bleeding, a fever with a sore throat or mouth ulcers, or yellow skin or eyes. Call 999 for difficulty breathing.
If you need treatment, see our trimethoprim 200mg tablets or cystitis treatment online.
Concerned About Side Effects? Talk to a Prescriber
Our GPhC-registered pharmacist independent prescribers check your health and other medicines before prescribing, and choose the antibiotic that suits you. 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.
Overview
Trimethoprim is one of the two first-choice antibiotics for cystitis (a lower urinary tract infection, or UTI) in women in UK guidance. For a short course it’s generally safe, but like any medicine it can cause side effects in some people.
This guide walks you through which side effects are common, which are rare but serious, and what to do if you notice them. For how trimethoprim works, the usual dose and how long it takes to work, see our trimethoprim guide.
Common side effects of trimethoprim
The common side effects are generally mild, and they tend to go away once treatment ends:
Feeling or being sick
One of the most common side effects. Taking trimethoprim with food can help.
Diarrhoea
Usually mild and short-lived. Drink enough fluid. If it’s severe, watery or bloody, get help the same day.
Itching or a mild rash
Usually settles after the course. If the rash spreads, blisters or affects your mouth or eyes, stop and get help straight away.
Headache
Usually mild and temporary. Paracetamol can help if it’s suitable for you.
Thrush
Antibiotics can upset the natural balance of the vagina. See our guide to thrush after antibiotics.
Raised potassium
The product information lists this as very common, based on blood tests. It rarely causes symptoms on a short course, but matters if you have kidney problems or take medicines that raise potassium.
Dizziness is listed in the product information as very rare. If you do feel dizzy, don’t drive or use machinery until it passes, and get advice if it carries on.
How long do trimethoprim side effects last?
Most mild side effects — feeling sick, a headache, an upset stomach — settle within a few days and clear once you finish the course. A mild rash can take a little longer to fade after the last dose.
Side effects that carry on after the course ends, or that get worse rather than better while you’re taking it, aren’t typical: contact your prescriber, GP or a pharmacist. And if a rash is spreading or blistering at any point, stop taking trimethoprim and get medical advice the same day, as it can occasionally be the start of a more serious reaction.
In short: mild side effects usually last a few days. Anything still there a few days after your last tablet is worth a call to your prescriber or GP.
Serious side effects
These are rare. Stop taking trimethoprim and get medical help if you notice any of them:
| Side effect | Symptoms to watch for | What to do |
|---|---|---|
| Severe allergic reaction | Difficulty breathing, swelling of the face, lips, tongue or throat, feeling faint | Call 999 straight away |
| Serious skin reaction | A widespread rash with blistering, peeling, or sores in the mouth, eyes or genitals | Stop and get urgent help: A&E or NHS 111 |
| Blood problems | Unusual bruising or bleeding, a fever with a sore throat or mouth ulcers, extreme tiredness, frequent infections | Stop and contact your GP or NHS 111 the same day |
| Liver problems | Yellowing of the skin or whites of the eyes, dark urine, pale poo | Stop and contact your GP or NHS 111 the same day |
| High potassium | Muscle weakness, an irregular or fluttering heartbeat, unusual tiredness | Contact your GP or NHS 111 the same day |
| Severe diarrhoea | Watery or bloody diarrhoea, stomach cramps, a high temperature | Contact your GP or NHS 111 the same day |
Trimethoprim 200mg side effects: are they dose-related?
200mg twice a day is the standard adult dose for cystitis, so it isn’t a “high” dose. The usual course is 3 days, and the prescriber may give 7 days if it’s needed. Over a short course like this, side effects are usually mild and short-lived. If you’ve been prescribed trimethoprim 200mg, you’re on the normal treatment strength.
The risk of more significant effects — changes to blood cells, low folate, raised potassium — rises mainly with long-term or repeated use, such as the low-dose prevention a GP manages for repeated UTIs, rather than with a short course. That’s why people on long-term trimethoprim may be offered blood tests. If you take it long term and haven’t had a review recently, it’s worth asking your GP.
How to take trimethoprim to reduce side effects
- Take one tablet twice a day, about 12 hours apart, ideally morning and evening.
- Take it with food if it upsets your stomach.
- Swallow each tablet with a glass of water.
- Finish the course you’re given, even if you feel better: stopping early can let the infection come back.
- If you miss a dose, take it when you remember, unless it’s nearly time for the next one. Don’t take two at once.
What to do if you get side effects from trimethoprim
For mild side effects like feeling sick, a mild rash or diarrhoea, carry on with the course if you can, and have a word with your prescriber, GP or a pharmacist if they bother you. They can advise whether to continue or switch to another antibiotic. Don’t stop without advice unless you have a serious reaction, as the infection may come back.
For serious reactions — difficulty breathing, a blistering rash, unusual bruising, yellow skin or eyes, or a fever with a sore throat — stop taking trimethoprim and get medical help straight away. Call 999 for difficulty breathing or swelling of the face or throat.
If trimethoprim doesn’t suit you, nitrofurantoin is the other first-choice antibiotic. Our nitrofurantoin guide explains how it’s taken and who it suits.
You can report any suspected side effect to the MHRA through the Yellow Card scheme. It helps keep medicines safe for everyone in the UK.
Trimethoprim side effects in older people
Older people are more likely to have significant side effects from trimethoprim, for three main reasons: kidney function naturally declines with age, and trimethoprim is cleared by the kidneys; raised potassium (hyperkalaemia) is more likely, especially alongside common medicines such as ACE inhibitors, ARBs or spironolactone; and effects on blood cells are more likely on longer courses.
That’s why prescribers usually check kidney function and other medicines before prescribing trimethoprim to older people, use the shortest effective course, and may recommend blood tests for anyone on long-term prevention. Women aged 65 and over with urine symptoms should see their GP, who can arrange a urine culture: our online service is for women aged 18 to 64.
Symptoms to report promptly in older people: new muscle weakness, unusual tiredness, confusion, or an irregular or fluttering heartbeat during a course of trimethoprim. These can be signs of raised potassium and need a same-day call to the GP or NHS 111.
Precautions and drug interactions
Tell your prescriber about every medicine you take. These need particular care with trimethoprim:
- Methotrexate: both lower folate, which can make methotrexate much more toxic.
- Medicines that raise potassium, such as ACE inhibitors (for example ramipril), ARBs (for example losartan), spironolactone, potassium supplements and salt substitutes: a higher risk of high potassium.
- Warfarin: trimethoprim can increase its effect, so your INR may need checking.
- Phenytoin, digoxin, ciclosporin, azathioprine, repaglinide and pyrimethamine: levels or side effects can change, so the prescriber needs to know.
- Health conditions: tell the prescriber if you’re pregnant or breastfeeding, have kidney or liver problems, a blood disorder, low folate or porphyria.
Long-term side effects of trimethoprim
For cystitis, trimethoprim is usually taken for 3 days. If a GP prescribes it for longer, for example to prevent repeated UTIs, they may arrange checks for:
- Changes to blood cells, such as anaemia or low white cell counts;
- Low folate, because trimethoprim works by blocking folate in bacteria and can lower it in people too over time;
- Raised potassium, especially with kidney problems or medicines that also raise potassium.
Trimethoprim allergy and resistance
If you’ve had an allergic reaction to trimethoprim before, don’t take it again: you’ll need a different antibiotic. Resistance is a growing problem too. The bacteria that cause UTIs are more often resistant to trimethoprim than to nitrofurantoin, and resistance is more likely if you’ve taken trimethoprim in the last 3 months. That’s exactly why UK guidance (NICE) recommends trimethoprim when the risk of resistance is low, and nitrofurantoin when your kidneys work well enough for it.
Alternatives to trimethoprim for UTIs
If trimethoprim isn’t suitable because of an allergy, side effects or resistance:
- Nitrofurantoin: the other first-choice antibiotic for cystitis, if your kidneys work well (eGFR 45 or more).
- Pivmecillinam or fosfomycin: second-choice options in NICE guidance, usually prescribed by a GP, often after a urine culture.
Our guide to UTI antibiotics in the UK compares the options, and how to treat cystitis covers self-care alongside antibiotics.
Cystitis treatment from Access Doctor
If you’re a woman aged 18 to 64 with cystitis symptoms, a pharmacist independent prescriber reviews your consultation and chooses the antibiotic that suits you, usually as a 3-day course.
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.
Get Cystitis Treatment Online
Concerned about trimethoprim side effects, or need an alternative? Our GPhC-registered pharmacist independent prescribers will check your health and medicines and recommend the right treatment. 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 causes, symptoms and every treatment option, see our complete cystitis guide.
Frequently Asked Questions
Can trimethoprim cause nausea?
Yes. Feeling sick is one of the most common side effects of trimethoprim. Taking each dose with or just after food can help, and it usually settles within a couple of days. If you’re being sick and can’t keep doses down, contact your prescriber, GP or NHS 111, as a different antibiotic may suit you better.
Can trimethoprim cause a severe allergic reaction?
Yes, but it’s rare. Signs of a severe reaction include difficulty breathing, swelling of the face, lips, tongue or throat, a fast heartbeat, dizziness and collapse. If this happens, stop trimethoprim and call 999 straight away. A mild itchy rash is more common and usually settles when the course finishes.
Can I drink alcohol while taking trimethoprim?
Alcohol doesn’t stop trimethoprim working, but it can irritate the bladder, make cystitis symptoms worse and add to feeling sick. It’s best to avoid it, or keep it to a minimum, until you’ve finished the course and feel better.
Can trimethoprim make you tired?
Tiredness isn’t one of the common side effects, and feeling run down is often caused by the infection itself. Get advice the same day if you feel unusually tired with bruising, a sore throat or mouth ulcers, which can be signs of a blood problem, or with muscle weakness or an irregular heartbeat, which can be signs of raised potassium.
How long does a trimethoprim rash last?
A mild, itchy rash usually fades within a few days of finishing the course. Stop taking trimethoprim and get help straight away if the rash spreads, blisters or peels, or if you have sores in your mouth or eyes, as these can be signs of a serious skin reaction.
Does trimethoprim interact with warfarin?
Yes. Trimethoprim can increase the effect of warfarin and raise the risk of bleeding. If you take warfarin, tell the prescriber and your anticoagulation clinic before starting, as your INR may need checking during and after the course. Our consultation asks about warfarin and other medicines.
Can trimethoprim cause thrush?
Yes. Like other antibiotics, trimethoprim can upset the natural balance of the vagina and let yeast (candida) overgrow, causing a vaginal yeast infection, or thrush. The product information lists this as common. If you get itching, soreness or a thick white discharge during or after the course, a pharmacist can recommend an antifungal treatment. See our guide to thrush after antibiotics.
What are the common side effects of trimethoprim?
The common side effects are feeling or being sick, diarrhoea, headache, itching or a mild rash, and thrush. Blood tests can also show raised potassium. These are usually mild and settle after the course. Serious side effects are rare and include severe allergic reactions, serious skin reactions, blood problems and liver problems.
Can trimethoprim cause antibiotic resistance?
Yes. Using any antibiotic, especially when it isn’t needed or the course isn’t finished, helps resistant bacteria spread, and UTI bacteria are now often resistant to trimethoprim. Only take it when it’s prescribed for you, finish the course, and never share antibiotics or use leftovers.
What are the alternatives to trimethoprim for UTIs?
The main alternative is nitrofurantoin, the other first-choice antibiotic for cystitis, as long as your kidneys work well enough. If neither is suitable, a GP may prescribe pivmecillinam or fosfomycin, often after a urine culture. The prescriber chooses based on your health, recent antibiotics and allergies.
References
- 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 (lower): antimicrobial prescribing (NG109). nice.org.uk
- NICE Clinical Knowledge Summaries. Urinary tract infection (lower) – women. cks.nice.org.uk
- NHS. Trimethoprim. nhs.uk
- UK Health Security Agency. English Surveillance Programme for Antimicrobial Utilisation and Resistance (ESPAUR) report. gov.uk
- NICE. Urinary tract infection (recurrent): antimicrobial prescribing (NG112). nice.org.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.