Trimethoprim Side Effects UK: Common, Serious and Long-term Risks Explained

Overview

Trimethoprim is a commonly prescribed antibiotic in the UK, most often used for UTIs and cystitis. It is generally considered safe for short-term use, but like all medications, it can cause side effects in some people.

This guide provides comprehensive information about trimethoprim side effects — what is common, what is rare but serious, and what you should do if you experience any concerning symptoms. For information on dosage and course lengths, see: Trimethoprim for UTI: dosage and course length. For what trimethoprim is and how it works, see: What is trimethoprim? Uses and how it works.

Common Side Effects of Trimethoprim

Common side effects are generally mild and resolve once treatment ends:

How Long Do Trimethoprim Side Effects Last?

Most mild trimethoprim side effects — nausea, headache, mild rash and stomach upset — settle within 2–3 days as your body adjusts to the medicine, and clear completely within a few days of finishing the course. A mild rash can occasionally take a little longer to fade after the final dose.

Side effects that persist beyond the end of the course, or that worsen rather than settle while you are taking it, are not typical — contact your prescriber. If itching or rash is worsening or spreading at any point, seek medical advice the same day, as this can occasionally be the start of a more significant reaction.

Serious Side Effects

The following are rare but serious — seek immediate medical attention if any occur:

Trimethoprim 200mg Side Effects — Are They Dose-Related?

200mg twice daily is the standard adult dose for urinary tract infections, and at this dose over a short 3–7 day course, side effects are usually mild and short-lived — the profile described above. You are not taking a “high” dose: 200mg is the normal treatment strength.

The risk of more significant side effects — effects on blood cells, folate depletion and raised potassium — rises mainly with long-term or repeated use, such as low-dose prophylaxis for recurrent UTIs, rather than with the standard short course. Patients on long-term trimethoprim may be offered periodic blood monitoring for this reason. If you have been prescribed trimethoprim long-term and haven’t had a review recently, raise it with your prescriber.

How to Take Trimethoprim Safely to Minimise Side Effects

• Take doses at the same time each day — ideally morning and evening
• Take with food if nausea is a concern
• Swallow tablets whole with a full glass of water — do not chew
• Complete the full prescribed course — stopping early increases resistance risk
• Avoid missing doses — if you miss one, take it as soon as remembered (unless near next dose time)

What to Do If You Experience Side Effects from Trimethoprim

For mild side effects (nausea, rash, diarrhoea) that are not severe, contact your prescriber — they can advise whether to continue at the current dose, reduce the dose, or switch to an alternative antibiotic. Do not stop taking trimethoprim without prescriber advice unless you experience a serious reaction, as this risks the infection returning.

For serious reactions (difficulty breathing, unusual bruising, jaundice, high fever with sore throat), stop trimethoprim immediately and seek emergency medical care. If you are having an anaphylactic reaction, call 999.

Report any trimethoprim side effects via the MHRA Yellow Card scheme at mhra.gov.uk/yellowcard. This helps the MHRA monitor medication safety across the UK.

Trimethoprim Side Effects in the Elderly

Older adults are more likely to experience 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 cell production are more likely on longer courses.

For this reason, prescribers often check recent kidney function before prescribing trimethoprim to older patients, use the shortest effective course, and may recommend blood monitoring for anyone on long-term prophylaxis.

Precautions and Drug Interactions

• Warfarin — trimethoprim enhances anticoagulant effect; INR monitoring required
• Methotrexate — combined folate inhibition can increase methotrexate toxicity significantly
• ACE inhibitors, ARBs and spironolactone — combined use raises the risk of high potassium (hyperkalaemia), particularly in older patients and those with reduced kidney function
• Cyclosporine — potential for reduced cyclosporine efficacy
• Pyrimethamine — combined use further depletes folate; avoid unless directed
• Before starting trimethoprim, inform your prescriber if you are pregnant or breastfeeding, have kidney or liver problems, have a folate deficiency, are at risk of high potassium levels (elderly, or on potassium-raising medications), or have porphyria

Long-Term Side Effects of Trimethoprim

Trimethoprim is typically prescribed for short 7-day courses for UTIs. If prescribed for longer periods (recurrent UTI prophylaxis or acne), additional monitoring may be needed for:

• Bone marrow suppression — reduced blood cell production
• Folate deficiency — trimethoprim’s mechanism can deplete folate over time; folic acid supplementation may be recommended
• Peripheral neuropathy — numbness, tingling, or pain in hands and feet (rare)

Trimethoprim Allergy and Resistance

If you have a known allergy to trimethoprim, do not take this medication. An alternative antibiotic will be prescribed. Antibiotic resistance is a growing public health concern — trimethoprim resistance has been increasing in the UK, which is why NICE now recommends nitrofurantoin as first-line ahead of trimethoprim.

Alternative Treatments for UTIs

If trimethoprim is not suitable due to allergy, resistance, or side effects:

• Nitrofurantoin — NICE first-line for uncomplicated UTIs; lower current resistance rates
• Cephalexin — a cephalosporin antibiotic; useful where both nitrofurantoin and trimethoprim are unsuitable
• Fosfomycin — single-dose option for uncomplicated UTIs; used in specific resistance situations

For more information on cystitis medicines, see: Cystitis Medicines UK: Nitrofurantoin and Trimethoprim Explained. For the complete trimethoprim guide, see: What is trimethoprim? Uses and how it works.

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