Trimethoprim
Is It Right for You?
A complete guide to Trimethoprim — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Trimethoprim?
Trimethoprim is an antibiotic used primarily to treat urinary tract infections (UTIs) and certain respiratory tract infections. It belongs to the diaminopyrimidine class of antibiotics and works by interfering with the synthesis of folic acid in bacteria – a process essential for bacterial DNA production and cell replication. Trimethoprim is one of the most commonly prescribed antibiotics for uncomplicated lower UTIs in UK primary care and is recommended as a first-line treatment option in national prescribing guidelines.
First introduced in the 1960s, trimethoprim has a well-established safety record and a decades-long track record in the treatment of UTIs. It is frequently used as a standalone antibiotic for uncomplicated lower urinary tract infections and is also combined with sulfamethoxazole to form co-trimoxazole (Septrin) – a combination antibiotic used for more complex infections and in immunocompromised patients.
Trimethoprim is available in tablet and oral solution forms and is suitable for adults and children. It is available generically and is sometimes referred to informally as "TMP."
Trimethoprim is a prescription-only medication in the UK. A licensed clinician must assess your symptoms and medical history before it can be prescribed – which can now be done quickly and conveniently through a telehealth consultation.
What Conditions Is Trimethoprim Used For?
Trimethoprim is prescribed for bacterial infections, with a particular focus on urinary tract and respiratory infections:
the most common indication; first-line treatment for uncomplicated bacterial cystitis in women, and a widely used option for men with lower UTI symptoms.
low-dose trimethoprim taken nightly is used as long-term prophylaxis in patients who suffer from frequent recurrent UTIs to reduce the frequency of episodes.
trimethoprim is used in some patients with COPD or chronic bronchitis where a susceptible organism is the likely cause of an acute exacerbation.
as part of co-trimoxazole regimens, trimethoprim is used to prevent Pneumocystis jirovecii pneumonia (PCP) in immunocompromised patients, including those with HIV.
certain lower respiratory tract infections caused by susceptible organisms, particularly in patients where other antibiotics are contraindicated.
trimethoprim (usually as co-trimoxazole) has been used for bacterial traveller's diarrhoea, though fluoroquinolones or azithromycin are now more commonly preferred.
Trimethoprim resistance among urinary tract pathogens – particularly Escherichia coli – has been rising in the UK and now exceeds 30% in some regions. A clinician will take local resistance patterns and your individual history into account when deciding whether trimethoprim is appropriate for your UTI. If your symptoms do not improve within 48 hours of starting trimethoprim, contact your clinician as resistance may be a factor.
How Does Trimethoprim Work?
Trimethoprim works by selectively and competitively inhibiting dihydrofolate reductase (DHFR) – a bacterial enzyme that is essential for the conversion of dihydrofolate to tetrahydrofolate, a biologically active form of folic acid.
Bacteria, unlike humans, cannot absorb folic acid from their environment and must synthesise it internally. Tetrahydrofolate is an essential cofactor required for the synthesis of thymidine, purines, and certain amino acids – all of which are necessary building blocks for bacterial DNA replication and protein synthesis. By blocking DHFR, trimethoprim prevents bacteria from producing the tetrahydrofolate they need, starving them of the molecular components required for DNA synthesis and halting their ability to replicate.
Trimethoprim's selectivity for bacterial DHFR over human DHFR is approximately 50,000 to 100,000 times greater, which is why it can inhibit bacterial folic acid synthesis at concentrations that have minimal effect on human cells. Humans obtain folic acid from dietary sources rather than synthesising it, and our DHFR enzyme has a significantly different structure from the bacterial version, making it far less susceptible to trimethoprim.
At standard therapeutic doses, trimethoprim is primarily bacteriostatic – it inhibits bacterial growth and replication rather than directly killing bacteria, allowing the immune system to clear the infection. At higher concentrations achieved in the urine, it may be bactericidal. It achieves particularly high urinary concentrations relative to blood levels, making it especially well suited to treating infections within the urinary tract.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Trimethoprim can be taken with or without food. If it causes mild nausea or stomach upset, taking it with a meal can help reduce this.
Take doses at evenly spaced intervals – twice-daily dosing is most effective when taken approximately 12 hours apart to maintain consistent antibiotic levels in the blood and urine throughout the day and night.
Drink plenty of fluids throughout treatment to help flush bacteria from the urinary tract and maintain adequate urine flow.
Always complete the full prescribed course even if UTI symptoms – such as burning, urgency, and frequency – resolve within the first day or two. Stopping early allows surviving bacteria to return and may contribute to the development of antibiotic resistance.
For prophylactic use, the single nightly dose is typically taken at bedtime. Consistent timing each night is important to maintain protective antibiotic levels throughout the overnight period when urine is most concentrated in the bladder.
Oral solution should be measured carefully using the measuring device provided and not a household spoon.
Side Effects of Trimethoprim
Trimethoprim is generally well tolerated for short courses. Most side effects are mild. Serious adverse effects are uncommon but require prompt medical attention.
- Nausea
- Vomiting
- Diarrhoea
- Abdominal discomfort
- Skin rash (mild)
- Itching (pruritus)
- Headache
- Oral or vaginal thrush
- Severe allergic reaction (anaphylaxis) – hives, facial or throat swelling, difficulty breathing, collapse
- Severe skin reactions – Stevens-Johnson Syndrome or Toxic Epidermal Necrolysis; blistering and widespread skin peeling
- Blood disorders – trimethoprim inhibits folate metabolism, which with prolonged use can lead to megaloblastic anaemia, thrombocytopenia (low platelets), or leukopenia (low white blood cells); symptoms include unusual fatigue, pallor, easy bruising, or increased infections
- Hyperkalaemia (high potassium) – trimethoprim blocks renal potassium excretion in a similar way to potassium-sparing diuretics; high potassium levels can cause cardiac arrhythmias; risk is higher in elderly patients and those with kidney disease
- Hepatotoxicity – jaundice, dark urine, severe fatigue; rare
- C. difficile-associated diarrhoea – persistent watery or bloody diarrhoea, abdominal cramping, fever
- Aseptic meningitis – rare; severe headache, neck stiffness, photophobia
Because trimethoprim inhibits folate metabolism, long-term use – particularly in patients who are already folate-deficient, malnourished, elderly, pregnant, or taking other folate antagonists such as methotrexate – can lead to clinically significant folate deficiency. Clinicians may recommend folic acid supplementation during prolonged trimethoprim therapy. For prophylactic long-term use in women of childbearing age, folic acid supplementation is particularly important.
Drug Interactions
Always disclose all prescription drugs, over-the-counter medicines, vitamins, and supplements to your clinician or pharmacist before starting trimethoprim.
Both trimethoprim and methotrexate inhibit dihydrofolate reductase. Concurrent use dramatically increases the risk of serious folate deficiency, bone marrow suppression, megaloblastic anaemia, and methotrexate toxicity. This combination should be avoided wherever possible. If both must be used, close monitoring of blood counts and folate levels is essential.
Trimethoprim significantly increases plasma levels of dofetilide (an antiarrhythmic drug) by inhibiting its renal excretion, raising the risk of potentially fatal QT prolongation and Torsades de Pointes. This combination is contraindicated.
Trimethoprim reduces renal potassium excretion, effectively acting as a potassium-sparing agent. When combined with potassium-sparing diuretics (e.g. spironolactone, amiloride) or ACE inhibitors/ARBs (which also raise potassium), the risk of dangerous hyperkalaemia is significantly increased. Potassium levels should be monitored closely in patients on these medications.
Trimethoprim may enhance the anticoagulant effect of warfarin by inhibiting its metabolism via CYP2C9, increasing the risk of bleeding. INR should be monitored closely during and after trimethoprim treatment in patients on warfarin.
Trimethoprim inhibits the hepatic metabolism of phenytoin, potentially increasing phenytoin blood levels to toxic concentrations. Phenytoin levels should be monitored if trimethoprim is prescribed concurrently.
Concurrent use with trimethoprim increases the risk of haematological toxicity including bone marrow suppression. Blood counts should be monitored closely.
Trimethoprim may increase digoxin plasma levels by reducing its renal excretion. Digoxin toxicity should be monitored in elderly patients and those on long-term digoxin therapy.
As with other antibiotics, there is a theoretical risk that trimethoprim may reduce the efficacy of hormonal contraceptives. Use an additional contraceptive method during treatment as a precaution.
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Trimethoprim may potentiate the effects of some oral antidiabetic agents, increasing the risk of hypoglycaemia. Blood glucose should be monitored more closely in diabetic patients starting trimethoprim.
Important Warnings
Trimethoprim blocks the renal epithelial sodium channel (ENaC) in a manner similar to potassium-sparing diuretics, reducing urinary potassium excretion and raising serum potassium levels. This can be clinically significant in patients who are elderly, have chronic kidney disease, or are taking other drugs that raise potassium – including ACE inhibitors, ARBs, spironolactone, and amiloride. Hyperkalaemia can cause life-threatening cardiac arrhythmias. Potassium and kidney function should be checked in at-risk patients when trimethoprim is prescribed.
Long-term trimethoprim use can cause folate depletion leading to megaloblastic anaemia, thrombocytopenia, and leukopenia. These effects are more likely in patients who are already folate-deficient, elderly, malnourished, alcoholic, or taking other folate antagonists. Full blood count should be monitored in patients on long-term trimethoprim prophylaxis.
Trimethoprim resistance among common UTI pathogens is increasing. If you have had a trimethoprim-resistant UTI in the past, or if your symptoms do not improve within 48 hours of starting treatment, contact your clinician. A urine culture may be required to identify the causative organism and confirm antibiotic sensitivity.
Trimethoprim is primarily excreted by the kidneys and can accumulate to toxic levels in patients with significantly reduced kidney function. Dose reduction or an alternative antibiotic may be required. Always disclose any history of kidney disease before starting trimethoprim. In patients with eGFR below 15 mL/min, trimethoprim is generally contraindicated.
Trimethoprim is a folate antagonist and should be avoided in the first trimester of pregnancy, as folate is essential for neural tube development and trimethoprim's anti-folate activity theoretically increases the risk of neural tube defects. It should also be avoided at term. If antibiotic treatment is necessary during the first trimester, alternative agents such as nitrofurantoin or cefalexin are preferred. Always inform your clinician if you are pregnant or trying to conceive before starting trimethoprim.
Trimethoprim passes into breast milk. While it is generally considered compatible with breastfeeding at standard short-term doses, prolonged use should be discussed with a clinician, particularly if the infant is premature or has jaundice, as trimethoprim can displace bilirubin from protein binding.
Older patients are at higher risk of adverse effects from trimethoprim, including hyperkalaemia, blood disorders, and drug interactions. Kidney function should be assessed before prescribing and monitored during long-term use.
Speak to a Clinician About Treatment
Getting advice no longer means sitting in a waiting room. Through The GP Service, you can consult with a licensed clinician in minutes — from home, on your lunch break, or wherever works for you. If treatment is clinically appropriate, your clinician can issue a prescription during the consultation. A consultation does not guarantee a prescription.
The treatment you need, when you need it.
Trimethoprim FAQs
Yes. A licensed clinician can assess your symptoms via a telehealth consultation and prescribe trimethoprim if it is clinically appropriate. Prescriptions are typically sent to your preferred pharmacy the same day, often within minutes of your consultation.
Most patients notice a significant improvement in UTI symptoms – including reduced burning, urgency, and frequency – within 24 to 48 hours of starting trimethoprim. If symptoms are not improving after 48 hours, contact your clinician, as this may indicate a trimethoprim-resistant infection requiring a different antibiotic.
For uncomplicated lower UTIs in women, trimethoprim is typically prescribed for either 3 or 7 days depending on local guidelines, clinical assessment, and individual circumstances. A 7-day course is recommended in most current UK guidelines. For men with UTIs and for complicated infections, longer courses of 14 days or more are usually required. Always complete the full prescribed course.
Trimethoprim remains effective against many UTI-causing bacteria, but resistance rates – particularly among E. coli – are rising. In some areas of the UK, more than 30% of UTI isolates are now resistant to trimethoprim. Your clinician will consider local resistance patterns and your personal history of UTIs and antibiotic use when deciding whether trimethoprim is the most appropriate choice.
Both are first-line antibiotic options for uncomplicated lower UTIs in UK guidelines. Trimethoprim works by inhibiting bacterial folate synthesis and is taken twice daily for 7 days. Nitrofurantoin works by damaging bacterial DNA and metabolic processes and is usually taken twice daily for 5 days in its modified-release form. Nitrofurantoin cannot be used if kidney function is significantly impaired (eGFR below 45 mL/min), and trimethoprim cannot be used in the first trimester of pregnancy. Your clinician will choose the most appropriate option based on your circumstances, local resistance data, and any contraindications.
Yes. Low-dose trimethoprim (typically 100 mg taken nightly) is used as long-term prophylaxis in patients who suffer from frequent recurrent UTIs. This approach can significantly reduce the frequency of UTI episodes. Long-term use requires monitoring of kidney function and blood counts due to the risk of folate depletion and hyperkalaemia with extended treatment.
Trimethoprim should be avoided in the first trimester of pregnancy due to its anti-folate activity and the potential risk to neural tube development. It should also be avoided at term. Alternative antibiotics such as nitrofurantoin (in the first and second trimesters) or cefalexin are usually preferred for treating UTIs during pregnancy. Always inform your clinician if you are pregnant before starting any antibiotic.
Take the missed dose as soon as you remember. If it is almost time for your next scheduled dose, skip the missed one and continue as normal. Try to maintain approximately 12-hour spacing between doses for twice-daily regimens. Do not take a double dose.
There is no known dangerous direct interaction between alcohol and trimethoprim. However, alcohol can irritate the bladder and potentially worsen UTI symptoms, and it can also suppress the immune system and slow recovery. It is generally advisable to avoid or minimise alcohol while treating a UTI.
As with other antibiotics, there is a theoretical risk that trimethoprim may reduce the effectiveness of hormonal contraceptives. The clinical evidence is limited, but using an additional contraceptive method during treatment and for 7 days after completing the course is a reasonable precaution. Discuss this with your clinician or pharmacist.