Trimethoprim – Specialist antibiotic for urinary and respiratory infections.
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What is Trimethoprim?
Trimethoprim is a bacteriostatic antibiotic that’s particularly effective for urinary tract infections and some respiratory infections. It’s often chosen for UTI treatment due to its effectiveness and relatively good tolerance profile.
Book an online consultation if you’re experiencing symptoms of a urinary tract infection such as burning during urination, frequent urination, or pelvic discomfort. Our GPs can assess your symptoms and determine if Trimethoprim is the appropriate antibiotic for your condition.
What is Trimethoprim used for?
- Urinary tract infections (UTIs)
- Cystitis and bladder infections
- Chest infections and bronchitis
- Pneumocystis pneumonia (in combination)
- Prevention of recurrent UTIs
- Some skin and ear infections
How does Trimethoprim work?
A bacteriostatic antibiotic that inhibits bacterial folate synthesis, preventing bacteria from producing essential proteins needed for growth and reproduction.
Dosage and Administration
Adults
200 mg twice daily for acute infections, or 100 mg once daily for prevention.
Children
4 mg per kg twice daily—GP will calculate appropriate dose based on weight.
Important Notes
- Take with plenty of water to help prevent kidney stones.
- Can be taken with or without food.
- Complete the full course even if symptoms improve.
- For prevention, usually taken as a single bedtime dose.
Side Effects
Common Side Effects
- Nausea and vomiting
- Diarrhoea
- Loss of appetite
- Skin rash
- Headache
- Mouth ulcers
Serious Side Effects
Seek immediate medical attention if you experience:
- Blood disorders (low blood cell counts)
- Severe skin reactions (Stevens-Johnson syndrome)
- Kidney problems
- Liver damage
- Severe allergic reactions
- Folate deficiency with long-term use
Important Warnings
- Tell your GP if you have kidney or liver problems—dose adjustment may be needed.
- Not suitable if you have severe kidney disease or blood disorders.
- May worsen folate deficiency—folic acid supplements sometimes needed.
- Regular blood tests required for long-term use to monitor blood counts.
- Avoid excessive sun exposure—may increase skin sensitivity.
Drug Interactions
Tell your GP if you're taking any of the following medications:
- Warfarin—may enhance anticoagulant effects.
- Methotrexate—increased risk of blood disorders and folate deficiency.
- Phenytoin—may increase phenytoin levels.
- Digoxin—may increase digoxin levels in elderly patients.
- ACE inhibitors—increased risk of high potassium levels.
Frequently Asked Questions
How is Trimethoprim different from other antibiotics for UTIs?
Trimethoprim is often preferred for simple UTIs because it concentrates well in urine and has fewer side effects than some alternatives like nitrofurantoin.
Why do I need to drink lots of water?
Adequate fluid intake helps prevent kidney stone formation and ensures good urine flow to help clear the infection.
Can I take Trimethoprim long-term for recurrent UTIs?
Yes, it can be used for prevention, but requires regular blood monitoring to check for side effects on blood cells and liver function.
What should I do if I develop a rash?
Stop taking the medication and contact your GP immediately, as this could indicate a serious allergic reaction.
Medically Reviewed by: Mohammed Hassan - Independent Prescriber
Last Updated: 2025-01-14