Trimethoprim — NZ Medication Guide
📋 Reviewed by a Registered Pharmacist NZ — This information is for general educational purposes only and does not replace professional medical advice. Always consult your doctor or pharmacist before starting, stopping, or changing any medication.
What is Trimethoprim?
Trimethoprim is an antibiotic used specifically for urinary tract infections, particularly uncomplicated cystitis. This medication is funded by Pharmac for UTIs.
What is Trimethoprim Used For?
Trimethoprim is used for uncomplicated urinary tract infections (UTIs/cystitis) in women and for short-term prophylaxis of recurrent UTIs.
How Does Trimethoprim Work?
Trimethoprim inhibits dihydrofolate reductase — a bacterial enzyme essential for the conversion of dihydrofolate to tetrahydrofolate (required for synthesis of nucleotides and DNA). By blocking this enzyme, trimethoprim inhibits bacterial cell growth and replication. It is selectively toxic to bacterial dihydrofolate reductase vs. mammalian enzyme.
How to Take Trimethoprim
For acute uncomplicated UTI: 300 mg once daily or 150 mg twice daily for 3–7 days. For recurrent UTI prophylaxis: 100 mg once nightly for 3–6 months. Take with or without food.
Common Side Effects of Trimethoprim
- Nausea and vomiting
- Rash (maculopapular — relatively common)
- Headache
- Elevated creatinine (trimethoprim reduces tubular creatinine secretion — does not represent true renal impairment at therapeutic doses)
Serious Side Effects — Seek Medical Attention
Contact your doctor or call 111 immediately if you experience any of the following:
- Folate deficiency (with prolonged use — particularly in malnourished patients or those on methotrexate)
- Hyperkalaemia (trimethoprim has a similar effect to potassium-sparing diuretics — significant when combined with ACE inhibitors, ARBs, or spironolactone)
- Serious skin reactions (Stevens-Johnson syndrome — rare)
Drug Interactions
Trimethoprim may interact with other medicines. Always inform your doctor and pharmacist of all medications you are taking. Key interactions include:
- ACE inhibitors, ARBs, potassium-sparing diuretics (additive hyperkalaemia — significant risk)
- Methotrexate (folate antagonism — increased toxicity)
- Warfarin (enhanced anticoagulant effect)
- Ciclosporin (increased nephrotoxicity risk)
New Zealand Prescribing Information
Trimethoprim (150 mg, 300 mg tablets) is funded by Pharmac. It remains a first-line antibiotic for uncomplicated cystitis in NZ, though local resistance patterns should be considered — E. coli resistance to trimethoprim is approximately 20% in NZ. BPAC NZ recommends checking local resistance data or obtaining a urine culture if infection does not respond to initial treatment.
Frequently Asked Questions
Does trimethoprim cause a false rise in creatinine?
Yes — trimethoprim partially blocks the tubular secretion of creatinine, causing a modest rise in measured creatinine without affecting actual kidney function. This can mislead clinicians into thinking kidney function has deteriorated. Inform your doctor you are taking trimethoprim if your creatinine is checked during treatment.
Is there resistance to trimethoprim for UTIs in NZ?
Trimethoprim resistance in E. coli (the most common UTI organism) is approximately 20% in NZ. If your UTI does not respond within 2–3 days, a urine culture should be done to check antibiotic sensitivity. Your GP may switch to nitrofurantoin or cefalexin if resistance is confirmed.
⚠️ Disclaimer: This page is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your doctor or a qualified health provider. In New Zealand, medication availability and funding may vary — check with Pharmac or your pharmacist for current information.
Reviewed by a Registered Pharmacist NZ
References & Further Information
The following New Zealand and international resources were used to inform this page: