Trimethoprim vs Nitrofurantoin for UTI: Which Antibiotic and When (UK)
A side-by-side comparison of the two most common UTI antibiotics — how each works, what NICE recommends first, and when a prescriber picks one over the other.
Part of the Cystitis & UTI in Women: Complete Clinical Guide.
Key fact: NICE NG109 makes nitrofurantoin the preferred first-line antibiotic for an uncomplicated UTI in non-pregnant women, with trimethoprim as an effective second-line option — the choice between them comes down to your kidney function, allergies, pregnancy status and local resistance data.
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Start Consultation →The Quick Answer: Which and When
For a straightforward bladder infection (uncomplicated lower UTI) in a non-pregnant woman, nitrofurantoin is the preferred first-line antibiotic under NICE guideline NG109. It clears the most common UTI bacteria reliably, resistance to it has stayed low, and it concentrates strongly in the urine where the infection sits. Trimethoprim is a well-established second-line alternative — still effective for many women, but reserved for situations where nitrofurantoin is not suitable.
The single most common reason to choose trimethoprim over nitrofurantoin is reduced kidney function. Nitrofurantoin is not recommended when the estimated glomerular filtration rate (eGFR) falls below 45 mL/min, because it may no longer reach effective levels in the urine. A nitrofurantoin allergy, or a urine culture confirming the bacteria are sensitive to trimethoprim, are the other main reasons.
How Each Antibiotic Works
The two medicines attack bacteria in completely different ways, which is part of why one can still work when the other fails.
Nitrofurantoin
Concentrated in the urine, it is converted inside bacteria into reactive compounds that damage bacterial DNA, proteins and cell walls at multiple points. Acting on several targets at once makes resistance slow to develop.
Trimethoprim
Blocks the bacterial enzyme dihydrofolate reductase, cutting off the folate the bacteria need to build DNA. With a single target, a small genetic change can let bacteria bypass it — which is how resistance has spread.
Because nitrofurantoin hits several targets, uropathogens have found it much harder to develop resistance to it over decades of use. Trimethoprim relies on one target, so widespread prescribing has allowed resistant strains to emerge and spread — the core reason the guidance changed.
Why NICE Makes Nitrofurantoin First-Line
NICE guideline NG109 recommends nitrofurantoin as the preferred first-line choice for an uncomplicated lower UTI in non-pregnant women for three practical reasons:
- Low resistance. Resistance among the E. coli that cause most UTIs remains low, so empirical treatment (before any culture result) is more likely to succeed.
- Bladder-focused action. It concentrates in the urine rather than the bloodstream, targeting the infection where it is while limiting wider effects on the body.
- Short, effective course. A 3-day course is enough for most uncomplicated infections, which supports antibiotic stewardship.
Diagnosis first: Antibiotic choice only matters once a UTI is the right diagnosis. Burning, frequency and urgency have other causes — and in women over 65, or where symptoms are vague, a urine culture is often needed before treatment. Neither antibiotic should be taken speculatively.
When Trimethoprim Is the Better Choice
Second-line does not mean second-rate. Trimethoprim remains a good option, and there are clear situations where a prescriber will choose it ahead of nitrofurantoin:
- Reduced kidney function (eGFR below 45 mL/min). Nitrofurantoin is not recommended here because it may not reach effective urinary levels; trimethoprim is often the practical alternative.
- Nitrofurantoin allergy or intolerance. A documented reaction to nitrofurantoin rules it out, making trimethoprim a logical next step.
- Culture-confirmed sensitivity. When a urine culture shows the specific bacteria are sensitive to trimethoprim (and perhaps resistant to alternatives), targeted treatment is preferred over guesswork.
- Low local resistance and no recent use. Where local resistance is low and you have not taken trimethoprim in the past few months, the chance of it working is higher.
Good to know: Recent use raises the odds of resistance. If you were prescribed trimethoprim in the last three months, a prescriber will usually avoid it and choose a different antibiotic.
Side-by-Side Comparison Table
| Feature | Nitrofurantoin | Trimethoprim |
|---|---|---|
| NICE NG109 position | Preferred first-line | Second-line alternative |
| Typical dose (uncomplicated UTI) | 100 mg modified-release, twice daily | 200 mg, twice daily |
| Typical course | 3 days | 3 days (low resistance risk) |
| How it works | Damages bacterial DNA, proteins and cell wall (multiple targets) | Blocks folate synthesis (single target) |
| E. coli resistance in the UK | Low | Roughly 30–40% in some areas |
| Main reason to avoid | eGFR below 45 mL/min; at term in pregnancy; G6PD deficiency | First trimester of pregnancy; folate deficiency; interactions (e.g. warfarin, methotrexate) |
| Harmless but notable effect | May turn urine dark yellow or brown | — |
Dosage and Course Length Compared
For an uncomplicated lower UTI in a non-pregnant woman, the standard regimens are:
1
Nitrofurantoin
100 mg modified-release capsule, taken twice daily (morning and evening) for 3 days. Take it with food to reduce the chance of nausea and improve absorption.
2
Trimethoprim
200 mg tablet, taken twice daily for the course your prescriber specifies — commonly 3 days where the risk of resistance is low. It can be taken with or without food.
Whichever you are given, finish the entire course even if symptoms settle within a day or two. Stopping early leaves surviving bacteria that are more likely to be resistant next time. For a deeper look at trimethoprim dosing, missed doses and recurrent-UTI prophylaxis, see our trimethoprim tablets dosage and safety guide.
Side Effects and Cautions Compared
Both antibiotics are generally well tolerated, and most side effects are mild and short-lived. The important differences lie in who should avoid each one.
Nitrofurantoin
Common effects include nausea and headache; it can harmlessly turn urine dark yellow or brown. It should be avoided when kidney function is reduced (eGFR below 45 mL/min), at term in pregnancy (from around 38 weeks and during labour) because of a risk of newborn haemolysis, and in people with G6PD deficiency.
Trimethoprim
Common effects include nausea, rash and headache. It is avoided in the first trimester of pregnancy because it interferes with folate, and it interacts with several medicines — notably warfarin (raising bleeding risk) and methotrexate. For the full picture, see our dedicated guide to trimethoprim side effects.
Seek urgent care: A high temperature, pain in the back or side (loin), shivering, nausea and vomiting, or feeling very unwell can mean the infection has reached the kidneys (pyelonephritis). This needs in-person assessment, not online prescribing. In an emergency, call 999.
Which One Will I Be Prescribed?
You do not have to decide — that is the prescriber’s job. During an assessment they weigh up several factors before choosing:
- Your kidney function (which steers whether nitrofurantoin is suitable)
- Any drug allergies or previous reactions to these antibiotics
- Whether you are or might be pregnant
- Any antibiotics you have taken in the past three months
- Local resistance patterns and, where available, a urine culture result
- Other medicines you take that could interact
In most uncomplicated cases the answer is nitrofurantoin first. Trimethoprim steps in when nitrofurantoin is ruled out, or when a culture points to it specifically.
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Complete a short GPhC-regulated online consultation. Our pharmacist independent prescribers review your answers, choose the right antibiotic under NICE NG109, and dispatch it for next-day delivery.
Treat Your UTI Online →Getting Treated Online
GPhC-registered pharmacist independent prescribers can assess an uncomplicated lower UTI remotely and prescribe either antibiotic where appropriate. The process is straightforward:
1
Complete the consultation
Answer a short questionnaire about your symptoms, medical history and any recent antibiotic use. It takes a few minutes.
2
Prescriber review
A pharmacist independent prescriber checks your answers and, if suitable, selects nitrofurantoin or trimethoprim in line with NICE NG109.
3
Discreet delivery
Your antibiotic is dispensed from our GPhC-registered pharmacy and dispatched for next-day delivery in plain packaging with a patient information leaflet.
For more on how remote prescribing works and who qualifies, see UTI Antibiotics Online UK for Women.
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Nitrofurantoin or trimethoprim, prescribed by GPhC-registered pharmacist independent prescribers after a quick online assessment. Discreet next-day delivery across the UK.
Start Your Consultation →Frequently Asked Questions
Which is better for a UTI, trimethoprim or nitrofurantoin?
For an uncomplicated lower UTI in a non-pregnant woman, NICE NG109 positions nitrofurantoin as the preferred first-line antibiotic because resistance rates remain low and it concentrates well in the bladder. Trimethoprim is an effective second-line choice, but E. coli resistance now reaches roughly 30–40% in parts of the UK, so it is usually reserved for cases where nitrofurantoin is unsuitable — for example reduced kidney function (eGFR below 45 mL/min) — or where a recent urine culture confirms the bacteria are susceptible. Neither is universally ‘better’; the right choice depends on your kidney function, allergies, pregnancy status and local resistance data.
When would a prescriber choose trimethoprim over nitrofurantoin?
A prescriber typically chooses trimethoprim when nitrofurantoin is contraindicated or unsuitable. The most common reason is reduced kidney function — nitrofurantoin is not recommended below an eGFR of 45 mL/min because it may not reach effective levels in the urine. Trimethoprim may also be preferred if you have had a nitrofurantoin allergy or intolerance, or when a urine culture has already confirmed the organism is sensitive to trimethoprim and resistant to alternatives.
Can I switch from trimethoprim to nitrofurantoin if it is not working?
If your symptoms have not improved within 48 hours of starting either antibiotic, contact your prescriber — do not switch or stop on your own. A lack of response often means the bacteria are resistant to the antibiotic you were given, and a urine culture may be needed to guide the next choice. Your prescriber can then switch you to an alternative that the culture shows is effective, rather than guessing.
Do trimethoprim and nitrofurantoin have the same course length?
No. For an uncomplicated lower UTI in a non-pregnant woman, nitrofurantoin is usually prescribed as 100 mg modified-release twice daily for 3 days. Trimethoprim is commonly prescribed as 200 mg twice daily; a 3-day course is used when the risk of resistance is low, while a longer course may be used in some circumstances. Always complete the full course you are given, even if you feel better after a day or two.
Is nitrofurantoin or trimethoprim safer in pregnancy?
Both need caution in pregnancy and neither should be self-selected. Trimethoprim is generally avoided in the first trimester because it interferes with folate, which is essential for early foetal development. Nitrofurantoin is generally avoided at term (from around 38 weeks and during labour) because of a risk of newborn haemolysis. Pregnant women with UTI symptoms should always be assessed in person by a GP or midwife rather than treated online.
Can I get either antibiotic online without seeing a GP?
Yes. GPhC-registered pharmacist independent prescribers can assess an uncomplicated lower UTI through an online consultation and, where appropriate, prescribe nitrofurantoin or trimethoprim in line with NICE NG109. The prescriber selects the antibiotic based on your kidney function, allergies, recent antibiotic use and pregnancy status. The service is for adult women with uncomplicated symptoms only; anyone with fever, loin pain or who is pregnant needs in-person care.
Access Doctor provides both first-line and second-line UTI antibiotics following a GPhC-regulated online consultation. Our pharmacist independent prescribers assess your symptoms and history and dispatch the appropriate medicine for next-day delivery in discreet packaging.
UTI Antibiotic · Rx
Nitrofurantoin (Macrobid)
NICE first-line antibiotic for uncomplicated cystitis and UTI in women.
View product →UTI Antibiotic · Rx
Trimethoprim
Second-line antibiotic used where nitrofurantoin is not suitable.
View product →References
- National Institute for Health and Care Excellence. Urinary tract infection (lower): antimicrobial prescribing. NICE guideline NG109. Updated 2022. nice.org.uk/guidance/ng109
- NICE. Urinary tract infection (lower) — women. Clinical Knowledge Summaries. cks.nice.org.uk
- UK Health Security Agency. English Surveillance Programme for Antimicrobial Utilisation and Resistance (ESPAUR) report. gov.uk
- Electronic Medicines Compendium. Nitrofurantoin 100mg m/r capsules — SmPC. medicines.org.uk/emc
- Electronic Medicines Compendium. Trimethoprim 200mg tablets — SmPC. medicines.org.uk/emc
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. Nitrofurantoin and trimethoprim are prescription-only medicines — a medical consultation is required before they can be dispensed. If you have a fever, loin or back pain, or are pregnant, seek urgent in-person medical care. In a medical emergency, call 999.