Trimethoprim vs Nitrofurantoin for UTI: Which Antibiotic and When (UK)
A clinical comparison of the two most-prescribed UTI antibiotics — how each attacks bacteria, why NICE ranks one above the other, and the factors that decide which you receive.
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 deciding factors between them are your kidney function, your allergies, whether you could be pregnant, and how much trimethoprim resistance there is where you live.
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Access Doctor supplies nitrofurantoin and trimethoprim after a GPhC-regulated online consultation. Our pharmacist independent prescribers decide which of the two is clinically correct for you.
Start Consultation →The Quick Answer: Which and When
For a straightforward bladder infection in a non-pregnant woman, nitrofurantoin is the preferred first-line antibiotic under NICE guideline NG109. It clears the bacteria that cause most UTIs reliably, resistance to it has stayed remarkably low over decades of use, and it concentrates strongly in the urine, exactly where the infection sits. Trimethoprim is a well-established second-line alternative — still effective for many women, but held back for situations where nitrofurantoin cannot be used.
The single most common reason to choose trimethoprim instead is reduced kidney function. Nitrofurantoin is not recommended when the estimated glomerular filtration rate (eGFR) falls below 45 mL/min, because too little of it reaches the urine to be effective. A nitrofurantoin allergy, G6PD deficiency, or a urine culture confirming the bacteria are sensitive to trimethoprim are the other main reasons.
How Each Antibiotic Attacks Bacteria
The two medicines work by completely different mechanisms, which is why one can still succeed where the other fails.
Nitrofurantoin
Filtered into the urine and concentrated there, it is converted inside bacterial cells into reactive compounds that damage DNA, proteins and the cell wall simultaneously. Hitting several targets at once makes it very hard for bacteria to adapt.
Trimethoprim
Blocks a single bacterial enzyme, dihydrofolate reductase, cutting off the folate that bacteria need to build DNA. Because everything depends on one target, one genetic change can let bacteria bypass it entirely.
That structural difference explains the resistance picture. Bacteria have found nitrofurantoin difficult to defeat because there is no single mutation that solves the problem. Trimethoprim’s single point of attack made it vulnerable, and decades of heavy prescribing gave resistant strains the opportunity to spread — which is precisely why national guidance was revised.
Why NICE Makes Nitrofurantoin First-Line
NG109 recommends nitrofurantoin as the preferred first choice for an uncomplicated lower UTI in non-pregnant women for three practical reasons:
- Resistance has stayed low. Because most UTIs are treated before any culture result is available, the first antibiotic is chosen on probability. Nitrofurantoin gives the better odds.
- It acts where the infection is. It concentrates in urine rather than circulating widely in the bloodstream, which targets the bladder and limits effects elsewhere in the body.
- A short course is enough. Three days clears most uncomplicated infections, which is good antibiotic stewardship — less exposure means less selection pressure for resistance.
NG109 sits alongside other prescribing options that come into play less often, including pivmecillinam and fosfomycin. Our summary of the latest UK UTI antibiotic guidance covers where each of those fits, and Cystitis Medicines UK compares prescription options against the over-the-counter products sold for symptom relief.
Diagnosis comes before antibiotic choice: None of this comparison matters if a UTI is not the right diagnosis. Burning, frequency and urgency also occur with thrush, sexually transmitted infections, bladder pain syndrome and vaginal atrophy. In women over 65, or where the picture is unclear, a urine culture is usually needed before any antibiotic is started.
When Trimethoprim Is the Better Choice
Second-line does not mean second-rate. There are clear clinical situations where a prescriber will reach for trimethoprim ahead of nitrofurantoin:
- Reduced kidney function (eGFR below 45 mL/min). Nitrofurantoin is not recommended here because too little reaches the urine to work, making trimethoprim the usual practical alternative.
- Nitrofurantoin allergy or intolerance. A documented reaction rules it out entirely, and trimethoprim is the logical next step.
- G6PD deficiency. Nitrofurantoin can trigger destruction of red blood cells in people with this inherited enzyme deficiency, so it is avoided.
- Culture-confirmed sensitivity. When a urine culture shows the organism is sensitive to trimethoprim, targeted treatment beats empirical guesswork every time.
- Low local resistance and no recent exposure. Where local surveillance shows low trimethoprim resistance and you have not taken it recently, its chance of working is considerably higher.
Recent use is a strong signal: If you have taken trimethoprim in the past three months, the bacteria causing your current infection are more likely to be resistant to it. Prescribers generally avoid repeating the same antibiotic within that window.
Side-by-Side Comparison Table
| Feature | Nitrofurantoin | Trimethoprim |
|---|---|---|
| NICE NG109 position | Preferred first-line | Second-line alternative |
| Mechanism | Damages bacterial DNA, proteins and cell wall (multiple targets) | Blocks folate synthesis (single target) |
| Typical dose | 100 mg modified-release, twice daily | 200 mg, twice daily |
| Typical course (uncomplicated UTI) | 3 days | 3 days |
| Food | Take with food | With or without food |
| E. coli resistance in the UK | Low | Roughly 30–40% in some areas |
| Main reasons to avoid | eGFR below 45 mL/min; at term in pregnancy; G6PD deficiency | First trimester of pregnancy; folate deficiency; interactions with warfarin and methotrexate |
| Harmless but notable effect | May turn urine dark yellow or brown | None commonly reported |
Dose, Course Length and How to Take Each
For an uncomplicated lower UTI in a non-pregnant woman, the standard NG109 regimens are:
1
Nitrofurantoin
100 mg modified-release capsule, twice daily — morning and evening — for 3 days. Take it with food or a snack. This improves absorption and substantially reduces the nausea that is its most common side effect.
2
Trimethoprim
200 mg tablet, twice daily for 3 days. It can be taken with or without food. Space the doses roughly 12 hours apart to keep the level in your urine steady across the day.
With either medicine, finish the whole course even once symptoms settle, which is often after a day or two. Stopping early leaves behind the bacteria that were least sensitive to the drug — exactly the population you do not want to select for.
Both preparations are stocked at Access Doctor: trimethoprim 200 mg tablets and nitrofurantoin 100 mg modified-release capsules. For dosing detail, missed doses and low-dose prophylaxis in recurrent UTI, see our trimethoprim tablets dosage and safety guide and the equivalent guide to nitrofurantoin for UTI.
How Quickly Each One Works
Neither antibiotic is meaningfully faster than the other when the bacteria are sensitive to it. Nitrofurantoin reaches high concentrations in the urine within a few hours of the first dose, but that is not the same as symptom relief: most women notice the burning and urgency easing over the first 24 to 48 hours, with the last of it fading as the course finishes.
The 48-hour mark is the clinically useful checkpoint. Improvement by then suggests the antibiotic is working and you should complete the course. No change at all by then rarely means you need to wait longer — it usually means the bacteria are resistant, and the answer is a urine culture rather than a second course of the same thing.
Side Effects, Cautions and Interactions
Both are generally well tolerated over a 3-day course, and most side effects are mild and short-lived. The meaningful differences are in who should avoid each one.
Nitrofurantoin
Nausea and headache are the most common complaints, and taking each dose with food helps considerably. It harmlessly turns urine dark yellow or brown, which is expected rather than a warning sign. 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 haemolysis in the newborn, and in people with G6PD deficiency. Rare but important adverse effects on the lungs and liver are associated with long-term use rather than short treatment courses.
Trimethoprim
Nausea, rash and headache are the common effects. It is avoided in the first trimester of pregnancy because it interferes with folate metabolism at a critical stage of development. It also has more drug interactions than nitrofurantoin: it raises bleeding risk with warfarin, increases toxicity with methotrexate, and can raise potassium levels when combined with ACE inhibitors, ARBs or potassium-sparing diuretics. For a fuller account, see our guide to trimethoprim side effects.
Neither antibiotic is the answer here: If you develop a high temperature, pain in your back or side, shivering, nausea or vomiting, or you simply feel very unwell, the infection may have reached a kidney. That is pyelonephritis, and it needs same-day in-person assessment and possibly different antibiotics altogether. Contact NHS 111, or call 999 if you are severely unwell.
How the Choice Is Made for You
You do not need to decide — that is the prescriber’s job, and it is a genuine clinical judgement rather than a preference. The factors weighed up are:
- Your kidney function, which largely determines whether nitrofurantoin is usable at all
- Documented allergies or previous reactions to either medicine
- Whether you are, or could be, pregnant — and if so, how far along
- Any antibiotic you have taken in the past three months, and whether it worked
- Local resistance surveillance, and any urine culture result available
- Your other medicines, particularly warfarin, methotrexate and potassium-raising drugs
In most uncomplicated cases that calculation lands on nitrofurantoin. Trimethoprim steps in when nitrofurantoin is ruled out, or when a culture points to it specifically.
Both antibiotics can be prescribed remotely by GPhC-registered pharmacist independent prescribers for adult women with uncomplicated symptoms. If you want the detail on how remote prescribing works, who qualifies and what happens after approval, that is covered in UTI Antibiotics Online UK for Women, with the consultation itself walked through in Online UTI Consultations for Women. Where an antibiotic is not yet warranted, how to treat cystitis sets out the self-care options.
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Nitrofurantoin or trimethoprim, selected under NICE NG109 by GPhC-registered pharmacist independent prescribers. Discreet next-day delivery across the UK.
Treat Your UTI Online →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 to 40% in parts of the UK, so it is usually reserved for cases where nitrofurantoin is unsuitable, for example reduced kidney function, or where a 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, if you have G6PD deficiency, or when a urine culture has already confirmed the organism is sensitive to trimethoprim and resistant to alternatives.
How long does each antibiotic take to start working?
Both usually begin to ease symptoms within 24 to 48 hours, and neither is meaningfully faster than the other when the bacteria are sensitive to it. Nitrofurantoin reaches high concentrations in the urine within a few hours of the first dose, but you should still expect a day or so before the burning and urgency settle noticeably. If there is no improvement at all by 48 hours, that usually points to resistance rather than to needing more time.
Can I switch from trimethoprim to nitrofurantoin if it is not working?
Not on your own. If symptoms have not improved within 48 hours of starting either antibiotic, contact your prescriber rather than switching or stopping. A lack of response usually means the bacteria are resistant to the antibiotic you were given, and a urine culture may be needed to identify what will work. Your prescriber can then move you to an antibiotic the culture shows is effective, which is far more reliable than trying the other one and hoping.
Do trimethoprim and nitrofurantoin have the same course length?
For an uncomplicated lower UTI in a non-pregnant woman, NICE NG109 recommends a 3-day course of either, so the length is usually the same. What differs is the preparation and the dose: nitrofurantoin is normally 100 mg modified-release twice daily and should be taken with food, while trimethoprim is 200 mg twice daily and can be taken with or without food. Longer courses are used in some circumstances, such as a complicated infection, and your prescriber will tell you if that applies.
Is nitrofurantoin or trimethoprim safer in pregnancy?
Both need caution 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 haemolysis in the newborn. Pregnant women with UTI symptoms should always be assessed in person by a GP or midwife, who can arrange a urine culture and select an antibiotic appropriate to the stage of pregnancy.
Access Doctor stocks both the first-line and second-line UTI antibiotics discussed above — Macrobid-brand nitrofurantoin capsules and generic trimethoprim tablets — alongside the wider range of UTI treatments available online. Which one you receive is decided by a pharmacist independent prescriber on the clinical detail you provide.
UTI Antibiotic · Rx
Nitrofurantoin (Macrobid)
100 mg modified-release capsules — the NICE first-line choice.
View product →UTI Antibiotic · Rx
Trimethoprim
200 mg tablets — 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. 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 modified-release 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.