Anti-infective
UTI antibiotics
Nitrofurantoin and trimethoprim — First-line antibiotics for urine infections — choice shaped by kidney function and pregnancy.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
These are the common first-line antibiotics for uncomplicated urinary-tract infections. The usual choice is between nitrofurantoin and trimethoprim, guided by kidney function, pregnancy and local resistance.
How it works
Nitrofurantoin concentrates in the urine and damages bacterial components there, which is why it treats bladder infections but not kidney or bloodstream infection. Trimethoprim blocks a folate-processing enzyme bacteria need to multiply.
In practice
In practice the two everyday first-line options for an uncomplicated lower urinary-tract infection are nitrofurantoin and trimethoprim, and the choice hinges on two factors. Kidney function: nitrofurantoin needs a reasonable eGFR to work and is avoided when it is low. Pregnancy: trimethoprim is best avoided in early pregnancy (it is a folate antagonist), while nitrofurantoin is avoided near term. Local resistance patterns and a short course guided by current guidance keep these drugs effective; longer or recurrent problems prompt review and culture.
Examples
Practical use
How to take it & use it well
- Take antibiotics for urine infections, such as nitrofurantoin or trimethoprim, exactly as prescribed and complete the full course even if you feel better quickly.
- Nitrofurantoin is usually best taken with food, which can improve how it is absorbed and reduce stomach upset.
- Drink plenty of fluids during treatment unless you have been advised otherwise.
- Space your doses evenly through the day so the antibiotic level stays steady.
- Tell your clinician if you are pregnant or might be, as the choice of antibiotic for a urine infection can change at different stages of pregnancy.
- Do not save leftover antibiotics or use someone else's, and contact your clinician if symptoms do not improve or you develop fever, back pain or feel very unwell.
Common uses
- Uncomplicated lower urinary-tract infection
- Some prevention regimens for recurrent UTI
Monitoring
- Renal function (especially for nitrofurantoin)
- Symptom response; culture if recurrent or complicated
- Potassium with trimethoprim in at-risk patients
Weighing it up
Advantages & disadvantages
Advantages
- They effectively treat common bladder infections and relieve symptoms such as burning and frequency.
- Short courses are often enough for an uncomplicated infection.
- They are well-established options for urine infections in the UK.
- Prompt treatment can prevent an infection spreading up to the kidneys.
Disadvantages
- Their suitability depends on kidney function, and some, like nitrofurantoin, work less well or are avoided when kidneys are weaker.
- Each has pregnancy cautions that differ by trimester, so the right choice varies.
- Side effects such as nausea, and harmless urine colour changes with nitrofurantoin, can occur.
- Bacterial resistance means the chosen antibiotic does not always match the bug, sometimes needing a change.
Key safety principles
What to watch for
- Nitrofurantoin needs adequate kidney function — avoid when eGFR is low; avoid near term in pregnancy.
- Trimethoprim is a folate antagonist — avoid in the first trimester and with methotrexate; can raise potassium.
- Reserve for genuine infection and keep courses short to limit resistance.
Key interactions
What to avoid or check alongside
- Trimethoprim taken with methotrexate can dangerously increase methotrexate toxicity and is avoided.
- Trimethoprim can raise potassium levels, more so when combined with certain blood pressure medicines like ACE inhibitors or potassium-sparing diuretics.
- Trimethoprim can enhance the effect of warfarin, increasing bleeding risk.
- Some indigestion remedies and antacids can reduce absorption of nitrofurantoin.
- Trimethoprim with other medicines that affect folate may add to effects on blood cells.
Patient & carer advice
- Finish the course as prescribed and drink plenty of fluids
- Nitrofurantoin can turn the urine dark — this is harmless
- Tell us if you are or might be pregnant
Use with
Related clinical calculators
Dose and risk decisions for this class often depend on renal function, weight or bleeding/stroke risk. These tools help:
Answers
UTI antibiotics: frequently asked questions
Why does my urine look discoloured on nitrofurantoin?
Nitrofurantoin can turn urine a dark yellow or brownish colour, which is harmless and expected, fading once the course is finished. If you also have fever, back pain or feel very unwell, contact your clinician.
Do I need to finish the whole course of antibiotics?
Yes. Take the full course as prescribed even if you feel better after a day or two. Stopping early can let the infection return and contributes to antibiotic resistance, making future infections harder to treat.
Can I take these antibiotics in pregnancy?
Both nitrofurantoin and trimethoprim have specific cautions in pregnancy that depend on the stage, and the preferred choice changes accordingly. Always tell your midwife, GP or pharmacist if you are pregnant or might be, so a safe option is chosen.
Why does kidney function matter for UTI antibiotics?
Some of these antibiotics rely on the kidneys to reach the urine and clear the body. When kidney function is reduced, certain options like nitrofurantoin may not work well or could build up, so your clinician selects accordingly.
What if my symptoms do not improve?
If you are no better after a couple of days, or you develop fever, chills, back pain, vomiting or feel very unwell, contact your clinician. The infection may need a different antibiotic or further assessment, as it could be spreading.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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