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How Long Does It Take For Trimethoprim To Work?

Dealing with a urinary tract infection (UTI) can be unpleasant and uncomfortable but finding the right treatment can get rid of symptoms and the infection quickly. Cystitis, a common type of UTI, can be treated with an antibiotic called Trimethoprim.

This article explores Trimethoprim, including how quickly it gets to work for cystitis symptom relief, how long you’ll take it, factors that could affect how well it works, and what to do if you miss a dose.

How Effective is Trimethoprim in Treating UTIs?

Trimethoprim is an effective antibiotic used to treat UTIs like cystitis. It works by killing the bacteria, stopping the production of a substance it needs to survive called folic acid. According to clinical studies, trimethoprim has shown high cure rates for uncomplicated UTIs.

How Quickly Does Trimethoprim Work?

Trimethoprim usually improves UTI symptoms within a few days, with some people starting to feel better within 24 hours. You might notice reduced urinary pain, less frequent urination, and improved comfort as it starts to work.

It’s important to still finish your course, as reduced symptoms don’t mean the infection has gone. If you stop treatment too early, even if you feel better, the infection could come back. This means you’d need another treatment course.

How Long Do You Take Trimethoprim?

How long you take Trimethoprim and at what dose can differ depending on the severity of your UTI, age (doses are lower for children and older people) and whether you have any underlying kidney problems. Complicated or severe UTIs are likely to need a longer treatment duration of 14 days.

Treatment for non-complicated UTIs typically lasts 3 to 7 days, though if you’re taking Trimethoprim to prevent infection, you could be advised to take it for at least 6 months. The treatment duration for Trimethoprim depends on your sex, where your infection is, how bad your infection is and whether you have any other health issues.

Men who have a UTI causing swelling of the prostate gland could be prescribed treatment for 4 to 6 weeks. Pregnant or breastfeeding women should always speak to their doctor or midwife before commencing treatment.

Women with a straightforward UTI will usually take 200mg twice a day for 3 days. Men and pregnant women will take the same dose for 7 days. Your healthcare provider will determine the appropriate time length based on your symptoms and medical history.

How Long Does it Take Trimethoprim to Leave the System?

Trimethoprim has a short half-life (how long it takes half the active ingredient to leave your system) of 12-20 hours. It continues to work on the infection even as it exits your body, as it leaves your body through your urine.


Trimethoprim is usually prescribed at a dose of 200mg, twice a day. This may differ depending on whether you’re taking it to relieve a current infection or prevent one.

What to Do If You Forget a Dose

If you miss a dose of Trimethoprim, take it as soon as you remember. If it's about time for your next dose, skip the missed one. Don’t double up on doses, as this increases your risk of side effects, although this is not likely to be severe. Missing doses of Trimethoprim could affect how well it works, especially if you miss several doses.

Additional Considerations

You should speak to your doctor or prescriber if you’re not feeling better after taking treatment for 3 days, or if you start to feel worse during treatment. This could indicate a more serious condition, such as a kidney infection.

Antibiotics like Trimethoprim can sometimes cause a fungal infection called thrush. Speak to a pharmacist or doctor if you get symptoms of thrush, as you may need additional treatment.


Trimethoprim is an effective UTI treatment with symptom relief within a few days of treatment. How long you’ll take it for and how quickly it gets to work can vary depending on the severity of your UTI and other medical history. You should always take the full treatment course, even if you start to feel better.

For any other questions or concerns about how Trimethoprim works, speak to your healthcare provider.

Authored & Reviewed By

Mohamed Imran Lakhi

MPharm - Lead Pharmacist
Imran Lakhi is the superintendent pharmacist and founder at Prescription Doctor. He has been at the core of our team.

Published on: 11/09/2023 Reviewed on: 01/11/2023
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