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 help alleviate symptoms and eliminate the infection quickly. Cystitis, a common type of UTI, can be treated with an antibiotic called Trimethoprim.

Given that fast relief is paramount, when comparing UTI treatments, you’ll probably want to know “how long does it take for Trimethoprim to work?” among other important questions. This is where we can help.

This article explores Trimethoprim, including how quickly it works for cystitis symptom relief, how long you should take it for, factors that may affect its effectiveness, and what to do if you miss a dose. Let’s get started. 

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How Effective is Trimethoprim in Treating UTIs?

Trimethoprim is an effective antibiotic used to treat UTIs like cystitis. It works by killing the bacteria and stopping the production of a substance it needs to survive, called folic acid.

So, how well does it work? Clinical studies show impressive results. 

A large UK study involving 538 patients from 45 general practices compared different antibiotics for UTIs. After seven days of treatment, trimethoprim successfully cleared the infection in about 77% of cases. This was slightly lower than the alternative antibiotic nitrofurantoin (82%), but still showed good effectiveness. The results were particularly strong for E. coli infections – the most common cause of UTIs – with a cure rate of 78%.

How Quickly Does Trimethoprim Work for UTI?

Trimethoprim typically improves UTI symptoms within a few days, with some people experiencing relief within 24 hours. You may notice reduced urinary pain, less frequent urination, and improved comfort as it begins to take effect.

It’s essential to complete your course of Trimethoprim tablets, as reduced symptoms don’t necessarily mean the infection has cleared. 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 to Take Trimethoprim for UTI Infections

The duration and dose of Trimethoprim you take can differ depending on the severity of your UTI, age (doses are lower for children and older adults), and whether you have any underlying kidney problems. Complicated or severe UTIs typically require 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 length of time based on your symptoms and medical history.

How Long Does it Take for Trimethoprim to Leave the System?

Trimethoprim has a short half-life (the time it takes for half the active ingredient to leave your system) of 8 to 10 hours, although this may be prolonged for individuals with renal dysfunction. The vast majority of the drug leaves the body as urine within 24 hours of administration.

Around 10% to 20% of ingested Trimethoprim is metabolised in the body, primarily in the liver. The medication continues to work on the UTI infection throughout this process, even as it exits your body. 

Trimethoprim Dosage for UTI

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

You'll typically take one dose in the morning and one in the evening, roughly 12 hours apart. The dose and duration depend on your situation. Women with straightforward UTIs usually take 200mg twice daily for 3 days, while men and pregnant women with straightforward UTIs typically take the same dose for 7 days. 

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If you have a particularly severe or complicated UTI, or you have a catheter, the NHS usually recommends a 14-day course. Men with a UTI that's caused prostate inflammation may need treatment for 4 to 6 weeks.

To prevent recurrent UTIs, your doctor may prescribe a lower dose of 100mg taken at night as a single dose, potentially for at least 6 months. Studies suggest that this approach is effective, reducing recurrence rates from 26 infections per 100 months to just 3.3 per 100 months during treatment.

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 these are unlikely to be severe. Missing doses of trimethoprim can affect its effectiveness, especially if several doses are missed.

What to Do if Trimethoprim is Not Working for UTI?

If Trimethoprim UTI antibiotics are not working after 3 days, you should speak to your doctor or prescriber for advice. This is especially the case if you start to feel worse during treatment, as it could indicate a more serious condition, such as a kidney infection. 

Ultimately, Trimethoprim doesn't work for everyone. You might have a bacterial strain that's resistant to trimethoprim, or you may experience side effects that make it difficult to continue. 

In areas where trimethoprim resistance is common, effectiveness can drop significantly. Research shows that while 86% of patients with trimethoprim-susceptible infections got better, only 42% with resistant bacteria were cured. 
Trimethoprim resistance is becoming increasingly common across the UK. National data shows that around 28 to 31% of E. coli bacteria causing UTIs in England are now resistant to trimethoprim.

The good news? Alternative treatments are available. At Prescription Doctor, we offer Nitrofurantoin – often the first-choice treatment nowadays due to its low risk of causing bacterial resistance. We also offer CanesOasis Cystitis Relief, which are sachets containing 4g of sodium citrate, a type of salt that alters the pH of your urine, making it more alkaline. This can provide symptomatic relief for any stinging and discomfort you may experience with cystitis, but it does not treat the infection. Please note that this treatment is not suitable for men. 

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If Trimethoprim isn't working or you're having side effects, our online consultation service can help you find a more suitable alternative. Visit our cystitis treatment page to explore your options.

Frequently Asked Questions

Here are answers to some of the most common questions about Trimethoprim:

Can Trimethoprim Cause Thrush?

Yes, antibiotics like Trimethoprim can cause thrush. This occurs when the drugs kill the “good” bacteria that normally keep yeast levels in check. When the balance gets disrupted, yeast can overgrow and cause vaginal infections or oral thrush (white patches in your mouth).  If you develop thrush, speak to your doctor, as they can recommend treatments to resolve the issue.

Can Trimethoprim Cause Constipation?

Constipation is a very rare side effect of trimethoprim, affecting less than 1 in 10,000 people. You're much more likely to experience nausea, vomiting, or diarrhoea if you do get side effects. If you do encounter persistent constipation or any other worrying symptoms while taking trimethoprim, get in touch with your healthcare provider.

Which is Stronger, Nitrofurantoin or Trimethoprim?

Neither Nitrofurantoin nor Trimethoprim is inherently "stronger" – they're both effective antibiotics that simply work in different ways. The best choice depends on factors like bacterial resistance patterns, potential side effects, and your individual circumstances. A large study comparing the two found nitrofurantoin cleared infections in 82% of cases compared to trimethoprim's 77%. 

These days, nitrofurantoin is often preferred because it's less likely to cause antibiotic resistance, though it can cause more stomach upset. Your healthcare provider will determine which antibiotic is best suited for you.

Is Trimethoprim Safe in Pregnancy?

Trimethoprim generally isn't recommended during the first trimester of pregnancy. The issue is that it interferes with folic acid metabolism, which is crucial for your baby's early development. Research suggests that taking trimethoprim in early pregnancy is linked to increased risks, including a doubled risk of miscarriage and potential associations with birth defects like neural tube defects and heart problems.

If you need Trimethoprim during pregnancy:

• You'll be offered high-dose folic acid supplements if taking Trimethoprim in early pregnancy
• Your doctor will usually try other antibiotics first, especially in the first trimester
• Remember – untreated UTIs can be dangerous in pregnancy, so treatment is essential

Always tell your doctor if you're pregnant or trying to conceive before taking Trimethoprim.

Support When You Need It Most

If you’re dealing with a condition like cystitis, you’re likely to want fast and reliable relief with as little hassle as possible. Privacy will also be important, alongside a comprehensive support network that can be accessed at any time.

Prescription Doctor can help with all of this, and much more. Our mission is to make access to healthcare seamless, which is why we don’t require time-consuming face-to-face meetings. Simply complete a medical questionnaire, and our clinicians will assess whether your requested medication is suitable for your specific circumstances. This can be done day or night, from the comfort of your own home.

Orders approved before 3pm on a weekday are eligible for next-day delivery, and everything dispatched from our General Pharmaceutical Council-registered dispensing pharmacy is sent in plain packaging for complete discretion.

Our service extends beyond just dispensing medication, however. We provide a wealth of information about Trimethoprim, cystitis, and a range of other medical conditions on our website, offering you essential knowledge and guidance. From covering questions such as “How long does it take for Trimethoprim to work?” to advising on what men can do to tackle cystitis, we have you covered.

Ready to get started? Begin your Trimethoprim consultation today. 
 

Authored By

Sam Jeans

Medical Content Writer

Sam has been a freelance writer since 2015, with extensive experience in the medical, scientific, and technology fields. He holds an MSc in International Development and Social Anthropology, bringing

Published on: 11/09/2023 Re-Published on: 12/11/2025

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.

Reviewed on: 12/11/2025