Why Take Trimethoprim at Night?

Trimethoprim is a popular antibiotic for treating UTIs and cystitis. It inhibits bacterial growth and often relieves symptoms within a few days. If you’ve been prescribed trimethoprim for a urinary tract infection (UTI) or cystitis, you will have been instructed to take it at night. But why is this the case?

In this article, we’ll answer the question “Why take Trimethoprim at night?”, explore the science behind it, and discuss what might happen if you took the medication during the day. Let’s get started. 

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The Importance of Timing

Timing is always important when it comes to taking medication, and Trimethoprim is no exception. There are several reasons for this, including preventing accidental overdoses and ensuring the active ingredients work as efficiently as possible.  

Moreover, timing your medication appropriately ensures you maintain a steady, consistent concentration in your system and helps form habits so you don’t miss a dose. 

The Best Time to Take Trimethoprim

For treating cystitis and other urinary tract infections, the NHS recommends taking Trimethoprim twice a day – once in the morning and once in the evening. The typical dose is 200mg twice daily, and it's important to space these doses evenly throughout the day.

Ideally, you should take your doses at least 10 to 12 hours apart. For example, you might take your first dose at 8am and your second dose at either 6pm or 8pm. 

It's often recommended that you take your evening dose just before bedtime. This allows the medication to work overnight, as urine naturally remains in the bladder for extended periods, providing the antibiotic with maximum contact time against any bacteria present.

You can take Trimethoprim with or without food; however, taking it with food may help alleviate any stomach upset.

Why is Trimethoprim Taken at Night?

There are several good reasons to take trimethoprim at night, including improved absorption, reduced side effects, and greater convenience. However, it’s always best to follow your doctor’s instructions, as they will have considered other factors before determining the best way to take trimethoprim in your specific situation.

Here are the main reasons for taking Trimethoprim at night in more detail:

• Maximum Absorption: When you sleep, urine stays in your bladder longer, giving the antibiotic more time to work against the bacteria. Taking Trimethoprim at bedtime allows the medication to concentrate in your bladder overnight, maximising its effectiveness against the infection.
• Patient Convenience: In simple terms, taking a pill just before bedtime is a convenient way for most people to manage their prescriptions. The routine makes it easy to remember, and regardless of what else you might be doing or how busy you are, you won’t risk accidentally skipping a dose, which is unwise.
• Reduced Side Effects: Some people experience mild side effects when taking Trimethoprim, such as dizziness and nausea. If you take Trimethoprim at night, these side effects will be reduced because you’ll be asleep for most of the time the medication is in your system.

The Science Behind Taking Trimethoprim at Night

The reason Trimethoprim works so effectively for urinary tract infections lies in how your body processes and eliminates the medication. Trimethoprim is effective against UTIs because it concentrates efficiently in the urine, killing bacteria where the infection is located. It is primarily excreted by the kidneys, with 50% to 60% of the dose eliminated in the urine within 24 hours. 

When you take Trimethoprim at bedtime, you're taking advantage of natural overnight bladder retention. During sleep, most people don't urinate for 6-8 hours, which means the concentrated antibiotic stays in contact with the bacteria in your bladder for a longer period. This is thought to enhance the medication's ability to eliminate the infection.

Can I Take Trimethoprim During the Day?

While taking Trimethoprim at night around bedtime is usually recommended, there will always be exceptions to the rule, and certain medical conditions or other factors, such as other medications, might make daytime a better option.

It’s essential to follow your healthcare provider's advice regarding dosage, as they will consider your specific situation when prescribing medication. 

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Frequently Asked Questions

Understanding how to take Trimethoprim correctly is essential for treating your UTI effectively. Here are answers to the most common questions about Trimethoprim dosing, timing, and what to do if an issue arises.

How Many Times a Day Do You Take Trimethoprim?

For treating urinary tract infections, the NHS recommends taking Trimethoprim twice daily, once in the morning and once in the evening. The standard dose for adults is 200mg twice a day. If you're taking Trimethoprim to prevent infections rather than treat an active UTI, you'll usually take a lower dose of 100mg once daily at bedtime.

How Many Hours Apart Should You Take Trimethoprim?

Ideally, you should space your Trimethoprim doses 10-12 hours apart. Proper spacing helps maintain consistent levels of the antibiotic in your system throughout the day and night, giving it the best chance to fight the infection and help you feel better as soon as possible.

What Happens If You Take Too Much Trimethoprim?

Taking too much Trimethoprim can cause overdose symptoms, including nausea, vomiting, dizziness, headaches, confusion, or depression. Overdose can occur from taking too much at once, or slowly over time if your daily doses are too high. It’s therefore essential to only take the amount of Trimethoprim prescribed by your clinician. 

If you've accidentally taken more than your prescribed dose, contact 111 for advice or call 999 if you're experiencing serious symptoms like difficulty breathing or loss of consciousness. Have your medication packaging with you when you seek help.

Is a 3 Day Course of Trimethoprim Enough?

For most women with straightforward, uncomplicated UTIs, a 3-day course of Trimethoprim is usually sufficient to clear the infection. However, men typically need a longer 7-day course, and those with more complicated UTIs, kidney involvement, or recurrent infections may require 14 days or more of treatment. 

Always complete the full course your doctor has prescribed, even if you start feeling better after a few days.

Reliable Relief from Cystitis with Prescription Doctor

If you’re looking for cystitis relief you can depend upon, Trimethoprim is one of the most effective available. It’s even suitable for treating cystitis in men, making it effective in all kinds of circumstances. But where should you get it from?

Since 2013, Prescription Doctor has made it our mission to take the hassle out of healthcare. We understand that obtaining treatment can sometimes be challenging, whether it’s scheduling an appointment or dealing with the potential embarrassment of discussing certain conditions. With us, none of that is an issue.

All it takes is the completion of a quick online medical questionnaire, and we’ll sort out the rest. You can even fill out the form at any time, from the comfort of your own home; it’s that straightforward. Once completed, our specialist clinicians will review your information to determine whether Trimethoprim is suitable for your circumstances or recommend an alternative if necessary.

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Start your Trimethoprim consultation with Prescription Doctor 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: 19/11/2025

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: 19/11/2025