Trimethoprim Side Effects and Safety

Trimethoprim is one of the most commonly prescribed antibiotics for urinary tract infections (UTIs), such as cystitis, in the UK. When you have been prescribed such a treatment, it's natural to question its safety, and what side effects you may experience.

Here we provide clear, evidence-based safety information regarding trimethoprim side effects, drinking alcohol whilst on treatment, taking it during pregnancy and standard dose schedules.

What Is Trimethoprim and How Does It Work

Trimethoprim is an antibiotic, designed to target and kill the bacteria that cause a UTI, most often E.Coli. It works by blocking the bacteria from making folic acid - a chemical required for it to grow and multiply.

In doing so, it allows the body’s immune system to clear the infection without worrying about the infection worsening or spreading.

Is Trimethoprim a Strong Antibiotic

A common question patients ask is : is trimethoprim a strong antibiotic?

From a medical perspective, antibiotics are not classified based on how strong they are but rather how effective they are against a range of different bacteria. Trimethoprim is known as a broad-spectrum antibiotic, which means it is sensitive against many common bacteria strains.

Trimethoprim is a highly effective cystitis treatment in the UK, used either as a first-line option or second-line, where a patient cannot tolerate other antibiotics.

Common Trimethoprim Side Effects

Like all medications, trimethoprim can cause side effects, though not everyone experiences them.

The most common side effects affect between 1 in 10 and 1 in 100 people. These are generally mild and go away on their own as the body adjusts to treatment:

• Nausea and stomach upset - Taking the dosage after a meal can help.

• Mild diarrhoea - Stay hydrated to make up for fluid loss.

• Appetite loss - Eating smaller, light meals throughout the day can help.

• Mild skin rash.

If any mild side effects persist, or get worse, you should speak to a pharmacist for advice. Importantly, you should never stop taking your medication early unless a healthcare professional has advised you to do so.

Dangers of Trimethoprim and Rare Side Effects

The following side effects require immediate medical attention.

Severe Allergic Reactions (Anaphylaxis)

Although very rare, severe allergic reactions can occur. If you experience any of the following, you should stop taking trimethoprim and seek immediate medical attention by calling 999 or going to A&E:

• Swelling of the tongue, lips, face or throat.

• Sudden itching, a severe skin rash or hives.

• Difficulty breathing, wheezing or a tight chest.

Severe Skin Reactions

Trimethoprim can cause serious skin reactions like Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN).

If you develop a severe peeling or blistering rash, particularly if accompanied by fever or flu-like symptoms, you should contact a doctor immediately.

Blood Disorders and Folate Levels

As trimethoprim affects folic acid levels, prolonged use can affect blood cell counts, causing conditions like anaemia or leukemia. This is rare for a standard 3-day course but for those who have an existing folate deficiency or those who take trimethoprim long term, clinicians exercise caution.

High Potassium Levels (Hyperkalemia)

Trimethoprim can cause a build up of potassium, by affecting the kidneys which help to clear it. This risk is higher in elderly patients, those with existing kidney problems, and those taking other medicines which also increase potassium.

High potassium levels can affect how the heart and other muscles work.

Can You Drink Alcohol With Trimethoprim

Amongst the most common queries clinicians receive are concerning alcohol intake whilst on trimethoprim.

There is no dangerous interaction between the two, (unlike with metronidazole) and a moderate alcohol consumption will not cause any toxic reactions.

Why Should You Still Limit or Avoid Alcohol

Whilst not strictly prohibited, clinicians advise limiting or avoiding alcohol whilst you are recovering from a UTI for a few reasons:

• Bladder irritation: Alcohol is known to irritate the bladder. Having even an occasional drink may worsen cystitis symptoms.

• Dehydration: Alcohol is a diuretic (makes you pee more often). When treating a UTI, hydration is important to help flush bacteria out of the urinary tract.

• Side effects: Both alcohol and trimethoprim can cause nausea, dizziness and stomach upset. Combining them increases the chance of experiencing these side effects.

If you do choose to drink alcohol, ensure to drink light, whilst also maintaining an adequate water intake.

Trimethoprim and Pregnancy: Safety Guidance

For those who are pregnant, using trimethoprim requires careful consideration by a healthcare professional, which can vary depending on how far along you are.

Is Trimethoprim Safe During Early Pregnancy?

Trimethoprim should be avoided during the first trimester (first 12 weeks) of pregnancy.

Folic acid is essential for proper development of the fetus’s brain and spine. Trimethoprim blocks folic acid, leading to developmental issues such as spina bifida.

If you are pregnant, think you might be, or are trying to conceive, you must inform your provider before starting any UTI treatment.

What Are The Alternatives During Pregnancy?

Alternative antibiotics that are known to be safe in pregnancy are nitrofurantoin (except if near term) and cefalexin.

Trimethoprim During Later Pregnancy and Breastfeeding

Trimethoprim can be considered during the second and third trimesters, only if other treatments are unsuitable or if urine culture tests show trimethoprim is the most effective option.

However, prescribing at any stage of pregnancy requires careful evaluation by a healthcare professional and is usually outside of the scope of a remote prescribing service.

Whilst small quantities of trimethoprim do pass into breastmilk, it is generally considered safe to use when breastfeeding though you should inform your prescriber prior to use.

Trimethoprim Dose for UTI and How To Take It

The standard trimethoprim dose for UTI in adults aged 18 to 65, when used for uncomplicated cystitis is:

• Dose: 200mg taken twice daily - usually every 12 hours.

• Duration: Usually a 3-day course.

Some patients may be prescribed a longer course of 7 or 14 days, particularly for men, pregnant women, those with complex infections or with certain underlying health conditions.

Key Tips For Taking Trimethoprim

1. Space doses evenly. The twice daily doses should be taken roughly 12 hours apart (i.e. 8AM and 8PM). This helps keep the concentration of medicine in the blood and urine steady.

2. The tablets should be swallowed whole with a full glass of water. You should not cut, crush or chew the tablets.

3. Taking the dose with food can help those who may experience stomach issues.

4. Complete the full course, even if you feel better. Stopping early lets surviving bacteria multiply bringing on another infection, which may even be resistant to treatment.

How Long Does Trimethoprim Take To Work?

When dealing with the uncomfortable burning symptoms of cystitis, knowing when to expect relief is important.

Expected Timeline For Relief

Most patients start to notice an improvement within 24 to 48 hours after their first dose. As the antibiotic starts to work to reduce the amount of bacteria in the bladder, inflammation reduces which settles pain and urinary urgency.

What If Your Symptoms Do Not Improve

If symptoms do not improve within 48 hours of starting trimethoprim, or if they continue to worsen, you should speak to your prescriber. This may mean:

• Resistance. The type of bacteria causing your infection may be resistant to trimethoprim.

• Alternative infection. Your symptoms may be caused by a different organism (not a bacteria). This needs alternative treatment.

• Upper UTI. If the infection spreads to the kidneys, a different approach is needed.

Red Flag Symptoms To Watch For

You should seek immediate medical attention if you develop any of the following:

• High fever, chills or uncontrollable shivering.

• Severe back pain, or pain in the side or flank.

• Visible blood in your urine.

• Nausea and persistent vomiting.

• Confusion.

For further information on how to manage bladder infections, read our guide to cystitis and UTIs. To compare trimethoprim with other treatments like nitrofurantoin, visit our in depth UTI and cystitis treatment comparison guide.

Where To Buy Trimethoprim

If you are experiencing cystitis symptoms, it’s important to get fast access to appropriate treatment.

Trimethoprim is available on private prescription from Quick Meds, a GPhC-registered online pharmacy.

Simply fill out a short form online. This asks about your symptoms, your medical history and if you take other medication, to determine suitability. All orders are reviewed by our in house UK-based prescribers before being packed securely, and discreetly delivered to your door using tracked delivery methods.