Trimethoprim

What Is trimethoprim?

Trimethoprim is an antibiotic used to treat bacterial infections by inhibiting bacterial growth. It is a prescription-only medicine available as a tablet or an oral liquid suspension[1]. 

What is trimethoprim used to treat?

Trimethoprim is commonly prescribed for[1]: 

• Urinary tract infections (UTIs), including bladder and kidney infections 

• Certain chest infections, such as acute bronchitis or pneumonia caused by susceptible bacteria 

• Acne (in specific cases, usually when other treatments are unsuitable) 

• Trimethoprim may also be used to prevent recurrent UTIs, especially in people who are prone to infections. 

How does trimethoprim work?

Trimethoprim kills bacteria by stopping them from making folic acid which they need to survive. Without folic acid, bacteria can't grow or reproduce, which helps your body fight off infection[1].  

Who can take trimethoprim?

Most adults and children can take trimethoprim, but it’s not suitable for everyone. Avoid or use with caution if you[1]: 

• Have low folate levels (e.g., due to megaloblastic anemia) 

• Have kidney or liver problems 

• Have blood disorders like anaemia or porphyria 

• Are pregnant, trying to conceive or breastfeeding 

• Are allergic to trimethoprim or similar medicines 

Always tell your doctor or pharmacist about any conditions you may have before you start treatment and always follow the instructions in the Patient Information Leaflet[2].

How do you take trimethoprim?

Trimethoprim is usually taken twice a day, in the morning and evening, approximately 12 hours apart. It can be taken with or after food. The tablets must be swallowed whole with a drink of water, do not chew or crush them[1]. 

The oral liquid should be measured using the provided syringe or dosing spoon to ensure the correct dose. Shake the bottle well before use, and do not use a kitchen teaspoon, as it may lead to incorrect dosing[1]. 

What is the typical dosage of trimethoprim?

 The dosage of trimethoprim depends on the infection, age, weight (for children), and kidney function. Typical adult doses include:[1]. 

• Acute UTIs: 200 mg twice daily for 3 to 7 days (sometimes 400 mg for the first dose) 

• UTI prevention: 100 mg once daily at bedtime 

• Post-sex cystitis: 100mg within two hours of intercourse 

• Acne: 300 mg twice daily (may be reduced over time) 

Can you stop taking trimethoprim early?

No, always complete the full course, even if you feel better. Stopping early can cause the infection to return or contribute to antibiotic resistance[1]. 

What do you do if you take too much trimethoprim?

Taking too much trimethoprim may increase the chance of side effects like nausea, vomiting, dizziness, or confusion. If you’ve taken more than one extra dose and you are worried or have side effects, call 111 or speak to a doctor. For large overdoses or severe symptoms, seek urgent medical attention by calling 999 or visiting A&E[1].  

What do you do if you forget to take your dose of trimethoprim?

If you miss a dose, take it as soon as you remember. If it's almost time for your next dose, skip the missed dose and continue with your regular schedule. Never take 2 doses at the same time[1]. 

What are the side effects of trimethoprim?

The most common side effects include[1]: 

• Nausea 

• Vomiting 

• Diarrhoea 

• Skin rash or itching 

• Thrush (oral or vaginal yeast infections) 

Rare, but more serious side effects include[1]: 

• Blood disorders (bruising, fatigue)  

• Signs of meningitis (e.g., headache, high temperature, stiff neck, light sensitivity) 

• High potassium (muscle weakness, irregular heartbeat) 

• Photosensitivity (increased sensitivity to sunlight, causing rash or burns) 

•  severe allergic reaction (swelling, difficulty breathing, rash).  

If you experience any of these rare but serious side effects, please call 999 or go to A&E immediately. 

If you experience any side effects, including those not listed, you are encouraged to report them via the Yellow Card Scheme or contact your doctor, pharmacist, or nurse. This information is intended as a summary for general awareness and does not replace professional medical advice. 

The information provided in the side effects section is not a comprehensive list of all possible side effects associated with this medication. For a complete list of side effects, please refer to the Patient Information Leaflet (PIL) provided with your medication or consult your healthcare professional[2].

Medicines you can’t take with trimethoprim

 Trimethoprim may interact with certain medicines, including[1]: 

• Methotrexate (increased risk of toxicity) 

• Phenytoin (affects levels in the blood) 

• Diuretics (especially in elderly, may increase potassium) 

• ACE inhibitors for high blood pressure (risk of high potassium) 

• Warfarin (may increase bleeding risk) 

Always inform your doctor or pharmacist about all medications and supplements you take before you start a new medicine, and follow the instructions in the Patient Information Leaflet[2]. 

Frequently asked questions about trimethoprim

Can I drink alcohol while taking trimethoprim? 
Yes, alcohol doesn’t interfere with trimethoprim[1]. 

How long does trimethoprim take to work? 
You should feel better in 2 to 3 days but always complete the full course[1]. 

Can I take trimethoprim while pregnant? 
It’s not usually recommended in early pregnancy as it may affect folic acid levels, which is important in the early stages of pregnancy. Always consult your doctor or pharmacist before taking[1]. 

Can I drive or operate machinery while taking trimethoprim? 
Trimethoprim is unlikely to affect driving or operating machinery. However, if you experience rare side effects like dizziness or feeling unwell, avoid these activities and consult your doctor[1].

For the official NHS guide to trimethoprim, visit their website.