Trimethoprim is a popular oral antibiotic against cystitis

Trimethoprim is the first choice among antibiotics for treating cystitis and other UTIs. The 200mg tablets are prescribed as a short course to prevent and cure cystitis.

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200mg6 tablets£13.95In Stock
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What is Trimethoprim?

Trimethoprim is an antibiotic primarily prescribed to treat urinary tract infections (UTSs) such as cystitis. In some cases, it is also prescribed treat chest infections and acne. 

How does Trimethoprim work?

Trimethoprim is an antibiotic used to treat bacterial infection, and will not work for viral infection such as flu. It works by inhibiting the production of folic acid by bacteria at the site of infection, preventing bacteria from being able to grow and spread.

How long does it take for Trimethoprim to work?

Trimethoprim will usually make you feel better within a few days. However, like all antibiotics, it is vital that you complete the full course prescribed to you, even if you feel better.

Other uses of Trimethoprim

Trimethoprim is primarily used to treat bacterial infections in the urinary tract. In some cases, it can be used for long-term prevention of recurrent UTIs. It can also be used to treat chest infections and acne.


Active ingredients

The active ingredient in Trimethoprim is trimethoprim.

Inactive ingredients

The inactive ingredients in Trimethoprim are colloidal silicon dioxide, lactose, macrogol, magnesium stearate, povidone, sodium starch glycollate, stearic acid, and microcrystalline cellulose (E460).

Please Note: Different manufacturers of generic Trimethoprim can use different inactive ingredients in their drugs that those listed above.


How to use Trimethoprim

Always take Trimethoprim as instructed by a doctor or pharmacist. Swallow the tablet or tablets at the same times each day with a glass of water. If you take more Trimethoprim than you should, contact a doctor immediately. If you forget to take a dose, take it as soon as you remember and then take your next dose at the usual time. Never take a double dose to make up for a missed one.

Trimethoprim dosage

Your doctor will prescribe the dosage that befits your symptoms and condition. For sudden or severe UTIs, treatment can last from 3 to 14 days, usually twice daily. For long-term prevention therapy, the recommended dose is 100 mg taken once daily at night for as long as a doctor sees fit.

What if I take too many Trimethoprim?

Always take the dose of Trimethoprim prescribed by your doctor. If you take too much, contact a doctor immediately. 

When can I stop taking Trimethoprim?

Your doctor will tell you when to stop taking Trimethoprim. It is vital that you follow instructions and continue to take Trimethoprim for the full course prescribed to you. Do not stop taking Trimethoprim just because you feel better.

Do I need to finish the course of Trimethoprim even if my symptoms have resolved?

You should always finish the course of antibiotics prescribed to you, even if you feel better after a few days. This is vital in preventing bacteria developing antibiotic resistance.

Side Effects

Trimethoprim side effects

Like all medications, Trimethoprim can cause side effects in some patients. If you experience any of the following rare but severe side effects, stop taking Trimethoprim and seek immediate medical attention:

  • Swelling of the face, lips, tongue or throat, or difficulty breathing
  • Chest pain, shock, fainting or collapse
  • Blistering or peeling of the skin, skin eruptions or lesions, or deep swelling of the skin known as angioedema
  • Pancreatitis, with sudden, severe upper abdominal pain

Other side effects tend to be mild, but you should alert your doctor if they are troubling you. The following side effects can affect up to 1 in 10 people:

  • High levels of potassium in the blood
  • Nausea, vomiting or diarrhoea
  • Headache
  • Skin rashes, urticaria
  • Anaemia, which may cause you to appear pale and feel tired
  • Sore throat or mouth ulcers
  • Thrush

Very rare side effects, affecting up to 1 in 10 000 people, include:

  • Constipation
  • Severe, watery or bloody diarrhoea
  • Cough
  • Agitation or abnormal behaviour
  • Aseptic meningitis, with symptoms such as headache, fever, stiff neck, tiredness, feeling unwell and sensitivity to bright lights
  • Kidney problems, painful urination or blood in the urine
  • Dizziness, tiredness
  • Convulsions or involuntary movements
  • Pins and needles, shakiness or tremors
  • Ringing in the ears, vertigo
  • Eye redness or pain
  • Sensitivity to sunlight
  • Low blood sugar levels, low levels of sodium in the blood, which may cause nausea, tiredness or muscle cramps
  • Anorexia
  • Depression, anxiety, sleeping difficulties, nightmares, confusion, hallucinations
  • Jaundice (yellowing of the skin and whites of the eyes), elevation of serum transaminases (an indication of liver damage), or elevation of bilirubin levels
  • Joint and muscle ache
  • Shortness of breath, wheezing, nosebleeds
  • Meningitis (stiff neck, fever, vomiting)
  • Lupus erythematosus
  • Purple discolouration of the skin
  • Fever
  • Anaemia or reduction in numbers of blood cells

Symptoms such as sore tongue and mouth, gastrointestinal disturbances, itching, and elevation of serum creatinine and blood urea nitrogen levels have also been known to occur, although their frequency cannot be measured.


Do NOT take Trimethoprim if:

  • You are allergic to trimethoprim or any of the other ingredients in this medication
  • You are pregnant
  • You have a blood disorder
  • You have severe kidney disease
  • You have severe liver disease

Before taking Trimethoprim, speak to your doctor if:

  • You have kidney disease or are having dialysis treatment
  • You have a deficiency of folic acid

Drug interactions

Before taking Trimethoprim, tell your doctor or pharmacist if you are taking any other medications, including those purchased over the counter without a prescription. In particular, tell your doctor or pharmacist if you are taking or have recently taken any of the following:

  • Antibiotics such as rifampicin
  • Anticoagulants such as warfarin
  • Digoxin for a heart condition
  • Phenytoin for epilepsy
  • Pyrimethamine or dapsone for malaria
  • Immunosuppressant drugs used in cancer treatment (such as methotrexate) or to treat organ rejection following a transplant (such as azathioprine or ciclosporin)
  • Bone marrow depressants
  • Spironolactone for fluid build up
  • Repaglinide for diabetes
  • Diuretics, such as water tablets furosemide or eplerenone
  • Procainamide for abnormal heart rhythm

How to cope with Trimethoprim side effects

Trimethoprim does not usually cause side effects, but when it does, they tend to be mild and easy to manage. Mild rashes and itching can be treated by taking antihistamines, which can be purchased over the counter from a pharmacy. If you experience nausea when you take your tablets, try taking it with or after a meal, and avoid rich or spicy foods. This is particularly important if your nausea leads to sickness, as Trimethoprim may not be absorbed by the body. Mild diarrhoea can be treated by staying hydrated and drinking plenty of water, but you should not take other medicines to treat diarrhoea without consulting a pharmacist. Headaches resulting from dehydration can be managed by staying hydrated, and by taking over-the-counter painkillers such as paracetamol and ibuprofen in needed.

Trimethoprim and alcohol

It is ok to drink alcohol while taking Trimethoprim.

Trimethoprim and pregnancy

Trimethoprim should not be used during pregnancy.

Trimethoprim and breastfeeding

If you are breastfeeding, speak to a doctor for advice before taking this medication.

Treatment Options

How to avoid UTIs

UTIs are a common infection in women caused by bacteria, usually E. coli from the large intestine, entering the urethra and travelling up the urinary tract. UTIs often clear up on their own, but do have a tendency to recur. There are some simple measures you can take to help prevent UTIs in the future. Avoid using perfumed soaps or bubble bath when cleaning your genitals, and heavily perfumed washing detergents when washing your underwear. Condoms and contraceptive diaphragms with spermicidal lubricant on them can also disturb the bacterial imbalance in the vagina, so try and avoid these. Go to the toilet as soon as you feel the urge to pee, and empty your bladder as soon as possible after having sex. Always wipe from front to back when you go to the toilet to avoid the spread of bacteria to the urethra. Avoid wearing tight jeans and trousers, and choose cotton underwear over synthetic materials such as nylon. Drinking plenty of water and staying well hydrated helps to flush out any bacteria in the urethra and prevent infection. 

Trimethoprim vs Nitrofurantoin

Nitrofurantoin is an antibiotic which is also prescribed for the treatment of UTIs. It is quickly filtered out of the body in urine, which is useful for treating infections of the urinary tract. Both Trimethoprim and Nitrofurantoin are effective treatments for UTI, but each may be more suited to different patients. Nitrofurantoin is recommended for use over Trimethoprim  if you have low levels of folic acid in your blood, and for this reason is preferred for treatment of UTIs in pregnant women. Trimethoprim is preferable where the patient has diabetes mellitus or vitamin B deficiency, although Nitrofurantoin may be prescribed in unique circumstances.

Alternatives to Trimethoprim

There are a range of antibiotics used in the treatment of UTIs. A doctor’s choice of antibiotic will depend on the the patient’s medical history as well as bacterial resistance in the area. Out of the alternative antibiotics used to treat UITs, doctors will often chose to prescribe quinolines, such as ciprofloxacin, ofloxacin and norfloxacin, amoxicillin, or nitrofurantoin before prescribing Trimethoprim.


Is Trimethoprim an antibiotic?

Trimethoprim is an antibiotic used to treat bacterial infection.

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