Trimethoprim is an antibiotic used to treat infections caused by bacteria.
In order to grow and multiply in numbers, bacterial cells need to produce genetic material (DNA). To produce DNA they require folic acid (folate). However, bacterial cells can't take up folic acid supplied in the diet like human cells can. Instead, they synthesise it themselves.
Trimethoprim works by preventing the bacteria from producing folate. Without folate, the bacteria cannot produce DNA and so are unable to increase in numbers. Trimethoprim therefore stops the spread of infection. The remaining bacteria are killed by the immune system or eventually die.
Trimethoprim is most commonly used to treat and prevent recurrent infections of the urinary tract such as cystitis.
The normal dose of trimethoprim for cystitis is 200mg twice a day for three days.
For recurrent infections the dose is 200mg twice a day for seven days.
To increase its effectiveness, try and take the trimethoprim tablets every twelve hours with the second dose being quite close to bedtime. This will allow the medication to concentrate in the bladder overnight since urine is not passed as often at this time.
If you forget to take a dose take it as soon as you remember, unless it is nearly time for your next dose. In this case just leave out the missed dose and take your next dose as normal.
Don't take a double dose to make up for a missed dose.
Unless your doctor tells you otherwise, it is important that you finish the prescribed course of this antibiotic medicine, even if you feel better or it seems the infection has cleared up. Stopping the course early increases the chance that the infection will come back and that the bacteria will grow resistant to the antibiotic.
Medicines and their possible side effects can affect individual people in different ways. Just because a side effect is stated here, it does not mean that all people using this medicine will experience that or any side effect. The following are some of the side effects that are known to be associated with trimethoprim:
- Nausea and vomiting.
- Overgrowth of the yeast Candida, which may cause infection such as thrush. Tell your doctor if you think you have developed a new infection while taking this antibiotic.
- Increase in the level of potassium in the blood (hyperkalaemia).
- Disturbance in the production of blood cells.
- Abnormal reaction of the skin to light, usually a rash (photosensitivity).
- Severe blistering skin reactions, e.g. Stevens-Johnson Syndrome, toxic epidermal necrolysis.
- Inflammation of the bowel lining (colitis).
- Liver or kidney disorders.
The side effects listed above may not include all of the side effects reported by the medicine's manufacturer.
For more information about any other possible risks associated with trimethoprim, please read the information provided with the medicine or consult your doctor or pharmacist.
Trimethoprim should be used with caution in:
- Elderly people.
- Newborn babies.
- Decreased kidney function.
- Decreased liver function.
- People at risk of folic acid deficiency.
- Hereditary blood disorders called porphyrias.
Trimethoprim should NOT to be used in:
- People with disturbances in the normal numbers of blood cells in the blood.
This medicine should not be used if you are allergic to one or any of its ingredients. Please inform your doctor or pharmacist if you have previously experienced such an allergy. If you feel you have experienced an allergic reaction, stop using trimethoprim and inform your doctor or pharmacist immediately.
The patient information leaflet (PIL) is a leaflet containing specific information about medical conditions, doses and side effects. You can download a copy of the PIL here:
Patient Info Leaflet