Family doctor



TRIMETHOPRIM - a patient's guide


This medicine is mainly used to treat urinary tract infections. This article provides an overview of the medication and how it should be taken.


  • Triprim
  • TMP
  • Monotrim
  • Ipral
  • Trimprex
  • Trimopan

Trimethoprim is also an ingredient in co-trimoxazole (Trisul, Septrin, Bactrim)


Trimethoprim alone is probably most commonly used for urinary tract infections (cystitis). It can be used both to treat a urinary tract infection and also sometimes to help prevent them (e.g. in people who keep getting this type of infection). Trimethoprim may also be used for some other bacterial infections; in this case it is often combined with another antibiotic as co-trimoxazole. There are some bacteria that are not affected by trimethoprim (resistant).

Trimethoprim works by stopping a step in the process of making DNA in bacteria.

Usual dose:

To clear up a urinary tract infection the usual adult dose is 300 mg once a day. Children take between 40 mg and 150 mg once daily depending on age. Usually trimethoprim is taken for a three to five day course. Higher doses as a single dose are also effective in treating uncomplicated urine infections.

To prevent a urinary tract infection the usual adult dose is 100 mg once a day for adults and children over 12 years. Children take between 25 mg and 50 mg once a day.

The tablets are usually taken at bedtime, and can be taken with food to help to prevent stomach upset.


Trimethoprim should be avoided if a person:

  • Has very bad kidneys
  • Is allergic to trimethoprim
  • Has serious blood problems
  • Is pregnant

Extra care should be taken if a person:

  • Has a bad liver
  • May be low in folate
  • Has porphyria or is at risk of porphyria
  • Is elderly

The doctor may want to take regular blood counts if you are using trimethoprim for a long time.

Side effects:

Most people do not have any side effects with this medicine, however the more likely side effects are:

  • Itching
  • Skin rash (talk to a doctor)
  • Stomach upset
  • Sore mouth

Trimethoprim can cause blood problems in rare cases.

There are other side effects, so if you have any unexpected symptoms while taking this medicine, please contact your doctor or pharmacist.


Possible drug interactions include:

  • Warfarin
  • Phenytoin
  • Pyrimethamine (a malaria preventer)
  • Cyclosporin
  • Procainamide
  • Amantadine
  • Digoxin
  • Methotrexate
  • Zidovudine
  • Lamivudine
  • Dapsone

Patient information:

  • Follow the instructions on the label of the medicine or as directed by your doctor.
  • Tell your doctor if you think you might be pregnant.
  • Talk to a doctor if you get a skin rash while taking this medicine.
  • Finish the course the doctor has given you.
  • If you have stomach upset while taking this medicine, take the dose with food.
  • Take at bedtime.
  • Do not share your medicines with anyone else, even if you think they may have the same problem as you.
  • You can buy a "urinary alkaliniser" from the pharmacy. If you have a urinary tract infection this will make it less painful to urinate (pee), but does not cure the problem so you will still need to take the antibiotics.
  • Keep drinking plenty of water, around 8 glasses a day.

See also:

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