What is Trimethoprim?

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  • Written By: Mary McMahon
  • Edited By: Kristen Osborne
  • Last Modified Date: 11 September 2019
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Trimethoprim is an antibiotic medication a doctor may prescribe for the treatment or prevention of bacterial infection. This medication is bacteriostatic in nature, working by blocking key biological functions in the bacteria to halt their growth and proliferation. Eventually, they will die because they are no longer able to access nutrients necessary for cell function. Manufacturers produce trimethoprim tablets in a variety of dosages and the medication is usually taken at home.

Urinary tract infections, pneumonia, and bacterial infections leading to diarrhea can all respond to trimethoprim. In a patient with a history of recurrent urinary tract infections, this medication may be used as prophylaxis to prevent infections, in which case the patient can take the medication in the long term, rather than in a short therapeutic course. A doctor may also prescribe trimethoprim for other reasons.

The most common trimethoprim side effects are nausea, diarrhea, and vomiting. Patients can also develop headaches, fatigue, and oral sores. These side effects are more serious, and a patient should discuss them with a doctor. The doctor may recommend a change in dosage or take the patient off the medication to prevent further complications. Patients with low-level gastrointestinal side effects should take note of any changes and report them to a doctor.


A course of trimethoprim may last for varying lengths of time, depending on the infection and the doctor's recommendations. It is important to complete the course of medication and to attend a follow-up appointment so the doctor can check to see if the infection is still present. If the patient's infection is still active, it may be necessary to go back on the trimethoprim or to switch to a different medication. The doctor may consider culturing a sample from the patient and testing for antibiotic susceptibility to see if the bacteria are resistant.

Antibiotics can sometimes interfere with each other. Unless a doctor specifically recommends it, a patient should not take multiple antibiotic drugs. Patients on antibiotics may also find other medications less effective. When a doctor prescribes trimethoprim, patients should ask about specific drug interactions to determine if they are at risk of an adverse drug reaction. It may be necessary to take special precautions or temporarily change a drug regimen while on antibiotics. The patient's pharmacist should also keep a complete list of the medications a patient takes, and if there is a question about a prescription, the pharmacist can bring it up with the doctor.


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