Trimethoprim: Uses, Mechanism and Side Effects
Trimethoprim is an antibacterial medication primarily used to treat urinary tract infections. It works by inhibiting bacterial folic acid synthesis, producing a bacteriostatic effect.
Things worth knowing about "Trimethoprim"
Trimethoprim is an antibacterial medication primarily used to treat urinary tract infections. It works by inhibiting bacterial folic acid synthesis, producing a bacteriostatic effect.
What is Trimethoprim?
Trimethoprim (abbreviated TMP) is a synthetic antibiotic belonging to the diaminopyrimidine class. It is primarily used to treat urinary tract infections (UTIs) and can be administered alone or in combination with sulfamethoxazole (as co-trimoxazol, commonly known as TMP-SMX or cotrimoxazole). Trimethoprim is active against a broad range of gram-positive and gram-negative bacteria.
Indications
Trimethoprim is used in the treatment of:
- Uncomplicated urinary tract infections (e.g., cystitis, bladder infections) in adults and children
- Acute otitis media (middle ear infection) in children, usually in combination with sulfamethoxazole
- Acute exacerbations of chronic bronchitis (in combination)
- Pneumocystis jirovecii pneumonia (PCP) – prophylaxis and treatment, especially in immunocompromised patients (in combination with sulfamethoxazole)
- Traveler's diarrhea and certain gastrointestinal infections (in combination)
Mechanism of Action
Trimethoprim selectively inhibits the bacterial enzyme dihydrofolate reductase (DHFR). This enzyme is essential for converting dihydrofolate into its active form, tetrahydrofolate, which is required for the synthesis of DNA building blocks (purines and thymidylate).
By blocking this enzyme, trimethoprim disrupts folic acid synthesis within the bacterial cell, preventing bacterial replication. The effect is therefore bacteriostatic (growth-inhibiting). Because human DHFR has a structurally different active site, trimethoprim is largely harmless to humans at therapeutic doses.
When combined with sulfamethoxazole (a sulfonamide), a synergistic effect is achieved: both agents target different steps of the folic acid synthesis pathway, resulting in a bactericidal (bacteria-killing) effect.
Dosage and Administration
Dosage depends on the indication, age, and renal function of the patient. General guidelines for adults include:
- Urinary tract infections: 200 mg twice daily or 100 mg three times daily for 7 days
- Prophylaxis of recurrent UTIs: 100 mg once daily (long-term prophylaxis)
- Dose adjustment is required in patients with reduced kidney function
Trimethoprim is available as tablets or oral suspension. It is taken orally and is well absorbed from the gastrointestinal tract.
Side Effects
Trimethoprim is generally well tolerated. Possible side effects include:
- Common: Nausea, vomiting, skin rash, itching
- Occasional: Elevated serum creatinine levels (due to inhibition of tubular creatinine secretion, without true kidney damage), hyperkalemia (elevated blood potassium levels)
- Rare: Blood count abnormalities (megaloblastic anemia, leukopenia, thrombocytopenia) due to interference with folate metabolism
- Very rare: Severe skin reactions (Stevens-Johnson syndrome), liver function disorders
Contraindications and Drug Interactions
Trimethoprim should not be used in patients with:
- Known hypersensitivity to trimethoprim
- Severe renal impairment (without appropriate dose adjustment)
- Megaloblastic anemia due to folate deficiency
Important drug interactions include:
- Methotrexate: Increased risk of bone marrow suppression
- Warfarin and other anticoagulants: Possible enhancement of anticoagulant effect
- ACE inhibitors and potassium-sparing diuretics: Increased risk of hyperkalemia
- Phenytoin: Possible increase in phenytoin plasma levels
Antibiotic Resistance
Growing antibiotic resistance is a significant concern with trimethoprim. Resistance commonly arises through mutations in the DHFR gene or through plasmid-mediated resistance genes. In some regions, up to 20-30% of E. coli strains (the most frequent causative agent of UTIs) are resistant to trimethoprim. Therefore, especially in recurrent infections, a susceptibility test (antibiogram) should be performed before starting treatment.
References
- World Health Organization (WHO): WHO Model List of Essential Medicines, 23rd Edition. Geneva: WHO, 2023. www.who.int
- Brunton, L. L. et al. - Goodman & Gilman's: The Pharmacological Basis of Therapeutics. 13th Edition. McGraw-Hill Education, 2017.
- Gupta, K. et al. - Infectious Diseases Society of America (IDSA): Clinical Practice Guideline for the Treatment of Acute Uncomplicated Cystitis and Pyelonephritis in Women. Clinical Infectious Diseases, 2011; 52(5): e103-e120.
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