Trimethoprim
Trimethoprim appears here as Bactrim, the sole listing on this page. It is paired with sulfamethoxazole in two combination strengths for Urinary Tract Infections and Pneumocystis Pneumonia.
Key points
- Trimethoprim is contraindicated in the first trimester of pregnancy; animal studies show a teratogenic effect and epidemiological data link early exposure to congenital malformations.
- Severe cutaneous reactions including Stevens-Johnson syndrome and toxic epidermal necrolysis have been reported with trimethoprim and can be life-threatening.
- The two listed strengths, 400 mg + 80 mg and 800 mg + 160 mg, maintain a fixed 5:1 ratio of sulfamethoxazole to trimethoprim.
How does the combination work?
Trimethoprim inhibits dihydrofolate reductase, the enzyme bacteria use to convert dihydrofolic acid into tetrahydrofolic acid, the active folate form needed to build amino acids. Sulfamethoxazole blocks an earlier step in the same pathway. Together they create a sequential blockade of bacterial folate synthesis at two distinct enzymatic steps. This Antibiotics combination is active against a wide range of Gram-positive and Gram-negative organisms, including E. coli, Klebsiella pneumoniae, and Staphylococcus aureus.