Trimethoprim is an antibacterial medication that inhibits bacterial folate synthesis. It blocks the enzyme dihydrofolate reductase which is required for the production of tetrahydrofolate. By preventing tetrahydrofolate formation the drug interferes with nucleotide and amino acid synthesis in bacteria. This inhibition leads to bacterial growth arrest and eventual death of susceptible organisms. In clinical practice Trimethoprim is often combined with sulfamethoxazole to enhance antibacterial activity. The combination produces a synergistic effect that improves treatment of various infections.

Trimethoprim is an oral antibacterial medication that belongs to the class of dihydrofolate reductase inhibitors. It is commonly prescribed for the treatment of bacterial infections of the urinary tract and for certain types of travel-associated diarrheal illnesses. The drug works by blocking the enzyme dihydrofolate reductase in bacterial cells, which is essential for the synthesis of tetrahydrofolate and consequently for nucleotide production. By interfering with folate metabolism the medication halts bacterial growth without having a direct bactericidal effect. This inhibition exploits a metabolic pathway that is largely absent in human cells, allowing selective targeting of microbes. The resulting bacteriostatic action enables the host immune system to clear the infection more effectively. Clinically the drug is employed to resolve infections and to prevent recurrence when used as part of combination therapy in certain settings.

Trimethoprim can cause nausea and vomiting in some patients. Headache and dizziness are also reported frequently. Diarrhea or loose stools may occur during treatment. Skin reactions such as rash or itching can develop. Photosensitivity to sunlight may be observed. Rarely patients experience elevated blood potassium levels. Most side effects are mild and resolve when the medication is stopped.

Trimethoprim may cause mild stomach upset and loss of appetite. It can sometimes lead to skin rash or itching. Some people experience dizziness or headache. Rarely, it may cause altered taste or dry mouth. These effects are usually temporary and not severe. Most patients tolerate the medication without serious complications.