Trimethoprim (INN) is a bacteriostatic antibiotic used mainly in the prevention and treatment of urinary tract infections. It belongs to the class of chemotherapeutic agents known as dihydrofolate reductase inhibitors. Trimethoprim was formerly marketed by GlaxoSmithKline under trade names including Proloprim, Monotrim, and Triprim; but these trade names have been licensed to various generic pharmaceutical manufacturers. Trimethoprim is on the World Health Organization’s List of Essential Medicines, a list of the most important medication needed in a basic health system. Trimethoprim and sulfamethoxazole are commonly used in combination.