Trimethoprim-13C3 is a 13C-labeled variant of Trimethoprim. Trimethoprim is an antibacterial antibiotic and an orally active inhibitor of dihydrofolate reductase. It is effective against a variety of Gram-positive and Gram-negative aerobic bacteria. Trimethoprim holds potential for studying urinary tract infections, shigellosis, and pneumocystis pneumonia. When combined with zinc, Trimethoprim can inhibit influenza A virus in chicken embryos.