Indication Trimethoprim is a potential sulfa drug with antibacterial properties, which is used in the treatment of many bacterial infections in dogs and cats. It is commonly used against the infections of the lungs, urinary tract, skin and gastrointestinal organs. Trimethoprim is found to be of use in treating conditions like Pneumonia, Enteritis, Otitis media, Prostatitis, Coccidiosis, Kennel cough, and Staphylococcal skin infections among others.