Description. Sulfadiazine/Trimethoprim-SMZ/TMP is a potentiated sulfonamide that is one of the most commonly used antimicrobials in the treatment of horses. Used as a combination drug it is more effective and has fewer side effects than when the drugs are used separately.rimethoprim sulfadiazine is FDA-approved for use in horses. SMZ/TMP is a prescription drug, restricted to use by or on the lawful written or oral order of a licensed veterinarian. SMZ/TMP is not permitted in drug-free competitions.