Trimethoprim Sulfa

Description

Trimethoprim sulfa (SMZ-TMP) is a potentiated sulfonamide used in combination with sulfadiazine and sulfamethoxazole as an antibiotic that is effective against a broad range of bacteria. As a combination drug, it is more effective and has fewer side effects than if used separately. Both drugs block folic acid metabolism by bacteria via different mechanisms. Used separately, they inhibit, but do not kill susceptible bacteria. In combination, they are bactericidal.

Usage

Trimethoprim sulfa is the most commonly administered antibiotic in horses, due to its low cost, effectiveness against a broad range of bacteria, and low incidence of side effects. SMZ-TMP is prescribed by veterinarians to treat minor wounds, bacterial dermatitis, and upper-respiratory infections.

Dosage and Administration

Side Effects

Diarrhea is the most common side effect associated with oral SMZ-TMP. Itching and rare allergic reactions have been associated with intravenous SMZ-TMP.

Precautions

Some veterinarians have reported rare fatalities after administration of intravenous SMZ-TMP. Because these reports are anecdotal, it is difficult to know the actual cause of the fatalities.

Injection site reactions can occur if the drug leaks out of the vein.

Anemia and decreased blood clotting ability can occur with long-term use.

SMZ-TMP should be used with caution in horses with decreased liver function, or with blood and bone marrow disorders.

Some veterinarians provide horses on long-term SMZ-TMP with folic acid supplements because of the concern that folic acid metabolism may be impaired.

Trimethoprim sulfadiazine is FDA-approved for use with horses, but trimethoprim sulfamethoxazole is not FDA-approved for horses. It is commonly used and is an accepted practice. Both drugs are prescription, 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. It is important to check with the proper regulatory group.

Interactions

SMZ-TMP can increase clotting time in horses on oral anticoagulants such as warfarin. Antacids can decrease the absorption of SMZ-TMP, and doses should be administered no less than two hours apart.

Overdose

SMZ-TMP is generally a safe drug, and no specific information is available concerning overdose in horses.