Equisul-SDT® (sulfadiazine/trimethoprim) for Horses

Published May 24, 2024
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PetMD’s medications content was written and reviewed by veterinary professionals to answer your most common questions about how medications function, their side effects, and what species they are prescribed for. This content shouldn’t take the place of advice by your vet.

What Is Equisul-SDT® for Horses?

Equisul-SDT® is an FDA-approved prescription veterinary medication used in horses for treating lower respiratory tract infections including bacterial pneumonia.

Equisul-SDT® for horses is an oral solution that is a combination of two antibiotics, sulfadiazine and trimethoprim, which work together to fight susceptible bacterial infections.

Equisul-SDT® is unique because, at the time of publication, it is the only oral liquid antibiotic that is FDA-approved for horses with proven effectiveness and safety, as compared to human prescription sulfamethoxazole/trimethoprim tablets used in an extra-label or off-label capacity.

Equisil-SDT® is apple-flavored for ease of administration.

Equisul-SDT® Considerations for Horses

Equisul-SDT® should not be used in horses that are allergic to sulfadiazine, sulfonamide, or trimethoprim antibiotics.

Equisul-SDT® should be used with caution in horses with liver disease and in highly stressed horses.

Safety studies evaluating the use of Equisul-SDT® in breeding, pregnant, or lactating horses or in horses less than one year of age had not yet been performed at the time of this publication.

How Equisul-SDT® Works in Horses

The two active ingredients in Equisul-SDT® are sulfadiazine and trimethoprim. antibiotics. Equisul-SDT® works by blocking the bacteria’s DNA from being able to replicate, thereby preventing the bacteria from multiplying and thriving.

Equisul-SDT® Directions for Horses

Follow the directions on the drug label or as provided by your veterinarian.

Draw up the prescribed amount of Equisul-SDT® into an oral syringe, and then slowly squirt the solution onto the back of your horse’s mouth on top of the tongue.

Go slowly and allow your horse to swallow during administration.

In general, your veterinarian may instruct you to give this medication twice daily by mouth.

Shake well before use.

Missed a Dose?

Speak with your veterinarian about what to do if you forget to give a dose of Equisul-SDT®. Generally, they may instruct you to give it when you remember, or if it is almost time for your horse’s next dose, to skip the missed dose and resume your normal dosing schedule. Do not give extra or double doses.

Possible Side Effects of Equisul-SDT® in Horses

Equisul-SDT® is generally well tolerated. Possible side effects may include:

Please call your veterinarian right away if you notice loose stools, diarrhea, or abdominal pain in your horse.

If your horse needs to be on a higher dose or a longer course of treatment of Equisul-SDT® than usual, your veterinarian may perform blood tests to evaluate the effects of the medication on your horse’s immune system function.

If your horse’s blood cell counts are too low (decreased platelets, red blood cells, or white blood cells) or if your horse’s blood takes longer than usual to clot (prolonged clotting times), your veterinarian may advise discontinuing Equisul-SDT®.

Human Side Effects

Equisul-SDT® for horses is not for use in human medicine, although other formulations of similar classes of antibiotics containing sulfonamides or trimethoprim are used for humans, frequently with dosages different from those prescribed for your animal by a veterinarian.

Due to possible side effects, humans should never use medicine dispensed for their animals and animals should not be given any medicine dispensed for a human’s use.

If you are allergic to sulfonamides or trimethoprim antibiotics, talk to your veterinarian about using another medication and avoid direct contact of the medication with your skin, eyes, mouth, and clothing.

If you accidentally ingest this medication or if an allergic reaction occurs (skin rash, hives, difficulty breathing, facial swelling), seek medical attention or call the national Poison Control Center hotline at 800-222-1222.

Call Your Vet If:

  • Severe side effects are seen (see above)

  • Your horse’s condition worsens or does not improve with treatment

  • You see or suspect an overdose

  • You have additional questions or concerns about the use of Equisul-SDT®

Equisul-SDT® Overdose Information for Horses

Signs of an overdose of Equisul-SDT® in a horse may include loose stools, an increased kidney level (creatinine), an increased protein level (albumin), an increased liver level (GGT), or an increased white blood cell level (neutrophils).

If you suspect an overdose, immediately contact your veterinarian, seek emergency veterinary care, or call an animal poison control center. Consultation fees often apply.

Pet Poison Helpline (855) 764-7661

ASPCA Animal Poison Control (888) 426-4435

Equisul-SDT® Storage for Horses

Always confirm storage requirements by reading the prescription label.

Equisul-SDT® oral suspension should be stored upright at 59-86 F. Brief periods up to 104 F are permitted. Protect from freezing.

Keep out of reach of children and pets.


What is Equisul used for in horses?

Equisul-SDT® is used in horses for the treatment of lower respiratory tract infections such as bacterial pneumonia.

How does Equisul work in horses?

Equisul-SDT® is an oral suspension composed of two antibiotics, sulfadiazine and trimethoprim, and works by blocking the bacteria’s DNA from being able to replicate, thereby preventing the bacteria from growing and multiplying.

No vet writer or qualified reviewer has received any compensation from the manufacturer of the medication as part of creating this article. All content contained in this article is sourced from public sources or the manufacturer.

Molly Price, DVM


Molly Price, DVM


Dr. Molly Price has practiced small animal medicine for over 20 years and is a graduate of Texas A&M College of Veterinary Medicine. She...

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