Most horse owners are well acquainted with phenylbutazone, better known as "bute". In many stables and barns, a bottle of phenylbutazone tablets is a familiar and integral part of the medical kit. Bute is a potent drug used to treat inflammation and pain, and we've all grown accustomed to using it to ease pain after injuries, or even routinely for chronically lame horses. It remains our most effective drug for treating musculoskeletal pain in horses, and is inexpensive and easily administered.
Despite bute's usefulness, however, all owners should be aware that phenylbutazone has a low threshold for toxicity, and overdoses or long-term use can result in life-threatening complications. Although bute is a prescription drug, it is frequently used without the advice of a veterinarian, and sometimes this results in inappropriate use. Here at Bend Equine Medical Center this year, for example, we saw several cases in which inadvertent overdoses resulted in kidney failure, gastric ulcers, colic, and fatal colitis.
How do these drugs cause such severe side effects? Phenylbutazone and Banamine, like other non-steroidal anti-inflammatory drugs (NSAIDs), work by blocking prostaglandin production throughout the body. Blockage of prostaglandins results in decreased inflammation and pain, which is the desired effect. Unfortunately, blocking prostaglandin pathways also results in decreased blood flow to some parts of the body, including the lining of the intestines and the kidneys. Decreased blood flow damages the tissues, and results in death of kidney cells (renal papillary necrosis) and formation of ulcers throughout the gastrointestinal tract (lips, tongue, stomach, small intestine, and large intestine/colon). Ulcers can cause loss of appetite, weight loss, diarrhea, lethargy, and severe colic. They can also perforate through the wall of the gastrointestinal tract and cause death.
The dose of phenylbutazone generally regarded to be relatively safe in adult horses weighing approximately 1000 pounds is 11/2 grams (tablets) twice per day for two to three days. For situations involving severe pain or inflammation, veterinarians may prescribe up to 2 grams twice per day for short periods. Doses exceeding 4 grams of phenylbutazone once per day or 2 grams twice per day consistently and quickly cause side effects, and should never be administered.
Banamine, commonly used to treat colic pain, is usually prescribed at 1.1 mg/kg bodyweight, or about 10 cc of the injectable form once per day. The oral paste form is dosed with a wormer-like tube that indicates the appropriate dose by weight on the plunger, making dosing relatively simple. When used to treat colic, remember that any horse that remains painful after one dose of Banamine should not be re-dosed for 12 hours. Your veterinarian should be called to examine the horse, and he or she will use other pain-relieving drugs if necessary.
I know some of you are thinking right now that you've given your horse 2 grams of bute twice a day regularly for years, and have never had a problem. In fact, what makes bute and the related drug Banamine so potentially dangerous is that they can be very well tolerated by some horses, even at high doses, yet very poorly tolerated by others. Despite proper dosages, some horses are highly sensitive to the side effects of bute and Banamine and can develop problems even when treated with a "safe" dose. The risk increases when they are administered in particular situations, such as when a horse is dehydrated, shows signs of "tying up", or is being treated with some types of injectable antibiotics. Side effects are also more common in foals and young horses.
In summary, in situations where the use of bute or Banamine is required, use them at low doses, for short periods, and be mindful of the potential side effects that can develop. If your horses show signs of lethargy, poor appetite, weight loss, increased thirst or increased frequency of urination after treatment with bute or Banamine, they should be examined by your veterinarian. Testing blood can help identify if damage is occurring to the kidneys or to the gastrointestinal tract. Blood work is also a good idea for any horse that is on a prolonged course of NSAIDs, even if they are not showing signs of problems. Horses that are suspected of having gastric ulcers can be examined with a video endoscope to determine if ulcers are present.
Remember, damage to sensitive kidney cells, intestinal lining, and stomach lining occurs before horses show clinical signs to their owners. NSAIDs are extremely valuable drugs used in the management of pain and inflammation in the horse when used correctly. Understanding the danger of these drugs will make them even more valuable to your horseand to you.
Wendy Krebs, DVM is completing a postdoctoral internship in equine medicine, surgery, and reproduction at Bend Equine Medical Center (BEMC) in Bend, Oregon. She is a graduate of the Oregon State University College of Veterinary Medicine. Dr. Krebs can be reached at (541) 388-4006 or firstname.lastname@example.org. The BEMC website is www.bendequine.com