Phenylbutazone, or Bute, is the most common pain medication used in horses and is generally safe if used at the correct dosage. However, its most common side effect is a condition called right dorsal colitis, which is a potentially deadly inflammation of the right dorsal colon. Similar drugs such as Banamine can also cause right dorsal colitis, but Bute is more commonly reported, probably because it is used more commonly. The first signs noted with right dorsal colitis are colic and diarrhea although more severely affected horses will have depression, lethargy and a decreased appetite. Surprisingly, one study revealed horses less than 15 years old with an average of 6 years were the most affected, probably because younger horses are in competition and need Bute due to lameness. Ponies and foals are also more susceptible but overall, but it is unknown why some horses develop the disease and others do not at the same dosages.
However, if the horse is off feed and dehydrated, is sick or has another type of gastrointestinal disease, Bute toxicity is more likely to occur. The toxicity causes damage to the colon wall that can lead to loss of blood and protein. Treatment includes fluid therapy and sometimes intravenous plasma to increase the protein level, plus giving medication to coat the intestine. Surgery can be performed by removing the affected area of the right dorsal colon but it is difficult. This is a serious disease as up to 40 percent of affected horses usually die, so if you are giving your horse Bute, watch for any abnormal symptoms and if noted, stop the bute and call your vet. Realize Bute and Banamine can cause serious and deadly side effects so check with your vet before using these drugs, especially in sick or dehydrated horses.