The large colon is a huge organ and very important in fermentation, digestion and absorption of roughage. It also plays a large role in water uptake and balance.

Generally, NSAID drugs have a negative effect on the health of the lining of intestine. These drugs, through their primary mechanism of action, reduce the production of "prostaglandin" chemicals in the body.
Most prostaglandins are products of inflammation, causing pain and inflammatory effects like swelling and attraction of toxic infection fighting cells to an inflamed area. NSAID drugs like bute, flunixin (Bamanime) and firocoxib (Equioxx) help reduce the amount of these prostaglandins produced and thus reduce the signs of disease like pain and swelling.

But there are also "beneficial prostaglandins" that are normally produced by the body, and that have PROTECTIVE effects on the intestine and kidney in particular. The problem is that many of the commonly used NSAIDS (like bute and flunixin (Banamine)) eliminate these along with the more damaging ones.

"Non-selective" NSAIDS like bute and Banamine reduce the levels of all the prostaglandins, so the effects are a mixed bag of mostly good, but some bad. The effect of overdose of these drugs is most dramatically seen in the right dorsal colon.

In horses that have had too much NSAID (especially phenylbutazone), Right Dorsal Colitis develops. In this condition, the right dorsal colon lining is swollen, thickened, inflamed and can become riddled with ulcers. This causes colic, protein loss, weight loss, and diarrhea, and is a life threatening condition.

The condition is usually diagnosed by clinical signs and a history of bute at high doses but some horses develop this condition without a history of NSAID overuse. Like people, horses vary in their sensitivity to all drugs. What seems to be a safe dose in one horse may cause toxicity in another.

Diagnosis is made by a combination of a history of NSAID drug administration, physical exam findings, diarrhea, and blood chemistry results. Ultrasound is used in some cases to visualize the thickened wall of the right dorsal colon.

Treatment depends on severity and chronicity of disease. Some acutely (suddenly) affected horses require emergency drugs like IV fluids, plasma and intensive care. For others more chronically affected, the primary consideration becomes nutrition to lessen the load on the colon, and stoppage of any NSAIDs.