Right Dorsal Colitis in Horses: NSAID-Associated Colon Injury

Right dorsal colitis is inflammation, ulceration, and sometimes scarring of the right dorsal colon, a section of the large colon in the horse. It is most strongly associated with nonsteroidal anti-inflammatory drugs (NSAIDs), especially when they are given at high doses, for too many days, or in combinations your vet did not intend. The condition can range from mild irritation to deep ulceration, protein loss, and life-threatening colitis.

NSAIDs help control pain and inflammation, but they also reduce protective prostaglandins in the gut. In some horses, that loss of protection can injure the colon lining and allow inflammation, edema, and ulcer formation. Merck notes that the large colon, especially the right dorsal colon, is one of the most sensitive sites for NSAID injury in horses.

This is not always an overdose problem. Some horses develop right dorsal colitis even when they received recommended doses. That is one reason any horse on NSAIDs should be monitored closely for appetite changes, manure changes, swelling under the belly, or signs of colic.

The good news is that many horses improve with early recognition and a treatment plan tailored by your vet. Recovery often depends on how quickly the drug is stopped, how severe the colon injury is, and whether complications like low albumin, dehydration, or impaction have developed.

The main cause is NSAID-associated injury to the colon lining. Common equine NSAIDs include phenylbutazone, flunixin meglumine, and firocoxib. These drugs reduce prostaglandin production. That helps with pain and inflammation, but prostaglandins also support blood flow, mucus production, and normal barrier function in the gastrointestinal tract. When that protection drops too far, the colon becomes more vulnerable to ulceration and inflammation.

Risk rises when a horse receives too much NSAID, NSAIDs for too many consecutive days, or more than one NSAID at the same time. Dehydration, poor perfusion, stress, and pre-existing illness may also make the gut more susceptible. Merck and retrospective case data both support that some horses can still develop right dorsal colitis at recommended doses, so individual sensitivity matters too.

Phenylbutazone is classically associated with this condition, but it is not the only drug involved. Flunixin and other NSAIDs can also contribute, especially if dosing instructions are not followed exactly. Giving extra doses because a horse still looks sore can unintentionally push the colon past its safety margin.

In practical terms, right dorsal colitis is often the result of a well-intended pain-control plan that became too aggressive for that horse's colon. That is why your vet may recommend the lowest effective dose, the shortest effective duration, and close monitoring rather than routine repeat dosing.

Diagnosis starts with a careful history. Your vet will want to know which NSAID was used, the dose, how often it was given, for how many days, and whether any other pain medicines were used at the same time. That medication history matters because right dorsal colitis can look like other causes of colic, diarrhea, weight loss, or low protein.

Bloodwork is often one of the first steps. Horses with right dorsal colitis commonly have low total protein and low albumin, and some also show inflammatory changes or dehydration. Merck specifically notes that monitoring fecal consistency and serum albumin concentrations is a practical way to detect developing disease in horses receiving NSAIDs.

Abdominal ultrasound can be very helpful. In horses with colitis, Merck describes a thickened colon wall greater than 4 mm, and studies of right dorsal colitis report more marked thickening in affected horses. Ultrasound does not diagnose every case by itself, but when it matches the history and lab work, it can strongly support the diagnosis.

Your vet may also use rectal examination, fecal testing, abdominal fluid analysis, or additional imaging to rule out other causes of colitis and colic. In severe or chronic cases, the workup may also focus on complications such as protein-losing enteropathy, dehydration, or large-colon impaction from scarring.

Prevention centers on careful NSAID use. Give these drugs only as your vet directs, at the exact dose and frequency prescribed. Do not combine NSAIDs unless your vet specifically tells you to, and do not extend treatment because a horse still seems uncomfortable. If pain is not controlled, the safer move is to call your vet and reassess the plan.

Hydration matters. Horses that are dehydrated, off feed, systemically ill, or already dealing with kidney or gastrointestinal disease may have less margin for NSAID safety. Before and during treatment, let your vet know if your horse is drinking poorly, has loose manure, or is showing any change in appetite or attitude.

For horses needing longer-term pain management, ask your vet about monitoring plans and alternative strategies. That may include periodic bloodwork, watching manure consistency closely, and using the lowest effective dose for the shortest practical time. Merck specifically recommends monitoring fecal consistency and serum albumin in horses receiving NSAIDs.

The most important prevention step is communication. Keep a written medication log, especially in barns where more than one person gives treatments. Many cases happen when doses overlap, a second NSAID is added, or treatment continues longer than intended. A clear plan with your vet helps protect both comfort and colon health.