Right Dorsal Colitis in Horses from Bute: Signs, Risks, and What To Do

By Dr Duncan Houston

Phenylbutazone, commonly called Bute, is one of the most useful pain and inflammation medications in equine practice. It can help a painful horse move, eat, stand, and recover more comfortably.

The danger is when Bute is treated like a routine stable supplement instead of a powerful medication. One of the most serious complications is right dorsal colitis, a potentially life threatening inflammatory and ulcerative disease of the right dorsal colon.

This matters because right dorsal colitis is not always obvious at the start. A horse may simply seem quieter, eat less, pass softer manure, or have mild recurring colic before the problem becomes severe.

Quick Answer

Right dorsal colitis is an inflammatory and ulcerative disease of the right dorsal colon, most commonly associated with NSAID use in horses, especially phenylbutazone. NSAID toxicity is more likely with high doses or prolonged use, but some horses can develop toxicity even at lower or recommended doses, particularly if they are dehydrated, unwell, not eating properly, or already have gastrointestinal disease. (MSD Veterinary Manual)

If a horse on Bute develops diarrhoea, soft manure, colic, depression, reduced appetite, weight loss, ventral swelling, fever, or a fast heart rate, stop guessing and contact a vet urgently.

What Is Right Dorsal Colitis?

Right dorsal colitis, often shortened to RDC, is inflammation, ulceration, and thickening of the right dorsal colon.

The right dorsal colon is part of the horse’s large colon. This area appears particularly sensitive to NSAID related injury. When the lining becomes damaged, the colon can leak protein, fluid, and inflammatory material into the gut. This can lead to low blood protein, diarrhoea, colic, dehydration, systemic inflammation, and in severe cases, life threatening deterioration. (MSD Veterinary Manual)

In practice, the danger is not just the diarrhoea. The real concern is that the horse may be developing a protein losing enteropathy, meaning protein is being lost through the damaged bowel wall.

How Does Bute Cause Right Dorsal Colitis?

Bute is an NSAID. NSAIDs reduce pain and inflammation by affecting cyclooxygenase pathways, but those pathways also help maintain normal gut protection and healing.

When protective prostaglandin activity is reduced, the gut lining becomes more vulnerable. The right dorsal colon can develop ulceration, oedema, inflammation, and thickening. MSD Veterinary Manual notes that NSAID injury can occur anywhere in the gastrointestinal tract, but the large colon, especially the right dorsal colon, and gastric mucosa appear especially sensitive in horses. (MSD Veterinary Manual)

The practical message is simple: Bute is useful, but the safety margin is not something to casually play with. Merck Veterinary Manual describes phenylbutazone as having a relatively narrow therapeutic index in horses, with dose adjustment recommended to the minimum needed to maintain comfort and avoid toxicity. (Merck Veterinary Manual)

Can Right Dorsal Colitis Happen at Normal Bute Doses?

Yes, it can.

The classic risk is too much Bute, too often, for too long. However, some horses appear more sensitive, and NSAID toxicosis can occur at lower than recommended doses in some patients. This is especially important in horses that are dehydrated, off feed, systemically unwell, already colicky, or already compromised. (MSD Veterinary Manual)

That is why the question is not only “what dose did the horse receive?”

The better questions are:

Was the horse eating normally?

Was the horse drinking normally?

Was the horse dehydrated?

Was the horse already colicky or systemically sick?

Was the horse receiving more than one NSAID?

How long had the medication been used?

Was bloodwork being monitored?

Those details often change the risk more than owners realise.

Early Signs of Right Dorsal Colitis

Right dorsal colitis can look vague at first. That is part of why it is dangerous.

Watch for:

1. Softer manure than normal

2. Watery diarrhoea

3. Intermittent colic

4. Recurrent mild abdominal discomfort

5. Reduced appetite

6. Depression or dullness

7. Weight loss

8. Ventral oedema, such as swelling under the belly

9. Fever

10. Increased heart rate

11. Dehydration

12. Poor response to ongoing pain relief

Clinical signs may appear days to weeks after NSAID therapy. More chronic cases can present with recurring colic, weight loss, and soft faeces rather than dramatic sudden collapse. (MSD Veterinary Manual)

How Worried Should You Be?

Lower Risk

The horse is bright, eating well, drinking normally, passing normal manure, and has only received a short, vet prescribed NSAID course at the correct dose.

This does not mean there is zero risk, but it is less concerning. The key is to use the medication exactly as directed and monitor appetite, manure, hydration, and attitude.

Moderate Risk

The horse is on repeated or longer term Bute, has softer manure, mild appetite changes, mild weight loss, or intermittent low grade colic.

This needs a vet check. Bloodwork, especially total protein and albumin, may be important because RDC can cause protein loss through the damaged colon.

High Risk

The horse has diarrhoea, recurring colic, depression, fever, a fast heart rate, ventral swelling, dehydration, or is not eating.

This should be treated as urgent. These signs suggest more than a mild medication side effect.

Critical Risk

The horse has severe diarrhoea, persistent colic, collapse, severe dehydration, toxic gum colour, marked weakness, or signs of shock.

This is an emergency. Referral to an equine hospital may be needed, especially if the horse requires intensive fluid therapy, plasma support, ultrasound monitoring, or advanced treatment.

Which Horses Are Most at Risk?

Right dorsal colitis can occur in any horse receiving NSAIDs, but risk increases when Bute is used in the wrong patient, at the wrong time, or for too long.

Higher risk situations include:

1. Long term Bute use for lameness, arthritis, laminitis, or chronic pain

2. High dose NSAID use

3. Accidental overdose

4. Combining NSAIDs, such as Bute and flunixin, without clear veterinary direction

5. Reduced appetite or poor feed intake

6. Dehydration

7. Colic or gastrointestinal disease

8. Systemic illness

9. Poor monitoring during ongoing treatment

10. Horses that seem unusually sensitive to NSAIDs

One of the biggest mistakes is giving Bute to a horse that is already not eating or drinking properly. That is exactly the type of horse where the gut and kidneys may already be vulnerable.

Bute, Banamine, and Other NSAIDs: Are They All the Same Risk?

No, but none should be treated casually.

Phenylbutazone is strongly associated with right dorsal colitis and has greater ulcerogenic potential in horses than flunixin or ketoprofen according to Merck Veterinary Manual. Firocoxib, a COX 2 selective NSAID, appears to have a safer gastrointestinal profile than some nonselective NSAIDs, but it is still an NSAID and still requires veterinary oversight. (MSD Veterinary Manual)

The practical rule: do not stack NSAIDs unless your vet has specifically directed it.

More pain relief is not always safer pain relief. In horses, combining NSAIDs can increase toxicity risk without giving a proportional safety benefit.

What Else Can Look Like Right Dorsal Colitis?

Right dorsal colitis is not diagnosed from symptoms alone. Diarrhoea, colic, weight loss, and depression can come from many different problems.

Important differentials include:

1. Infectious colitis, including Salmonella or clostridial disease

2. Larval cyathostominosis

3. Sand enteropathy

4. Antimicrobial associated diarrhoea

5. Dietary change or feed related colitis

6. Gastric ulcer disease

7. Large colon impaction

8. Right dorsal colon thickening or stricture

9. Peritonitis

10. Kidney injury associated with NSAID use

11. Severe systemic inflammatory disease

This is why the medication history matters so much. If a horse develops diarrhoea or recurring colic while receiving Bute, that detail should be mentioned immediately to the treating vet.

How Do Vets Diagnose Right Dorsal Colitis?

A vet will usually combine the history, clinical signs, physical examination, bloodwork, and imaging.

Important findings may include:

1. Recent or ongoing NSAID use

2. Diarrhoea or soft faeces

3. Recurrent colic

4. Low total protein

5. Low albumin

6. Dehydration

7. Electrolyte abnormalities

8. Thickening of the right dorsal colon on ultrasound

9. Evidence of systemic inflammation

10. Possible gastric ulceration or other gastrointestinal disease

MSD Veterinary Manual notes that a tentative diagnosis can be based on NSAID history, compatible clinical signs, and hypoproteinaemia. Ultrasound may detect colon thickening, and severe cases can show electrolyte and acid base abnormalities. (MSD Veterinary Manual)

The most important monitoring point for long term NSAID use is not just whether the horse still looks sore. It is whether the gut, hydration, kidneys, and blood protein are still coping.

When Is This an Emergency?

Treat this as urgent if a horse on Bute or another NSAID develops any of the following:

1. Diarrhoea

2. Recurrent colic

3. Depression or marked lethargy

4. Reduced appetite

5. Fever

6. Fast heart rate

7. Dehydration

8. Swelling under the belly

9. Weight loss

10. Signs of shock

11. Severe abdominal pain

12. Weakness or collapse

Do not keep adding more Bute because the horse still looks uncomfortable. If pain is persisting despite NSAID use, that is a reason to reassess the diagnosis, not simply increase the dose.

How Is Right Dorsal Colitis Treated?

Treatment depends on severity, but the first principle is usually stopping further NSAID exposure under veterinary direction.

Treatment may include:

1. Stopping Bute and avoiding further NSAIDs

2. IV fluids to correct dehydration and support circulation

3. Bloodwork monitoring, especially albumin, total protein, kidney values, and electrolytes

4. Plasma or colloid support in horses with significant protein loss

5. Gastrointestinal protectants such as sucralfate where appropriate

6. Prostaglandin analogue therapy such as misoprostol in selected cases

7. Dietary modification to reduce mechanical load on the colon

8. Ultrasound monitoring of right dorsal colon thickness

9. Hospitalisation for severe diarrhoea, systemic inflammation, or shock

10. Surgery in rare cases with severe colon damage, scarring, stricture, or complications

Dietary management can be a major part of treatment. MSD describes long term management using a low fibre complete pelleted ration fed several times daily with elimination of roughage, while the University of Melbourne equine guidelines describe avoiding further NSAID use, reducing stress, dietary modification, and targeted medication as core principles of RDC treatment. (MSD Veterinary Manual)

This is not a condition to manage casually at home. Severe colitis in adult horses can deteriorate quickly, and hospital level care may be needed.

What Is the Prognosis?

The prognosis depends on how early the disease is recognised, how severe the protein loss is, whether the horse is systemically unwell, and whether complications develop.

Mild cases recognised early may respond to stopping NSAIDs, dietary change, gut protectants, monitoring, and supportive care. Severe cases can be life threatening, particularly when diarrhoea, dehydration, fever, tachycardia, shock, or marked protein loss are present.

The University of Melbourne equine guidelines describe adult acute colitis as guarded overall, with reported mortality rates of 19 to 42 percent in retrospective studies, and note that right dorsal colitis may have a worse prognosis than some other causes of colitis. (Faculty of Science)

The takeaway: early recognition matters. Waiting until the horse is severely dehydrated, toxic, or profoundly protein depleted can dramatically reduce the chance of a good outcome.

Safer Bute Use: What Owners Should Know

Bute can be very appropriate when used correctly. The problem is unsafe use, repeated use without monitoring, or use in horses that are poor candidates for NSAIDs at that moment.

Safer use principles:

1. Use the lowest effective dose for the shortest appropriate duration

2. Never guess the dose

3. Do not combine NSAIDs unless your vet specifically instructs you

4. Do not use Bute in a dehydrated horse without veterinary guidance

5. Be very cautious if the horse is off feed

6. Monitor manure quality every day

7. Monitor appetite, water intake, attitude, and signs of colic

8. Arrange bloodwork for longer term use

9. Ask whether a COX 2 selective option is more appropriate for ongoing cases

10. Recheck the diagnosis if pain continues despite treatment

MSD specifically lists limiting NSAID dose and duration, considering COX 2 selective NSAIDs or alternative analgesic therapy, and monitoring faecal consistency and serum albumin as prevention strategies for RDC in horses receiving NSAIDs. (MSD Veterinary Manual)

Common Mistakes That Increase the Risk

Giving Bute because the horse “looks a bit sore”

Pain relief should have a reason, a dose, a plan, and a review point. Random repeat dosing is where problems start.

Using Bute when the horse is not eating

A horse that is off feed is already a higher risk patient. Adding NSAIDs without assessment can increase gastrointestinal and kidney risk.

Combining Bute and flunixin

This is one of the classic danger zones. Stacking NSAIDs can increase toxicity risk.

Ignoring soft manure

Soft manure in a horse on Bute is not just a messy stable problem. It can be an early warning sign.

Treating colic pain repeatedly without a diagnosis

If colic keeps returning, the horse needs reassessment. More pain relief can mask deterioration.

Not monitoring blood protein during long term use

Albumin can be a major clue. A horse can be losing protein through the colon before the owner fully appreciates how serious the problem is.

How To Monitor a Horse on Bute

If your horse is prescribed Bute, monitor the horse like the medication matters, because it does.

Daily checks should include:

1. Appetite

2. Water intake

3. Manure consistency

4. Number of droppings

5. Attitude

6. Gum colour

7. Signs of colic

8. Swelling under the belly

9. Weight or body condition

10. Whether pain is improving or worsening

For longer courses, ask your vet about bloodwork timing. Total protein, albumin, kidney values, hydration status, and electrolytes can help identify problems earlier.

Prevention: The Best Treatment Is Avoiding the Setup

Right dorsal colitis is often preventable.

The prevention mindset is not “never use Bute.” That is unrealistic. Bute is a valuable medication. The better mindset is “never use Bute casually.”

Preventive steps include:

1. Dose accurately based on body weight

2. Keep treatment duration as short as clinically reasonable

3. Recheck horses that need repeated pain relief

4. Avoid NSAIDs in dehydrated or anorexic horses unless specifically directed

5. Do not combine NSAIDs without a clear veterinary plan

6. Use bloodwork monitoring for longer term treatment

7. Consider alternative pain strategies where appropriate

8. Discuss COX 2 selective NSAIDs for some ongoing pain cases

9. Watch manure quality closely

10. Treat appetite changes seriously

In practice, the horses that worry me most are the ones getting repeated pain relief while the real problem is still unclear. Pain control is important, but it should never replace diagnosis.

FAQs

Can Bute cause fatal colitis in horses?

Yes. Bute can be associated with right dorsal colitis, which can become life threatening, especially if it causes severe diarrhoea, dehydration, protein loss, systemic inflammation, or shock.

How quickly can right dorsal colitis develop?

Signs can appear within days to weeks after NSAID therapy. Some horses develop vague early signs such as soft manure, reduced appetite, or intermittent colic before more severe diarrhoea or systemic illness appears. (MSD Veterinary Manual)

Is firocoxib safer than Bute?

Firocoxib is a COX 2 selective NSAID and is generally considered to have a safer gastrointestinal profile than some nonselective NSAIDs. However, it is still a medication that should be used under veterinary guidance, not as a casual substitute. (MSD Veterinary Manual)

Should I stop Bute if my horse develops diarrhoea?

Contact your vet urgently. In suspected NSAID related colitis, further NSAID exposure is usually avoided, but your vet should guide the safest pain control and treatment plan.

Can right dorsal colitis be treated?

Yes, some horses can recover with early recognition, stopping NSAIDs, dietary management, gut support, fluids, monitoring, and hospital care when needed. Severe cases have a guarded prognosis and may require intensive treatment.

Final Thoughts

Bute is not a bad medication. It is a powerful medication.

The danger comes when it is used without enough respect for dose, duration, hydration, appetite, gut health, and monitoring. Right dorsal colitis is one of the clearest examples of why equine pain relief should always be paired with clinical judgement.

If your horse is on Bute and develops diarrhoea, soft manure, colic, reduced appetite, depression, swelling under the belly, or weight loss, do not wait to see if it passes. Those signs deserve veterinary attention.

The safest approach is simple: use Bute carefully, monitor closely, and treat any gut changes early.

If you are unsure whether your horse’s symptoms are medication related, urgent, or safe to monitor, ASK A VET™ can help you make the next decision with clearer guidance.