Bute Side Effects in Horses: Warning Signs, Right Dorsal Colitis and Safe Use

By Dr Duncan Houston

Phenylbutazone, commonly called bute, is one of the most familiar medications in horse care. That familiarity can make it seem safer than it is.

Bute can be extremely effective for musculoskeletal pain and inflammation, but it is still a prescription non-steroidal anti-inflammatory drug. Incorrect dosing, prolonged treatment, dehydration or combining it with another anti-inflammatory can lead to gastrointestinal ulceration, right dorsal colitis, protein loss and kidney injury.

The dangerous mistake is not always one enormous overdose. It may be several slightly excessive doses, an inaccurately estimated body weight, two people medicating the same horse, or continuing treatment while the horse is dehydrated and eating poorly. (DailyMed)

Quick Answer

Bute can cause oral and gastric ulcers, right dorsal colitis, low blood protein, oedema, kidney injury and rare blood or liver abnormalities. Risk increases with excessive or prolonged dosing, dehydration, pre-existing disease, or combining bute with Banamine, Equioxx or corticosteroids.

Do not give another dose until you have spoken with your veterinarian if your horse develops reduced appetite, colic, diarrhoea, abnormal manure, mouth ulcers, lethargy, swelling or changes in urination. Suspected overdose, severe colic, profuse diarrhoea, collapse or markedly reduced urine production should be treated as an emergency. (DailyMed)

What Is Bute Used for in Horses?

Phenylbutazone is an NSAID used mainly to control pain and inflammation associated with the musculoskeletal system.

Veterinarians may prescribe it for conditions such as:

• Osteoarthritis

• Hoof pain

• Laminitis-associated pain

• Tendon or ligament inflammation

• Muscular or joint injuries

• Postoperative musculoskeletal discomfort

The United States label for one approved oral phenylbutazone paste specifically indicates it for inflammatory conditions affecting the musculoskeletal system. It is not a harmless general-purpose painkiller, and it should not be given “just in case” whenever a horse appears uncomfortable. (DailyMed)

Pain that repeatedly returns despite bute may mean the underlying diagnosis has not been addressed. Increasing the dose can temporarily hide the problem while increasing the risk of toxicity.

How Does Bute Work?

Phenylbutazone reduces pain and inflammation by inhibiting cyclooxygenase enzymes involved in producing prostaglandins.

Some prostaglandins contribute to pain, fever and inflammation. Others help maintain:

• Blood flow to the stomach and intestines

• Mucus and bicarbonate protection within the gastrointestinal tract

• Repair of the intestinal lining

• Kidney blood flow during dehydration or low blood pressure

• Normal platelet and vascular functions

This creates the central trade-off with NSAIDs. The same mechanism that reduces inflammation can also reduce the horse’s natural protection against gastrointestinal and kidney injury. Experimental work has shown that phenylbutazone can impair protective bicarbonate secretion and barrier function within the equine gastrointestinal tract. (PubMed)

What Are the Main Side Effects of Bute?

The most important potential complications include:

Gastrointestinal ulceration

Phenylbutazone may contribute to ulcers affecting the:

• Mouth and tongue

• Oesophagus

• Glandular stomach

• Small intestine

• Large colon

A controlled study found that phenylbutazone increased gastric ulceration scores and altered intestinal permeability and faecal microbiota in otherwise healthy horses. (PubMed)

Right dorsal colitis

Right dorsal colitis is inflammation and ulceration affecting the upper right portion of the horse’s large colon.

It is one of the most serious complications associated with bute and other NSAIDs.

Protein loss

A damaged colon may allow albumin and other proteins to leak into the intestinal tract. As blood albumin falls, fluid can move out of blood vessels and collect beneath the abdomen, around the sheath or in the lower limbs.

Kidney injury

NSAIDs can reduce the kidney’s ability to maintain blood flow when the horse is dehydrated, hypovolaemic or systemically unwell.

Rare blood or liver abnormalities

Official phenylbutazone labelling advises stopping treatment if gastrointestinal upset, jaundice or evidence of a blood dyscrasia develops. Black or tarry manure is also listed as a reason for immediate cessation and veterinary intervention. (DailyMed)

What Is Right Dorsal Colitis?

Right dorsal colitis is an NSAID-associated protein-losing enteropathy affecting the right dorsal colon.

The damaged colon becomes inflamed, thickened, ulcerated and less capable of maintaining its normal barrier. Protein and fluid may be lost into the bowel, while bacterial products and inflammatory mediators can cross the compromised intestinal lining.

Signs can range from mild and frustratingly vague to rapidly life-threatening.

Possible signs include:

• Reduced appetite

• Depression or lethargy

• Soft manure or diarrhoea

• Recurrent low-grade colic

• Weight loss

• Fever

• Increased heart rate

• Swelling beneath the abdomen

• Swelling of the limbs or sheath

• Dehydration

• Low blood albumin

• Laminitis

• Shock

Diarrhoea is not present in every case. A horse may initially show only reduced appetite, intermittent colic, weight loss or unexplained low albumin. (Wiley Online Library)

Severe ulceration may eventually heal with fibrosis and narrowing of the colon. This can create a stricture that causes recurrent obstruction and colic even after the active inflammation has settled. (PubMed)

How Common Is Right Dorsal Colitis?

The true incidence among all horses receiving phenylbutazone is unknown. Most studies describe hospitalised horses that were already affected, rather than following thousands of routinely treated horses.

A 2023 Australian multicentre study evaluated 35 horses diagnosed with right dorsal colitis. Where the NSAID dose was known, 84% had received an excessive dose or duration, and phenylbutazone was the most commonly used drug. However, four of the 25 horses with complete dosing information developed the condition despite receiving doses classified as recommended. (Wiley Online Library)

The study reported a 43% case fatality rate. That does not mean 43% of horses given bute will die. It reflects a selected population of horses sick enough to reach referral hospitals with established disease.

The practical message is that right dorsal colitis is uncommon but potentially severe. Correct dosing greatly reduces risk, but it cannot make the risk zero.

Can Bute Cause Damage at the Recommended Dose?

Yes, although severe clinical toxicity is more strongly associated with excessive dosing, prolonged treatment and additional risk factors.

A controlled 2024 study found that horses receiving a clinical phenylbutazone regimen for seven days developed:

• Lower albumin and total protein

• Increased right dorsal colon wall thickness

• Higher gastric ulcer scores

• Altered faecal pH

Importantly, these horses did not necessarily show obvious outward illness. (PubMed)

This does not mean every correctly treated horse will develop clinically significant disease. It means that a horse can appear normal while measurable gastrointestinal changes are beginning.

Older research also documented right dorsal colitis in horses receiving doses and treatment durations that were not considered unusually high. (PubMed)

Which Horses Are at Greater Risk?

Horses receiving too much bute

Overdose may occur because:

• Body weight was guessed incorrectly

• A pony received a full-sized horse dose

• A powder scoop was assumed to equal a specific dose

• More than one person administered medication

• The horse spat out part of a dose and was redosed

• Pain returned and the owner gave an additional dose

• The starting dose was continued for too long

• Different bute formulations were confused

A scoop is not a dose until the amount in grams and the horse’s body weight are both known.

Dehydrated horses

Dehydration is a major concern because prostaglandins help maintain kidney perfusion when circulating blood volume is reduced.

In an experimental study, horses receiving therapeutic phenylbutazone while deprived of water developed acute renal papillary necrosis, whereas horses receiving phenylbutazone without water deprivation did not develop the same lesion. (Sage Journals)

Extra caution is required when a horse:

• Has diarrhoea

• Is colicking

• Is not drinking

• Is sweating heavily

• Has recently undergone surgery

• Has blood loss or shock

• Is receiving a diuretic

• Is systemically unwell

Horses receiving another NSAID

Bute should not casually be combined with:

• Flunixin, including Banamine or Finadyne

• Firocoxib, including Equioxx

• Meloxicam

• Ketoprofen

• Aspirin

• Diclofenac

Giving two different NSAIDs does not create two separate safety pathways. Their effects on protective prostaglandins overlap.

In one study comparing phenylbutazone alone with phenylbutazone plus flunixin, one horse receiving the combined regimen died from acute necrotising colitis. Another small study found that combined phenylbutazone and firocoxib produced changes concerning for renal disease. (PubMed)

Horses receiving corticosteroids

Combining an NSAID with a systemic corticosteroid can increase gastrointestinal and kidney risk. Current equine NSAID labelling advises avoiding concurrent treatment and using an appropriate veterinarian-directed washout period when switching between NSAIDs and corticosteroids. (FDA.report)

Relevant corticosteroids include:

• Dexamethasone

• Prednisolone

• Triamcinolone

• Betamethasone

Even an intra-articular corticosteroid may have systemic absorption, so your veterinarian needs to know about recent joint injections as well as oral or injectable medications.

Horses with pre-existing disease

Risk may be higher in horses with:

• Previous gastric or intestinal ulceration

• Previous NSAID intolerance

• Kidney disease

• Liver disease

• Cardiovascular compromise

• Low blood protein

• Chronic diarrhoea

• Poor appetite

• Significant systemic illness

• Reduced hydration

A horse that reacted badly to one NSAID may also react to another. Changing the brand name does not guarantee safety.

What Are the Earliest Warning Signs?

Early toxicity is often less dramatic than owners expect.

Watch for:

• Leaving part of a meal

• Eating hay more slowly

• Becoming quieter or less interactive

• Soft manure that still holds some shape

• Reduced manure production

• Mild intermittent pawing or flank watching

• Drinking less

• Reduced urination

• Mouth or tongue sores

• Mild swelling under the abdomen

• Unexplained weight loss

The mistake I see most often is waiting for profuse diarrhoea. By that stage, gastrointestinal injury may already be advanced.

A change in appetite or manure during NSAID treatment deserves attention, even when the horse is still standing quietly and appears reasonably comfortable.

How Worried Should You Be?

Lower risk

The horse is:

• Receiving one NSAID exactly as prescribed

• Bright and eating normally

• Drinking and urinating normally

• Passing normal manure

• Well hydrated

• On a planned short course

• Showing the expected improvement

Continue according to the veterinary plan and check appetite, water intake, manure and behaviour at least daily.

Moderate concern

The horse:

• Is eating slightly less

• Seems unusually quiet

• Has developed softer manure

• Has been receiving bute beyond the original treatment period

• Has recently travelled, sweated heavily or drunk less

• Is not improving as expected

Contact the prescribing veterinarian the same day, ideally before giving the next dose.

High concern

The horse has:

• Recurrent colic

• Diarrhoea

• Mouth or tongue ulcers

• Swelling beneath the abdomen or limbs

• Noticeable weight loss

• Reduced urination

• Dark or tarry manure

• A suspected overdose

• Received two NSAIDs

• Received bute and a corticosteroid

Do not administer another dose. Arrange urgent veterinary assessment.

Critical

Treat the situation as an emergency if the horse develops:

• Severe or persistent colic

• Profuse watery diarrhoea

• Weakness or collapse

• Markedly abnormal gums

• A rapid resting heart rate

• Little or no urine production

• Severe dehydration

• Signs of shock

• Signs of laminitis

• Rapid deterioration over several hours

When Is Bute Toxicity an Emergency?

Call your veterinarian or nearest equine hospital immediately when:

• The horse has received a major accidental overdose

• Several doses may have been duplicated

• Bute has been combined with another NSAID

• The horse is severely dehydrated

• Colic is persistent or escalating

• Diarrhoea is profuse

• The gums are very pale, dark red, purple or tacky

• The horse becomes weak, uncoordinated or unable to stand

• Urine production is markedly reduced

• Swelling is rapidly developing

• The horse appears painful despite medication

• Laminitis signs develop

Do not wait for the horse to “sleep it off.” Serious gastrointestinal and kidney complications can worsen quickly once dehydration, protein loss and systemic inflammation develop.

What Should You Do If You Suspect a Side Effect or Overdose?

1. Do not give another dose

Withhold further phenylbutazone until you have spoken with the veterinarian.

Do not replace it with Banamine, Equioxx, meloxicam or another anti-inflammatory.

2. Gather the medication information

Have the following ready:

• Product name

• Active ingredient

• Strength

• Paste, powder, tablet or injectable formulation

• Amount administered

• Time of every dose

• Horse’s body weight

• Reason for treatment

• All other medications and supplements

• Whether another person may have administered a dose

Keep the container or packet. Photograph the label if the veterinarian is assessing the horse remotely.

3. Check the horse

When safe, record:

• Heart rate

• Respiratory rate

• Temperature

• Gum colour

• Capillary refill time

• Appetite

• Water intake

• Manure production

• Urination

• Signs of colic

• Swelling beneath the abdomen or limbs

4. Keep clean water available

Do not deliberately restrict water. Dehydration increases concern for kidney injury.

Do not force feed a horse that is actively colicking. Follow your veterinarian’s instructions regarding feeding, walking and transport.

5. Monitor for worsening signs

Changes over the next several hours matter.

Escalate immediately if the horse develops increasing pain, watery diarrhoea, weakness, abnormal gums, reduced urine production or rapid deterioration.

Could These Signs Be Caused by Something Else?

Yes.

Colic, diarrhoea, reduced appetite, weight loss and low albumin are not specific to phenylbutazone toxicity.

Your veterinarian may also need to rule out:

• Infectious colitis

• Salmonellosis or clostridial disease

• Equine coronavirus

• Parasitism

• Sand accumulation

• Inflammatory bowel disease

• Intestinal lymphoma

• Liver disease

• Kidney disease

• Primary gastric ulceration

• Another surgical or medical cause of colic

• Other protein-losing intestinal diseases

A recent bute history raises suspicion, but it should not end the diagnostic process. The 2023 multicentre right dorsal colitis study required clinicians to consider and exclude other explanations for diarrhoea and colic before accepting presumptive cases. (Wiley Online Library)

How Do Veterinarians Diagnose Bute Toxicity?

Diagnosis usually combines the medication history, physical examination, blood results and abdominal imaging.

Medication history

The veterinarian will calculate:

• Milligrams or grams administered

• Dose per kilogram

• Total daily exposure

• Number of treatment days

• Whether different NSAIDs overlapped

• Whether corticosteroids or nephrotoxic drugs were used

• Whether the horse was dehydrated

Blood testing

Testing may include:

• Packed cell volume

• White blood cell count

• Total protein

• Albumin

• Creatinine

• Blood urea nitrogen

• Electrolytes

• Calcium

• Lactate

• Liver-related values

Albumin is particularly important. In the 2023 right dorsal colitis study, hypoalbuminaemia was more common than general hypoproteinaemia, meaning total protein alone may miss some affected horses. (Wiley Online Library)

Urinalysis

Urine concentration, protein, blood, casts and other abnormalities may help assess hydration and kidney injury.

Abdominal ultrasound

The right dorsal colon can often be examined through the right side of the abdomen.

Increased wall thickness or oedema supports the diagnosis, but a normal ultrasound does not completely rule out early disease. In the multicentre study, approximately one-quarter of examined cases did not have visible right dorsal colon thickening. (Wiley Online Library)

Gastroscopy

Gastroscopy can identify concurrent gastric ulceration.

It cannot directly evaluate the right dorsal colon. A horse may have significant colonic disease even when gastric findings are mild.

How Is Right Dorsal Colitis Treated?

Treatment depends on severity and may include:

• Immediate discontinuation of all NSAIDs

• Intravenous fluid therapy

• Correction of electrolyte abnormalities

• Plasma or other protein support when albumin is severely reduced

• Carefully selected non-NSAID analgesia

• Dietary modification

• Gastrointestinal protectants selected by the veterinarian

• Monitoring for endotoxaemia

• Laminitis prevention and monitoring

• Repeated blood testing and abdominal ultrasound

Some mild cases respond to medication withdrawal, supportive care and dietary management. Severe cases may require prolonged hospitalisation, intensive fluid and protein support, or surgery when a stricture or irreversibly damaged section of colon develops. (PubMed)

What Is the Prognosis?

The prognosis depends heavily on how early the problem is recognised.

A horse with mild appetite changes and early laboratory abnormalities may recover well when treatment is stopped promptly and supportive care begins.

The prognosis becomes more guarded when there is:

• Severe diarrhoea

• Persistent tachycardia

• Marked dehydration

• Abnormal gums

• Very low albumin

• High lactate

• Kidney injury

• Laminitis

• Colonic stricture

• Shock

In the Australian multicentre study, a higher heart rate, increased packed cell volume and abnormal mucous membranes at presentation were associated with lower survival. (Wiley Online Library)

How Long Can a Horse Safely Take Bute?

There is no universal number of days that is safe for every horse.

One current United States oral-paste label directs veterinarians to use a higher initial dose for no more than the first 48 hours, then reduce gradually to the lowest effective dose. It also advises reassessing the diagnosis and treatment plan when there is no meaningful response within five days. Those instructions are product-specific and should not be copied into a home dosing plan without veterinary direction. (DailyMed)

There is no magical safety boundary at day five or day seven. Gastrointestinal changes can begin earlier, while some horses with chronic disease may receive longer treatment under close veterinary supervision.

Longer or repeated courses may justify:

• Baseline blood testing

• Repeat albumin and total protein

• Kidney and liver testing

• Urinalysis

• Regular weight assessment

• Review of appetite and manure

• Reassessment of whether bute remains necessary

The goal is the lowest effective dose for the shortest appropriate duration, not simply the shortest duration regardless of the horse’s pain.

Can You Give Banamine After Bute?

Not unless your veterinarian has specifically instructed you to do so.

Bute and Banamine remain active in the body beyond the moment they are administered. Giving one in the morning and the other later that evening may still create significant overlap.

If pain returns before the next planned dose, contact your veterinarian. Recurrent pain may indicate that:

• The diagnosis needs reassessment

• The condition is worsening

• A surgical problem is present

• The pain plan is inadequate

• A different class of analgesic is needed

Adding another NSAID at home is not a safe substitute for reassessment.

Is Firocoxib Safer Than Bute?

Firocoxib is more selective for COX-2 than phenylbutazone and may cause less gastrointestinal injury in some horses.

A controlled study found that both firocoxib and phenylbutazone produced evidence of gastrointestinal injury, but the changes were less severe in the firocoxib-treated horses at the doses evaluated. (PubMed)

The correct conclusion is:

Firocoxib may be a lower-risk option for selected horses, but it is not gut-safe or kidney-safe in every horse.

It must still be correctly dosed and should not overlap with bute. Because firocoxib remains in the body for an extended period, switching medications requires a veterinarian-directed washout plan rather than simply waiting until the following morning. (FDA.report)

Does Omeprazole Prevent Bute Side Effects?

Omeprazole may reduce acid-related gastric injury, but it does not protect the right dorsal colon or kidneys.

A small controlled study found that omeprazole reduced phenylbutazone-associated glandular gastric disease but was also associated with more intestinal complications in the combined-treatment group. The authors advised caution when prescribing the two drugs together. (PubMed)

This does not mean omeprazole should never be used with an NSAID. It means owners should not automatically add it as an insurance policy.

Omeprazole does not:

• Prevent right dorsal colitis

• Prevent kidney injury

• Make NSAID stacking safe

• Correct an overdose

• Allow appetite or manure changes to be ignored

The decision should be based on the individual horse, its gastric findings, diet, management and overall gastrointestinal risk.

What Are the Alternatives to Long-Term Bute?

A horse that repeatedly requires bute deserves a diagnosis-focused pain plan.

Depending on the condition, that plan may involve:

• Controlled rehabilitation

• Exercise modification

• Weight management

• Corrective trimming or shoeing

• Changes in footing

• Joint medication

• Local therapies

• Shockwave therapy

• Targeted surgery

• Regenerative treatments in selected cases

• A different veterinarian-supervised analgesic strategy

The answer is not necessarily to remove effective pain relief. Uncontrolled pain also damages welfare.

The aim is to reduce reliance on daily systemic medication by treating the source of pain wherever possible.

Common Bute Mistakes

Guessing the horse’s weight

Ponies, miniature horses and lightly built horses are especially vulnerable to accidental overdosing when owners use a standard “horse dose.”

Assuming every scoop contains the same amount

Different products and measuring devices can contain different quantities. Check the concentration and grams, not simply the number of scoops.

Allowing several people to medicate

Duplicate doses are easily given when owners, riders and stable staff do not use a written medication chart.

Alternating bute and Banamine

Alternating the names does not prevent the drugs from overlapping in the body.

Continuing after appetite or manure changes

A slightly quieter horse with softer manure may be showing the first stage of intolerance.

Using omeprazole as a protective shield

Omeprazole does not make unrestricted bute administration safe.

Reaching for bute whenever the horse is sore

Repeated pain needs a diagnosis. Bute can mask the sign without correcting the cause.

Continuing because the horse has tolerated it before

Previous tolerance does not guarantee that the horse is hydrated, healthy or equally tolerant during the next course.

How to Reduce the Risk of Bute Complications

• Use phenylbutazone only under veterinary direction

• Confirm the horse’s body weight

• Confirm the product strength and amount in grams

• Nominate one person to administer medication

• Record every dose and time

• Never combine NSAIDs without explicit instructions

• Disclose recent corticosteroid treatment or joint injections

• Maintain access to clean water

• Reassess treatment when appetite or water intake falls

• Use the lowest effective dose

• Do not extend the course without veterinary review

• Monitor appetite, manure, behaviour, swelling and urination daily

• Arrange blood testing when treatment is prolonged or risk is increased

• Reassess the diagnosis if pain is not responding

Frequently Asked Questions

Can a recommended dose of bute cause right dorsal colitis?

Yes. Most recognised cases involve excessive doses, prolonged treatment or additional risk factors, but right dorsal colitis has also been reported in horses receiving recommended regimens. Individual susceptibility varies. (Wiley Online Library)

How quickly can bute side effects begin?

Measurable gastrointestinal changes can begin within several days, sometimes before the horse shows obvious illness. Appetite, manure, water intake and behaviour should be monitored from the first dose. (PubMed)

Should I give the next dose if my horse has soft manure?

Contact the prescribing veterinarian before administering it. Soft manure may have another cause, but it can also be an early sign of NSAID intolerance.

Can I give Equioxx the day after stopping bute?

Do not switch without veterinary instructions. The appropriate washout period depends on the dose, duration, horse’s health and the pharmacology of both drugs.

Can a horse take bute every day for arthritis?

Some horses receive longer courses under veterinary supervision, but daily treatment requires an individual risk assessment and periodic review. The dose should not be continued indefinitely simply because it initially improved the lameness.

Final Thoughts

Bute remains one of the most useful medications in equine practice. Used correctly, it can significantly improve comfort, mobility and welfare.

The problem is not the existence of the drug. It is treating it as though familiarity equals safety.

Measure the dose accurately. Never combine anti-inflammatory drugs casually. Keep the horse hydrated. Watch appetite, manure, urination and behaviour from the beginning of treatment. Take subtle changes seriously rather than waiting for severe diarrhoea or collapse.

Most importantly, when pain is not responding as expected, reassess the horse instead of simply increasing the medication. The returning pain is information, not an invitation to reach deeper into the bute tub.

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Use ASK A VET™ to record your horse’s medication, dose times, appetite, water intake, manure changes and veterinary instructions. If you are uncertain whether a reaction is urgent, tailored veterinary guidance can help you decide what to do next.