Why Does My Horse Have Mouth Ulcers?

By Dr Duncan Houston

When a horse suddenly goes off feed, drops hay, drools, resists the bit, or looks painful around the mouth, oral ulcers should be on the list.

Mouth ulcers are not just a small surface problem. They can make chewing and swallowing painful, reduce water intake, cause weight loss, and sometimes point to something more serious than a sore tongue. The cause may be simple, such as sharp plant awns in hay, or more concerning, such as dental disease, chemical irritation, medication injury, choke, vesicular stomatitis, or another infectious or systemic condition.

The key is not just spotting the ulcer. It is working out why it is there, whether your horse can still eat and drink safely, and when the problem needs urgent veterinary attention.

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Quick Answer

Oral ulcers in horses can be caused by sharp forage such as foxtail or sandbur awns, dental disease, bit or foreign body trauma, chemical irritation, some medication problems, and infectious diseases such as vesicular stomatitis. A horse with mouth ulcers should be checked by a vet if they are not eating, drooling heavily, losing weight, bleeding, feverish, showing nasal discharge, struggling to swallow, or if lesions are also present on the muzzle, coronary bands, teats, sheath, or other animals on the property. Merck Veterinary Manual notes that stomatitis in horses can cause frothy drooling, reluctance to eat, difficulty swallowing, and resistance to mouth examination. (Merck Veterinary Manual)

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What Are Oral Ulcers in Horses?

Oral ulcers are breaks or erosions in the lining of the mouth.

They may affect the:

• Tongue

• Gums

• Lips

• Cheeks

• Bars of the mouth

• Roof of the mouth

• Corners of the lips

• Dental pad in cattle, if livestock are also affected

In horses, inflammation of the mouth is called stomatitis. It can happen after trauma, chemical irritation, sharp plant material, embedded foreign bodies, dental disease, and some infectious diseases. Merck Veterinary Manual specifically lists sharp awns from plants such as barley, foxtail, porcupine grass, and spear grass as causes of severe stomatitis in horses. (Merck Veterinary Manual)

The practical point is simple: an ulcer is a sign, not a diagnosis.

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What Do Mouth Ulcers Look Like?

Mouth ulcers can look different depending on the cause.

You may see:

• Red raw patches

• White or yellow-grey shallow sores

• Bleeding areas

• Swollen gums

• Sores along the tongue edges

• Ulcers inside the lips or cheeks

• Embedded grass awns or plant material

• Crusting or erosions around the lips

• Foul smell from the mouth

• Feed packed between the cheek and teeth

Sometimes owners never see the ulcers directly. Instead, they notice behaviour changes first.

Common signs include:

• Dropping feed

• Slow chewing

• Quidding, where partially chewed feed is dropped

• Drooling or frothing

• Reluctance to eat hay or hard feed

• Refusing treats

• Bad breath

• Weight loss

• Head tilting while chewing

• Bit resistance or head tossing

• Blood-tinged saliva

• One-sided nasal discharge if dental infection or sinus involvement is present

MSD Veterinary Manual lists difficulty or slowness in feeding, quidding, excessive drooling, bad breath, blood-tinged mucus, facial or jaw swelling, and one-sided nasal discharge as signs that may occur with equine dental disease. (MSD Veterinary Manual)

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How Worried Should You Be?

The severity depends on whether your horse can eat, drink, swallow, and stay comfortable.

Severity	What It Looks Like	What It May Mean	What To Do
Mild	Small sore, horse bright, eating reasonably well, no fever, no nasal discharge	Minor trauma, early forage irritation, mild bit rub	Remove obvious irritants, offer softer feed, monitor closely
Moderate	Drooling, slow chewing, dropping feed, multiple ulcers, reduced appetite	Painful oral disease, sharp forage, dental problem, medication irritation	Arrange a veterinary exam soon
Severe	Refusing feed, losing weight, foul smell, bleeding, facial swelling, marked pain, one-sided nasal discharge	Dental infection, deep trauma, foreign body, severe stomatitis, sinus involvement	Call your vet promptly
Critical	Mouth ulcers with fever, blisters, lesions on muzzle or coronary band, feed from nostrils, trouble swallowing, weakness, severe dehydration, multiple animals affected	Vesicular stomatitis, choke, toxicity, serious infection, neurologic disease, or another urgent condition	Treat as urgent and seek veterinary care immediately

The real concern is not just the ulcer. The concern is whether the horse is still able to chew, swallow, drink, and maintain hydration.

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Common Causes of Oral Ulcers in Horses

Mouth ulcers have several possible causes. Some are obvious once you look carefully. Others need sedation, a full oral exam, dental instruments, or diagnostic testing.

The most common categories include:

• Sharp forage or grass awns

• Dental disease

• Bit, bridle, or foreign body trauma

• Medication or chemical irritation

• Infectious disease

• Toxic plants or irritants

• Less common immune-mediated, systemic, or neoplastic disease

In practice, the history often gives the biggest clue. A new hay batch, recent medication, new bit, dental procedure, regional vesicular stomatitis outbreak, or several affected horses on the same feed can change the likely diagnosis quickly.

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Foxtail, Sandbur, and Sharp Grass Awns

Sharp plant material is one of the classic causes of mouth ulcers in horses.

Foxtail, sandbur, ticklegrass, barley awns, and similar seed heads can embed in the lips, gums, tongue, and cheeks. The barbed material can irritate the mouth every time the horse chews. University of Minnesota Extension warns not to feed hay containing moderate amounts of foxtail, sandbur, or ticklegrass seed heads because they can cause trauma to the mouth and gut of horses. (University of Minnesota Extension)

This is especially common when problem plants are baled into hay.

Clues include:

• Several horses affected after a new hay batch

• Ulcers on the lips, gums, or tongue

• Reluctance to eat hay but willingness to eat softer feed

• Drooling

• Head shaking while chewing

• Visible seed heads or sharp awns in hay

• Mouth pain that improves when the hay is changed

The first step is to stop feeding the suspect hay. If awns are embedded, your vet may need to sedate the horse and remove the material properly. Merck Veterinary Manual notes that mouth examination is often done under sedation to remove embedded foreign matter such as grass awns. (Merck Veterinary Manual)

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Dental Disease and Sharp Enamel Points

Dental problems can create or worsen oral ulceration.

Horses have teeth that continue erupting through life. Uneven wear can create sharp enamel points, hooks, ramps, waves, fractured teeth, periodontal pockets, and feed trapping. Sharp points, especially on the cheek side of the upper cheek teeth and tongue side of the lower cheek teeth, can cut the inside of the cheeks or tongue. MSD Veterinary Manual notes that sharp edges on the upper molars can cut the inside of the cheek and that dental discomfort can cause reluctance to take the bit, head shaking, or resistance when ridden. (MSD Veterinary Manual)

Dental-related mouth ulcers are more likely if your horse has:

• Quidding

• Weight loss

• Bad breath

• Slow eating

• One-sided chewing

• Feed packing in the cheeks

• Facial swelling

• Nasal discharge from one nostril

• Bit resistance

• No recent dental examination

This is why a quick look at the front teeth is not enough. Many painful lesions sit far back in the mouth and require sedation, a speculum, light, and a proper dental exam.

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Bute Paste and Medication Irritation

Phenylbutazone, commonly called Bute, is widely used in horses, but it is still a medication that needs respect.

The better-known concern with phenylbutazone is gastrointestinal injury, especially with high doses, prolonged use, dehydration, illness, or combining anti-inflammatory drugs. Toxic dose studies in ponies have reported signs including depression, anorexia, oral ulcers, and soft faeces. (PubMed)

There is also practical clinical concern that oral paste medications, including Bute paste, can irritate the mouth if they are retained against the tongue, gums, or cheeks instead of being swallowed promptly. Veterinary Partner notes that Bute paste can cause mouth ulcers and advises making sure the horse swallows the paste and does not retain it in the mouth. (Veterinary Partner)

This does not mean every horse on Bute will develop mouth ulcers. Most do not. But if a horse on oral medication suddenly becomes dull, drools, refuses feed, or develops mouth pain, the medication history matters.

Do not simply continue dosing and hope the appetite returns.

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Chemical Irritation and Caustic Products

Chemical irritation can cause painful stomatitis.

This may happen when a horse contacts or ingests irritants such as:

• Harsh topical products

• Caustic wound treatments

• Undiluted disinfectants

• Blistering agents

• Inappropriate home remedies

• Strong oral rinses

• Chemicals on fences, buckets, feeders, or stable surfaces

Merck Veterinary Manual notes that contact with chemical irritants can cause short-term inflammation of the mouth in horses. (Merck Veterinary Manual)

The mistake I see most often is owners trying to treat a sore mouth with something too harsh. A painful mouth does not need a stronger chemical. It needs the cause identified and the tissue protected while it heals.

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Bit, Bridle, and Foreign Body Trauma

Mechanical trauma can also cause oral ulcers.

Possible triggers include:

• Poorly fitted bit

• Rough bit handling

• Noseband pressure

• Sharp edges on tack

• Foreign material stuck in the mouth

• Wood splinters

• Wire or metal fragments

• Chewing inappropriate objects

• Recent dental work irritation

• Lip or tongue injury

Bit-related ulcers often occur at the corners of the mouth, bars, tongue, or cheeks. But do not assume the bit is the only cause. Dental points, wolf teeth, retained caps, pain elsewhere in the body, and rider contact can all contribute to contact problems.

If a horse suddenly resists the bit, check the mouth properly before changing equipment five times and blaming attitude. Horses are not dramatic for sport. Well, not only for sport.

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Vesicular Stomatitis

Vesicular stomatitis is an important cause of oral ulcers because it can affect horses and livestock, spreads through insects or contact, and can resemble more serious vesicular diseases in livestock.

In horses, excessive drooling is often one of the first signs. Blisters may rupture quickly, leaving ulcers and erosions on the tongue, lips, and mouth lining. Lesions can also occur around the muzzle, coronary bands, sheath, and udder, and lameness may occur if the coronary band is involved. (MSD Veterinary Manual)

USDA APHIS notes that vesicular stomatitis can cause drooling, blister-like lesions, fever, reluctance to eat, lesions on the mouth, nose, sheath, udder, ears, and coronary band, and it can affect animal movement and trade. (APHIS)

This matters because vesicular stomatitis is reportable in the United States, and laboratory testing may be needed to distinguish vesicular diseases. USDA APHIS states that vesicular stomatitis is reportable and that affected farms may be quarantined until 14 days after lesions appear on the last case at that location. (APHIS)

If you see mouth ulcers with fever, drooling, lesions on the muzzle or coronary band, or multiple animals affected, do not haul the horse to a show, clinic, sale, or another property. Call your vet first.

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Slobbers Is Different From Mouth Ulcers

This is an important distinction.

Slobbers, caused by slaframine toxin from fungus-infected clover or forage, can make a horse drool dramatically. But in a classic slaframine case, the mouth itself usually looks normal. Merck Veterinary Manual notes that with slaframine poisoning there are no abnormalities inside the mouth and profuse drooling may be the only sign. (Merck Veterinary Manual)

So:

• Heavy drooling with a normal mouth may fit slobbers.

• Heavy drooling with ulcers, bleeding, pain, fever, or trouble swallowing is not simple slobbers until proven otherwise.

That difference matters because slobbers is usually managed by removing contaminated forage, while true oral ulceration may need dental care, foreign body removal, infectious disease testing, medication changes, or wound management.

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Could It Be Choke Instead?

Yes. Choke can look like a mouth problem at first because the horse may drool, gag, retch, or stop eating.

Choke in horses means obstruction of the esophagus, not airway choking like in people. The horse can usually still breathe, but cannot swallow properly. Merck Veterinary Manual lists nasal discharge of feed material or saliva, difficulty swallowing, coughing, ptyalism, anxiety, and neck stretching as signs of esophageal obstruction in horses. It also warns that affected horses may continue to eat or drink, worsening the signs. (Merck Veterinary Manual)

Suspect choke if you see:

• Feed or saliva coming from the nostrils

• Green frothy nasal discharge after eating

• Repeated swallowing

• Coughing

• Gagging or retching motions

• Neck stretching

• Distress after feeding

• Sudden refusal to continue eating

If choke is possible, remove feed and water and call your vet promptly.

Do not syringe water, oil, medication, or anything else into the mouth. That is a fast way to turn one problem into an aspiration pneumonia problem.

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When Is This an Emergency?

Call your vet urgently if your horse has mouth ulcers plus any of the following:

• Refusal to eat

• Refusal to drink

• Feed or saliva coming from the nostrils

• Difficulty swallowing

• Coughing or choke-like signs

• Fever

• Marked drooling

• Severe pain

• Bleeding from the mouth

• Swelling of the face or jaw

• One-sided nasal discharge

• Bad smell from the mouth

• Rapid weight loss

• Weakness or depression

• Multiple animals affected

• Lesions on the muzzle, coronary bands, teats, sheath, or udder

• Recent travel, show, sale, or contact with new animals

• Suspected chemical or toxic plant exposure

• No improvement within 24 to 48 hours after removing the suspected irritant

The two situations that should make you act fastest are trouble swallowing and possible vesicular disease.

One can lead to aspiration and dehydration. The other can affect the whole property.

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What Should You Do Right Now?

If you suspect mouth ulcers, start with safe, practical steps.

1. Remove suspect hay or pasture access.
   If foxtail, sandbur, ticklegrass, mouldy hay, or sharp plant material is possible, stop feeding it.

2. Offer soft, palatable feed.
   Soaked pellets, soaked hay cubes, mashes, or softer forage may be easier to eat until your vet advises.

3. Provide clean water.
   Painful mouths can reduce drinking, so make water easy to access.

4. Do not use a bit.
   Rest the mouth until pain and lesions have been assessed.

5. Check for choke signs.
   Feed or saliva from the nostrils, coughing, repeated swallowing, or neck stretching changes the urgency.

6. Look for lesions elsewhere.
   Check the muzzle, lips, coronary bands, sheath, udder, and teats.

7. Do not apply harsh mouth products.
   Avoid undiluted disinfectants, caustic powders, human mouthwash, or random home remedies.

8. Call your vet if eating, drinking, swallowing, or comfort is affected.
   A proper oral exam is often needed, and that usually means sedation.

Merck Veterinary Manual notes that a veterinarian may need to examine the horse’s mouth under sedation to remove embedded foreign matter such as grass awns. (Merck Veterinary Manual)

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How Do Vets Diagnose Oral Ulcers?

Your vet will start with history and examination.

Important questions include:

• When did the signs start?

• Has the hay or pasture changed?

• Is foxtail or sandbur present?

• Has the horse had Bute, wormer, or another oral medication recently?

• Is the horse eating and drinking?

• Is there drooling?

• Is feed coming from the nostrils?

• Are other horses affected?

• Has the horse travelled recently?

• Are there lesions on the muzzle, coronary bands, sheath, teats, or udder?

• When was the last dental exam?

• Has the horse had recent bit, bridle, or training changes?

The exam may include:

• Temperature, pulse, and hydration check

• Oral exam under sedation

• Full dental exam with speculum and light

• Removal of embedded awns or foreign material

• Assessment for sharp teeth, ulcers, fractures, caps, and periodontal disease

• Testing for vesicular stomatitis if suspicious

• Bloodwork if systemic illness is suspected

• Endoscopy if choke or deeper swallowing problems are suspected

• Biopsy if lesions are chronic, unusual, mass-like, or not healing

A horse’s mouth is deep, dark, and full of opinions. A proper exam beats guessing every time.

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How Are Oral Ulcers Treated?

Treatment depends on the cause.

A simple traumatic ulcer from sharp hay may improve once the hay is removed and the mouth is rested. Embedded awns may need to be removed under sedation. Dental ulcers caused by sharp enamel points need proper dental correction. Medication-related irritation may require stopping, changing, or adjusting the medication plan under veterinary guidance.

Treatment may include:

• Removing sharp forage or foreign material

• Dental floating or treatment of diseased teeth

• Resting the mouth from the bit

• Soft feed or soaked feed

• Clean water and hydration support

• Pain relief prescribed by your vet

• Mild antiseptic rinses if recommended

• Medication changes if a drug is suspected

• Treatment for secondary infection when needed

• Testing, isolation, and supportive care if vesicular stomatitis is suspected

• Further diagnostics for chronic or unusual lesions

For vesicular stomatitis, MSD Veterinary Manual notes that no specific treatment is needed in uncomplicated cases, but affected animals should be isolated, feed can be softened, and mild antiseptic cleaning may help avoid bacterial infection. (MSD Veterinary Manual)

Do not use leftover antibiotics or human pain medication. The wrong treatment can hide the real problem or cause a new one.

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How Long Do Mouth Ulcers Take to Heal?

Healing time depends on the cause and depth of injury.

A mild traumatic ulcer may start improving within a few days once the source is removed. More severe ulcers, embedded awn injuries, dental wounds, medication burns, secondary infection, or vesicular disease can take longer.

The decision checkpoint is this:

• If your horse is bright, eating better, and the ulcers are shrinking after the cause is removed, that is encouraging.

• If your horse is still drooling, refusing feed, losing weight, bleeding, feverish, or painful after 24 to 48 hours, do not keep waiting.

Vesicular stomatitis in horses generally resolves within about 2 weeks in uncomplicated cases, but affected animals should still be isolated and managed according to veterinary and official guidance. (MSD Veterinary Manual)

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Common Mistakes Owners Make

Assuming drooling always means slobbers
Slobbers usually causes drooling without mouth lesions. Ulcers, bleeding, fever, nasal discharge, or trouble swallowing change the situation.

Continuing suspect hay
If sharp awns are causing trauma, every meal keeps damaging the mouth.

Using harsh chemicals in the mouth
Painful tissue needs protection, not a chemical assault.

Giving more oral medication when the horse cannot swallow properly
If choke or swallowing difficulty is possible, oral dosing can increase aspiration risk.

Blaming the bit without checking the teeth
Bit resistance may be caused by dental points, ulcers, wolf teeth, caps, back pain, lameness, or training factors.

Waiting too long when the horse stops eating
Horses are not designed to fast for long periods. Reduced intake can quickly become a bigger problem.

Ignoring possible reportable disease
Mouth ulcers with fever, drooling, lesions on the coronary bands, or multiple animals affected need veterinary guidance before movement.

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How To Prevent Oral Ulcers in Horses

You cannot prevent every mouth ulcer, but you can reduce the risk.

Helpful prevention steps include:

• Inspect every new hay batch

• Avoid feeding hay with moderate amounts of foxtail, sandbur, or ticklegrass seed heads

• Remove suspect hay immediately if several horses develop mouth pain

• Maintain regular dental exams

• Check young horses more often during tooth eruption years

• Use correctly fitted bits and bridles

• Avoid harsh rein contact and over-tight nosebands

• Make sure oral medications are swallowed properly

• Watch appetite closely after starting medications

• Keep buckets, feeders, and stable surfaces free of sharp edges

• Avoid caustic or unapproved oral treatments

• Monitor regional vesicular stomatitis alerts if you are in an affected area

• Isolate new or returning horses when appropriate

• Call early if mouth pain appears suddenly

University of Minnesota Extension recommends not feeding hay with moderate amounts of foxtail, sandbur, or ticklegrass seed heads, and notes that timely mowing can help prevent seed head production. (University of Minnesota Extension)

Regular dental care also matters. MSD Veterinary Manual notes that horses require regular dental care throughout life and that preventive care helps remove sharp edges and maintain normal chewing surfaces. (MSD Veterinary Manual)

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Will Your Horse Be Okay?

Most horses with mouth ulcers recover well once the cause is found and removed.

The outlook is best when:

• The horse is still eating and drinking

• The cause is obvious and removable

• There is no fever

• There are no choke signs

• There is no severe dental infection

• Lesions are improving within a few days

• The horse receives appropriate dental or veterinary care early

The outlook is more guarded when:

• The horse cannot eat or drink

• There is deep infection

• A dental root or sinus problem is present

• Lesions are widespread or recurring

• Vesicular stomatitis or another infectious disease is suspected

• There is weight loss or dehydration

• The ulcers are chronic, unusual, or mass-like

The main takeaway is that mouth ulcers are usually manageable, but they should not be ignored. A horse that cannot chew comfortably cannot maintain health for long.

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FAQs

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Can Bute cause mouth ulcers in horses?

Phenylbutazone can be associated with ulceration and toxicity, especially with inappropriate dosing or prolonged use. Oral paste may also irritate the mouth if it is retained rather than swallowed promptly. If your horse develops drooling, feed refusal, depression, or mouth pain while on Bute, contact your vet before continuing treatment. (PubMed)

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Can foxtail hay cause mouth ulcers?

Yes. Foxtail, sandbur, ticklegrass, and similar sharp seed heads can embed in the lips, gums, tongue, and mouth lining. This can cause painful ulcers, drooling, slow eating, and weight loss if exposure continues. (University of Minnesota Extension)

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Is drooling always a sign of mouth ulcers?

No. Drooling can be caused by mouth ulcers, dental disease, choke, vesicular stomatitis, medication irritation, or slobbers. Classic slaframine slobbers usually causes profuse drooling without visible mouth abnormalities. (Merck Veterinary Manual)

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Should I ride a horse with mouth ulcers?

Avoid riding with a bit until the mouth has healed and the cause has been addressed. If the ulcers are near the lips, bars, tongue, or cheeks, bitting can worsen pain and delay healing.

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When should I call the vet for mouth ulcers?

Call your vet if your horse is not eating, drooling heavily, bleeding, feverish, losing weight, showing nasal discharge, struggling to swallow, has lesions elsewhere on the body, or if ulcers do not improve quickly after removing the suspected cause.

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Final Thoughts

Oral ulcers in horses are easy to underestimate because they can be hidden deep in the mouth.

A small sore may simply reflect sharp hay or minor trauma. But a horse that drools, refuses feed, drops hay, resists mouth handling, has fever, shows lesions elsewhere, or cannot swallow normally needs a proper veterinary assessment.

The safest approach is to think in three steps: remove obvious irritants, protect the mouth with soft feed and no bit, and call your vet if eating, drinking, swallowing, or comfort is affected.

A healthy mouth is not a cosmetic detail. It is the front door to hydration, nutrition, performance, and welfare.

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If you are unsure whether your horse’s mouth ulcers are from hay, dental disease, medication irritation, choke, or something more urgent, ASK A VET™ can help you decide what to do next.