Sores and ulcers in a dog's mouth have many causes. The most common are gum disease and a condition where the cheeks and lips ulcerate where they touch the teeth, often called CUPS or "kissing ulcers."1,2 Other causes include kidney failure, burns from chewing electrical cords or caustic products, foreign bodies, immune diseases, and tumors.1,3 Signs include drooling, bad breath, pawing at the mouth and not wanting to eat.3,1 Because the causes range from mild to serious, any mouth sore that doesn't heal in a few days needs a vet visit, and an electrical cord bite needs one right away.4,5
Go to a vet right away if:
- Your dog chewed an electrical cord, even if he seems fine. Fluid can build up in the lungs 12 to 36 hours later.4
- He licked or chewed a caustic product such as a drain or oven cleaner. Call your vet or a poison line first.3,5
- He won't eat, or has mouth sores plus vomiting, heavy drinking or lethargy, which can point to kidney failure.6,1
- There's heavy bleeding, a string under the tongue, or something stuck in the mouth.3
Signs of Mouth Sores
- Drooling, sometimes thick and ropey, or blood-tinged saliva1,3
- Bad breath1
- Pawing at the mouth, dropping food, or not eating3,1
- Not wanting the mouth touched or teeth brushed7
- Swollen glands under the jaw, weight loss or behavior changes from ongoing pain1,8
Common Causes
| Cause | What it looks like | Notes |
|---|---|---|
| CUPS / chronic ulcerative stomatitis ("kissing ulcers") | Ulcers on the inside of the lips and cheeks where they touch the teeth, especially the upper canines and back teeth | Maltese, Cavaliers, Labradors, Greyhounds, Schnauzers and terriers are often affected.1,8,2 |
| Periodontal (gum) disease | Red, sore gums and contact sores next to dirty teeth | Most of these heal with dental treatment alone.2,6 |
| Kidney failure | Ulcers on the tongue and gums, ammonia-smelling breath | Waste products build up and damage the mouth lining. Bloodwork checks for it.1,9,3 |
| Electrical burns | Pale yellow, tan or gray burns on the tongue, lips or gums | Mostly young dogs chewing cords. Full damage may take 3 to 4 days to show.5,4 |
| Chemical or heat burns | Sores on the tongue, palate or lips | From caustic cleaners, some houseplants, or very hot food.5,3 |
| Foreign bodies | A cut under the tongue, swelling, or something embedded | String around the tongue, splinters, plant awns, quills.3,1 |
| Immune diseases | Deep, painful erosions on the gums, lips and nose | Pemphigus vulgaris causes mouth lesions in most affected dogs; lupus and drug reactions are others.10,11 |
| Eosinophilic granuloma | Raised plaques or ulcers, often on the palate or tongue | Rare; Siberian Huskies and Cavaliers may be at higher risk.12 |
| Tumors | A sore that keeps growing or bleeding | Melanoma, squamous cell carcinoma and fibrosarcoma can ulcerate.3 |
| Viral warts and infections | Pale, cauliflower-like warts in young dogs | Warts usually go away in weeks to months. Distemper can also cause mouth sores.3,1 |
See periodontal disease, kidney disease in dogs, signs of oral cancer and bad breath in dogs.
CUPS (Kissing Ulcers) Explained
CUPS stands for chronic ulcerative paradental stomatitis. Vets increasingly call it canine chronic ulcerative stomatitis, because about 40% of lesions occur away from the teeth.8,2 The cause isn't fully understood. It was long thought to be an overreaction to plaque, but newer research suggests an immune-mediated disease similar to oral lichen planus in people.1,13,8
Treatment used to mean pulling many or all of the teeth. A 2025 study of 42 dogs found that 81% had complete resolution, most with dental cleaning and gum treatment alone; only 17% needed all their teeth removed. The authors recommend a step-by-step approach.2 Merck notes that extraction helps but isn't always curative, and some dogs keep having lesions afterward.1
How Vets Diagnose It
- A full mouth exam, often under anesthesia, with dental X-rays.8,7
- Bloodwork and a urine test to check for kidney failure and other illness.3,6
- A biopsy to tell CUPS apart from immune diseases, eosinophilic granuloma and cancer.1,11,8
Treatment
- Treat the cause: kidney disease, burns, foreign bodies or tumors each need their own treatment.3,5
- Plaque control: a professional cleaning above and below the gumline, then daily brushing once the mouth is comfortable.1,7
- Antiseptic rinses or gels such as chlorhexidine, as your vet directs.3,6
- Extractions for teeth that can't be saved, or when other treatment fails.8,2
- Immune-modulating medicines for CUPS or autoimmune disease, such as cyclosporine, corticosteroids and certain antibiotic combinations. Many dogs need long-term treatment.8,10,2
- Pain relief prescribed by your vet.1,8
Home Care and Prevention
- Offer soft or canned food while the mouth heals.3,6
- Brush daily once your vet says it's comfortable. See how to brush your dog's teeth.3,8
- Use only the gels or rinses your vet recommends.6
- Keep electrical cords, cleaning products and toxic houseplants out of reach, especially with puppies.4,3
- Never use human mouth ulcer products on a dog unless your vet says to.
Dog General Diseases and Symptoms QuickLinks
Frequently Asked Questions
What causes sores in a dog's mouth?
Common causes are gum disease, CUPS (kissing ulcers), kidney failure, burns from cords or chemicals, foreign bodies, immune diseases and tumors.
What are kissing ulcers in dogs?
Ulcers on the inside of the lips and cheeks where they touch the teeth. They're part of a condition called CUPS, or chronic ulcerative stomatitis.
Can kidney disease cause mouth ulcers in dogs?
Yes. When the kidneys fail, waste products build up and damage the lining of the mouth, causing ulcers and ammonia-smelling breath.
How do you treat a dog's mouth ulcer at home?
Offer soft food and use only vet-recommended gels or rinses. A sore that doesn't heal in a few days needs a vet, because the cause matters.
Do dogs with CUPS need all their teeth pulled?
Usually not. In a 2025 study, most dogs got better with dental cleaning and gum treatment, and only about 1 in 6 needed full-mouth extraction.
Is a mouth ulcer in a dog an emergency?
It is after an electrical cord bite or a chemical exposure, or if your dog won't eat or seems ill. Otherwise, see your vet within a few days.
What does a dog mouth ulcer look like?
A raw, red, white or yellow patch on the gums, tongue, lips or cheeks, sometimes bleeding. Burns from cords may look pale gray or tan.
Can a dog get burned from chewing an electrical cord?
Yes. It can burn the tongue and lips and cause fluid in the lungs up to a day and a half later. Unplug the cord before touching your dog and go to a vet.
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Sources and further reading
This article was written and fact-checked on November 4, 2026 against current veterinary references and specialist guidelines:
- Mulherin BL, DVM, DAVDC. Oral Inflammatory and Ulcerative Disease in Small Animals. Merck Veterinary Manual (professional version), November 2024.
- Ford J, Bell C, Donovan TA, Martel D. Surgical outcomes in canine lymphoplasmacytic stomatitis: a retrospective study of 42 dogs (2012 to 2022). Frontiers in Veterinary Science, November 2025.
- Reiter AM, DAVDC. Disorders of the Mouth in Dogs. Merck Veterinary Manual (pet owner version), updated June 2026.
- MSPCA-Angell. Electrical Cord Injury. MSPCA-Angell, 2026.
- Mulherin BL, DVM, DAVDC. Soft Tissue Trauma of the Mouth in Small Animals. Merck Veterinary Manual (professional version), November 2024.
- PetMD Editorial. Mouth Inflammation and Ulcers (Chronic) in Dogs. PetMD, accessed November 2026.
- Bellows J, DVM, DAVDC. Chronic ulcerative paradental stomatitis: When kissing isn't a good thing. dvm360, 2015.
- van Velzen H. Canine chronic ulcerative stomatitis: what we know. Vet Times, January 2025.
- Dosenberry C, Arzi B, Palm C, Vapniarsky N, Soltero-Rivera M. An update on oral manifestations of systemic disorders in dogs and cats. Frontiers in Veterinary Science, January 2025.
- Torres SMF, DVM, PhD, DACVD. Pemphigus Vulgaris. Small and Large Animal Dermatology Handbook, University of Minnesota, 2025.
- Erosive and ulcerative stomatitis in dogs and cats: which immune-mediated diseases to consider? JAVMA 2023;261(S1):S48 to S57.
- Moriello KA, DVM, DACVD. Eosinophilic Lesions in Dogs. Merck Veterinary Manual (professional version), January 2025.
- Anderson JG, Peralta S, Kol A, Kass PH, Murphy B. Clinical and Histopathologic Characterization of Canine Chronic Ulcerative Stomatitis. Veterinary Pathology 2017;54(3):511 to 519.
About this article
Last updated : new page written using the Merck Veterinary Manual, the American Veterinary Dental College, MSPCA-Angell and recent published research on canine stomatitis.
This page is for general information and is not a substitute for an exam by your veterinarian.
