Mouth Ulcers: What They Are, What They Look Like, and When to Worry

A mouth ulcer is an open sore inside the mouth. The most common type is a canker sore — also called an aphthous ulcer — a noncontagious sore that forms on the soft tissues of the lips, cheeks, tongue, gums, or palate and usually heals on its own in 1 to 2 weeks.^1,2

If you keep getting mouth ulcers or want a deeper look at causes and triggers, see our full guide to canker sores. For treatment options, see how to treat a painful ulcer on the tongue or gums.

Mouth Ulcer at a Glance

• What is a mouth ulcer? An open sore on the soft tissue inside the mouth. The common noncontagious type is a canker sore, also called an aphthous ulcer.^1,2
• What does a mouth ulcer look like? A round or oval sore with a white, yellow, or gray center and a red, inflamed border, usually less than 1 cm across.²
• How long does a mouth ulcer last? Most heal on their own in 7 to 14 days; larger or recurring ulcers can take several weeks.¹
• What can be mistaken for a mouth ulcer? Cold sores (blisters on the lips, usually from HSV-1), oral thrush, traumatic injuries, and — rarely — oral cancer, especially when a sore lasts more than 3 weeks.⁴

What Does a Mouth Ulcer Look Like?

Mouth ulcers look like round or oval sores on the soft tissues inside your mouth. They most often appear on the lips or cheeks, and also develop:

• On the sides of the tongue
• Beneath the tongue
• Around the gums
• On the upper palate

A mouth ulcer is white, yellow, gray, or red, and the surrounding tissue typically looks swollen. You might have a single ulcer or several, and they’re usually small and shallow.²

A canker sore on the inner lower lip. Most mouth ulcers look similar: round or oval, with a white or gray center and a red border.

Mouth ulcers fall into three types based on size, number, and healing time:⁵

• Minor aphthous ulcers ⁠— Usually under 1 cm — many are just a few millimeters wide — and heal within 7 to 14 days without scarring. Around 80% of mouth ulcers are minor.
• Major aphthous ulcers ⁠— Usually larger than 1 cm, deeper, and slower to heal — sometimes over weeks or months. Major ulcers sometimes leave scars.
• Herpetiform ulcers ⁠— A cluster of 10 to 100 pinpoint ulcers that merge into larger sores and typically heal within one month.

Symptoms of Mouth Ulcers

Mouth ulcers usually start with a tingling or burning sensation a day or two before the visible sore appears.¹ Once the sore forms, common symptoms include:

• Pain that worsens when eating spicy, sour, or salty foods
• Loss of appetite due to mouth pain
• Discomfort while brushing your teeth
• Swelling around the sore

Symptoms tend to worsen during periods of stress, illness, or lack of sleep.¹

Mouth Ulcer vs. Cold Sore

Mouth ulcers and cold sores are not the same condition. Mouth ulcers appear inside your mouth on soft tissue, are not contagious, and come from triggers like injury, food sensitivity, or stress.²

Cold sores are blisters that appear on the lips or around the mouth. They’re caused by the herpes simplex virus — usually HSV-1, though HSV-2 can also cause oral cold sores — are contagious, and often start with a tingling sensation before forming a fluid-filled blister that crusts over.²

What Causes Mouth Ulcers?

Mouth ulcers don’t always have a single identifiable cause.⁵ Common triggers and associated factors include:

• Minor oral tissue trauma ⁠— Accidentally biting the inside of the cheek, sharp edges on dental appliances or teeth, or sports injuries.
• Food sensitivities ⁠— Coffee, chocolate, eggs, nuts, and acidic or spicy foods are common culprits.
• Nutritional deficiencies ⁠— Low levels of iron, folate, zinc, or vitamin B-12 are linked to more frequent mouth ulcers.⁵
• Sodium lauryl sulfate (SLS) ⁠— Some toothpastes and mouth rinses contain this foaming ingredient, which can worsen or trigger ulcers in sensitive people.⁶
• Allergies and oral hypersensitivity ⁠— Reactions to certain foods, dental materials, or ingredients in toothpaste or mouthwash trigger mouth ulcers in sensitive people.
• Medications ⁠— Chemotherapy, NSAIDs (like ibuprofen and naproxen), beta-blockers, and some antibiotics trigger or worsen mouth sores. If you started a new medication shortly before the ulcers appeared, ask your prescriber.³
• Hormonal changes ⁠— Mouth ulcers flare during puberty, pregnancy, and menopause.
• Health conditions ⁠— Inflammatory bowel disease, celiac disease, lupus, Behçet’s disease, immune disorders, and diabetes-related dry mouth or thrush cause or mimic mouth ulcers.²

Stress, lack of sleep, and stopping smoking also increase the risk of mouth ulcers in some people.⁷ If ulcers appear after you quit smoking, they’re usually temporary — continue your quit plan and ask a clinician if they’re severe or persistent.

When to See a Dentist or Doctor

Most mouth ulcers heal on their own. Use the three tiers below to decide what to do next.

Self-care is fine if the ulcer is small, painful but tolerable, and less than 2 weeks old. Salt-water rinses, OTC oral pain-relief products used as directed, and avoiding spicy or acidic foods help while it heals.¹

See a dentist or doctor if:

• The ulcer lasts longer than 2 weeks
• You get mouth ulcers frequently (3 or more times per year)
• The ulcer is unusually large (over 1 cm)
• A fever, swollen lymph nodes, or signs of infection come with it
• You suspect an SLS toothpaste or new medication is the trigger and cannot easily switch

Seek urgent evaluation if:

• A sore doesn’t heal within 3 weeks (oral-cancer red flag)⁴
• The ulcer is painless or has a hard, raised, or unusually colored border (oral cancer sometimes presents this way)⁴
• Sores spread rapidly or you develop multiple mouth sores along with severe pain or trouble swallowing

A dentist usually diagnoses a mouth ulcer with a visual exam. If the ulcer is unusual, doesn’t heal, or could be cancerous, they may take a small tissue sample (biopsy) for analysis.

Treating a Mouth Ulcer

Most mouth ulcers heal on their own without treatment. To ease the pain while you wait, use OTC oral pain-relief products as directed, rinse with warm salt water, and avoid acidic, spicy, or crunchy foods. Do not give benzocaine to children under 2 — it can cause a rare but serious blood disorder called methemoglobinemia. Check with a clinician before using any topical numbing product on a young child.³ If an ulcer is severe, keeps coming back, or doesn’t heal within 2 weeks, a dentist or doctor may prescribe a topical corticosteroid or recommend further evaluation.⁸

For a full walkthrough of home remedies, OTC options, and prescription treatments, see our guide on how to treat a painful ulcer on the tongue or gums.