June 6, 2026 · 3 min read

Canker Sores and Mouth Sores: Causes, Relief and When to Worry

Most mouth sores are harmless and heal in one to two weeks. Here's how to tell canker sores from cold sores, what eases the pain and which sores need a closer look.

Few things make eating and talking as uncomfortable as a sore in your mouth. The good news is that most mouth sores are harmless and heal on their own within one to two weeks. The key is knowing what kind of sore you have, how to ease the discomfort, and which signs mean it's time to have it checked.

Canker sores

Canker sores (aphthous ulcers) are small, round sores with a white or yellowish center and a red border. They appear inside the mouth, on the inner cheeks and lips, under the tongue or on the gums. They aren't contagious.

Common triggers include:

A minor injury, such as biting your cheek, brushing too hard or a sharp edge on a tooth
Stress and lack of sleep
Certain foods, especially acidic or spicy ones
Toothpastes containing sodium lauryl sulfate (SLS), for some people
Hormonal changes
Some nutritional deficiencies (such as iron, B12 or folate) or underlying conditions, in people with frequent sores

Most canker sores heal in 7 to 14 days.

Cold sores

Cold sores (fever blisters) are different. They're caused by the herpes simplex virus and usually appear on or around the lips as a cluster of small blisters that break, crust over and heal in about one to two weeks. They're contagious, especially while blisters are present, so avoid kissing and sharing cups, utensils or lip balm during an outbreak. Triggers include illness, stress, sun exposure and fatigue. Antiviral medications from your doctor can shorten outbreaks if started early.

Other common mouth sores

Irritation from appliances. A new denture, retainer or braces can rub and cause sore spots until they're adjusted.
Burns from hot pizza or coffee.
Thrush, a fungal infection that causes white patches that may wipe off, more common in babies, denture wearers, people with diabetes and people using inhaled steroids.

How to ease the discomfort

Rinse with warm salt water (half a teaspoon of salt in a cup of water) a few times a day.
Avoid irritating foods, such as citrus, tomatoes, spicy foods and crunchy snacks, until the sore heals.
Use over-the-counter numbing gels or protective pastes as directed on the label.
Brush gently with a soft brush, and consider an SLS-free toothpaste if you get canker sores often.
Take pain relievers as directed if needed.
Ask us about an adjustment if a denture, retainer or rough tooth edge is causing the irritation.

When to see your dentist

Most sores are minor, but some need a closer look. Call us if a sore:

Lasts longer than two weeks
Is unusually large or keeps coming back
Doesn't hurt but won't go away, especially a white or red patch
Comes with fever, swollen glands or trouble swallowing
Makes it hard to eat or drink enough

A sore that doesn't heal within two weeks should always be checked, because in rare cases it can be an early sign of oral cancer. Our post on oral cancer screenings explains what we look for. If you notice swelling with fever or trouble breathing, seek emergency medical care right away.