A canker sore is a small, shallow ulcer that forms inside the mouth, typically on the inner cheeks, lips, tongue, or soft palate. Unlike cold sores, canker sores are not contagious and are not caused by a virus. They’re the most common ulcerative condition of the mouth, and most heal on their own within about two weeks.
What a Canker Sore Looks and Feels Like
Canker sores usually appear as round or oval white or yellowish spots surrounded by a red, inflamed border. They can range from barely noticeable to genuinely painful, especially when you’re eating, drinking, or talking. The pain tends to be worst in the first few days and gradually fades as the sore heals.
They only form inside the mouth. If you have a sore on the outside of your lips or around your mouth, that’s more likely a cold sore (fever blister), which is a completely different condition caused by the herpes simplex virus.
Three Types of Canker Sores
Most canker sores are the minor type: small, oval-shaped, and gone within one to two weeks without scarring. These are what the vast majority of people experience.
Major canker sores are larger, deeper, and significantly more painful. They can take several weeks to heal and sometimes leave scars. Herpetiform canker sores are a third, less common variety that appear as clusters of tiny, pinpoint-sized ulcers. Despite the name, they have nothing to do with the herpes virus.
What Causes Them
Canker sores don’t have a single known cause. The current understanding is that the immune system plays a central role. In people who are prone to canker sores, certain immune cells attack the lining of the mouth, breaking down tissue and creating the ulcer. This appears to be driven by genetics: some people’s immune systems are simply more reactive to triggers in the oral environment.
One theory involves a type of cross-reactivity where immune cells responding to common mouth bacteria accidentally damage healthy tissue. People with active canker sores show elevated levels of inflammatory signaling molecules in their blood compared to people without them.
Common Triggers
Even if you’re genetically predisposed, canker sores usually need a trigger to appear. The most frequently reported ones include:
- Physical injury: biting the inside of your cheek, aggressive brushing, dental work, or braces rubbing against tissue
- Stress: both emotional and physical stress can precede outbreaks
- Certain foods: acidic or spicy foods like citrus, tomatoes, and chips are common culprits
- Nutritional deficiencies: low levels of vitamin B12, folate, iron, or vitamin C are linked to more frequent and severe sores
- Hormonal changes: some people notice outbreaks tied to their menstrual cycle
Folate deficiency is particularly relevant. Folate helps your body renew the cells that line the inside of your mouth. When levels drop, that lining becomes more fragile and vulnerable to ulceration. Research published in Frontiers in Oral Health found that people with low serum folate levels had more frequent and more severe canker sores compared to those with normal levels. Vitamin B12 deficiency produces similar effects, increasing mucosal sensitivity and predisposing the tissue to painful ulcers.
Canker Sores vs. Cold Sores
People confuse these two constantly, but they’re fundamentally different. Cold sores (fever blisters) are caused by the herpes simplex virus, usually type 1. They form on the outside of the mouth, around the lips, and they are highly contagious. Canker sores form only inside the mouth, are not caused by any virus, and cannot be spread to another person through kissing, sharing utensils, or any other contact.
If your sore is inside your mouth on soft tissue, it’s almost certainly a canker sore. If it’s on or around your lips on the skin surface, it’s more likely a cold sore.
How to Treat a Canker Sore
Most canker sores don’t require treatment. They heal within two weeks on their own. But if the pain is interfering with eating or talking, a few options can help.
Over-the-counter products containing benzocaine (a numbing agent) can temporarily dull the pain. These come as gels, pastes, and liquids that you apply directly to the sore. Hydrogen peroxide rinses designed for mouth sores can help keep the area clean and reduce irritation. For more severe or stubborn sores, prescription options include topical anti-inflammatory creams. The key with any topical product is applying it early, as soon as the sore appears, for the best results.
Simple home remedies also help. Rinsing with warm salt water several times a day can soothe the area and promote healing. A rinse made with a small amount of baking soda dissolved in water works similarly. Avoiding acidic and spicy foods while you have an active sore will reduce pain considerably.
Reducing How Often You Get Them
If you get canker sores regularly, your toothpaste might be part of the problem. Sodium lauryl sulfate (SLS), the ingredient that makes toothpaste foamy, has been linked to longer-lasting and more painful canker sores. In a 2012 clinical trial, 90 participants who switched to SLS-free toothpaste reported that their sores didn’t last as long or hurt as much compared to when they used standard toothpaste. Researchers in other studies found that switching to SLS-free formulas reduced the overall occurrence of sores. SLS-free toothpastes are widely available and easy to find.
Addressing nutritional gaps can also make a difference. If you get frequent canker sores, it’s worth evaluating whether you’re getting enough B12, folate, iron, and vitamin C through your diet. Leafy greens, legumes, citrus fruits (when you don’t have an active sore), and fortified cereals cover most of these bases. A blood test can confirm whether a deficiency is contributing to your outbreaks.
When a Canker Sore Needs Attention
A canker sore that lasts longer than two weeks is the main signal that something else may be going on. Unusually large sores, sores that keep coming back in rapid succession, sores accompanied by fever, or sores so painful that you can’t eat or drink enough fluids all warrant a visit to a healthcare provider. In rare cases, persistent mouth ulcers can be a sign of an underlying condition like celiac disease, an immune disorder, or oral cancer, so sores that don’t follow the normal healing pattern shouldn’t be ignored.