A canker is a small, shallow ulcer that forms inside your mouth. Officially called an aphthous ulcer, it’s one of the most common oral conditions, affecting anywhere from 5 to 25% of the general population. Unlike cold sores, canker sores are not contagious and not caused by a virus. They’re painful, sometimes annoyingly persistent, but almost always harmless.
What a Canker Sore Looks and Feels Like
Canker sores are round or oval with a shallow center covered by a yellowish-gray film. They’re surrounded by a red halo with slightly raised edges. Most are small, typically 2 to 3 millimeters across, though they can reach up to about a centimeter. They show up on the soft tissues inside your mouth: the inner cheeks, lips, the soft palate, the floor of the mouth, or along the base of your gums.
The pain usually starts before the sore is fully visible, often as a tingling or burning sensation. Once formed, eating, drinking, and even talking can irritate it. Minor canker sores tend to clear on their own within one to two weeks without leaving a scar.
Three Types of Canker Sores
Not all canker sores are the same. They fall into three categories based on size, depth, and number.
- Minor aphthae are by far the most common. They’re small (under 1 cm), shallow, and can appear alone or in small groups. These heal without scarring.
- Major aphthae are larger and dig deeper into the tissue. They take longer to heal and are more likely to leave a scar.
- Herpetiform aphthae appear as clusters of many tiny sores that can merge together. Despite the name, they have nothing to do with the herpes virus.
Common Triggers
The exact cause of canker sores is still unknown, but a long list of triggers can set them off. Physical injury to the mouth is one of the most common: biting your cheek, brushing too hard, getting poked during dental work, or taking a hit during sports. Even rough-textured foods like chips and pretzels can scratch the tissue enough to spark one.
Certain ingredients in oral care products play a role too. Sodium lauryl sulfate, a foaming agent found in many toothpastes and mouthwashes, has been linked to canker sore outbreaks. Switching to an SLS-free toothpaste is one of the simplest things you can try if you get sores frequently.
Food sensitivities are another well-known trigger. Acidic fruits like pineapple, grapefruit, and oranges are common culprits, along with chocolate, coffee, strawberries, eggs, nuts, cheese, and anything particularly spicy or salty. If you notice a pattern between certain foods and outbreaks, avoiding those foods can reduce how often sores appear.
Emotional stress is a trigger that’s easy to overlook. People who get recurrent canker sores often notice flare-ups during high-stress periods. The connection isn’t fully understood, but stress-reduction techniques like meditation can help in some cases.
Nutritional Deficiencies and Recurring Sores
If you keep getting canker sores, your diet may be part of the problem. Several nutrient deficiencies are clinically linked to recurrent outbreaks. Vitamin B12 deficiency is one of the most commonly identified. Low levels of iron, zinc, and folate (vitamin B9) are also associated with frequent sores. Zinc supports immune function and wound healing, folate is essential for cell division, and iron supports healthy blood and tissue repair.
Eating foods rich in these nutrients, or addressing a confirmed deficiency, can reduce how often canker sores come back. This is especially worth considering if your sores are frequent and you can’t identify an obvious trigger like a food sensitivity or mouth injury.
Canker Sores vs. Cold Sores
People mix these up constantly, but they’re entirely different conditions. The easiest way to tell them apart is location. Canker sores form inside the mouth. Cold sores (also called fever blisters) form on the outside, typically around the border of the lips.
Cold sores are caused by herpes simplex virus type 1 (HSV-1) and are highly contagious. Canker sores have no known infectious cause and cannot be spread to another person through kissing, sharing utensils, or any other contact. If the sore is inside your mouth and not on your lips, it’s almost certainly a canker sore.
How to Manage the Pain
Most minor canker sores don’t require treatment. They resolve on their own within a week or two. In the meantime, you can reduce discomfort by rinsing with warm salt water, avoiding acidic and spicy foods, and using a soft-bristled toothbrush to prevent further irritation. Over-the-counter numbing gels applied directly to the sore can take the edge off, especially before meals.
For sores that are large, unusually painful, or slow to heal, stronger treatments are available by prescription. These typically involve anti-inflammatory gels or rinses that reduce swelling and speed healing.
Signs a Canker Sore Needs Attention
A canker sore that hasn’t healed after two weeks is worth getting checked. The same goes for sores that are unusually large, spreading, or so painful that you can’t eat or drink normally. Frequent outbreaks, especially if they overlap so you’re rarely sore-free, can sometimes point to an underlying nutritional deficiency or immune system issue that’s worth investigating. Fever alongside mouth sores is another signal that something beyond a routine canker sore may be going on.