Canker sores are small, painful ulcers that form inside the mouth, most commonly on the inner cheeks, tongue, soft palate, or the base of the gums. Unlike cold sores, they are not caused by a virus and are not contagious. Most heal on their own within 10 to 14 days, but they can make eating, drinking, and talking genuinely miserable in the meantime.
What a Canker Sore Looks Like
A typical canker sore appears as a single round or oval sore with a white or yellow center and a red border. It sits on soft tissue inside the mouth, never on the outer lips or skin. That location is the quickest way to tell it apart from a cold sore (fever blister), which shows up as a cluster of small, fluid-filled blisters on or around the lips. Cold sores are caused by herpes simplex virus and are contagious. Canker sores are neither.
Three Types and How They Differ
Not all canker sores are the same size or severity. They fall into three categories.
- Minor aphthous ulcers are by far the most common. They measure less than 10 mm across, heal within 10 to 14 days, and leave no scar.
- Major aphthous ulcers are larger than 10 mm, can persist for up to 6 weeks, and may occasionally scar.
- Herpetiform ulcers (named for their appearance, not for herpes) arrive as clusters of tiny sores that can merge into larger, irregular ulcers. When they fuse together, scarring is possible.
Most people who get canker sores only ever deal with the minor type. If yours is unusually large, lasts more than two weeks, makes it very difficult to eat or drink, or comes with a fever, it’s worth getting evaluated by a doctor or dentist.
What Causes Them
The exact cause isn’t fully understood, but canker sores are driven by the immune system rather than an infection. Certain white blood cells (T cells) attack the lining of the mouth, destroying the surface tissue and creating the ulcer. This process is fueled by local inflammatory signals, and people with active canker sores show a higher proportion of specific immune cells involved in tissue destruction compared to people who don’t get them.
One theory is that the immune system overreacts to a common oral bacterium, mistaking mouth tissue proteins for bacterial proteins. There also appear to be abnormalities in how the immune system’s early warning pathways function, leading to an exaggerated inflammatory response in people who are prone to outbreaks.
Common Triggers
Even if the underlying mechanism is immune-related, specific triggers tend to set off individual episodes. The most widely reported include:
- Mouth injuries: biting the inside of your cheek, aggressive brushing, dental work, or braces rubbing against tissue.
- Acidic and irritating foods: oranges, lemons, tomatoes, vinegar, pineapple, and spicy foods are commonly linked to outbreaks.
- Food sensitivities: some people notice sores after eating chocolate, nuts, cheese, cow’s milk, or wheat flour.
- Nutritional shortfalls: people with recurrent canker sores tend to consume less vitamin B12, folate, and iron than people who don’t get them. One study found that canker sore sufferers consumed about 20% less of the recommended daily folate and 7% less B12 than controls.
- Stress and hormonal shifts: emotional stress and menstrual cycles are frequently cited triggers.
- Toothpaste containing sodium lauryl sulfate (SLS): a systematic review of clinical trials found that switching to an SLS-free toothpaste significantly reduced the number of ulcers, episode frequency, duration, and pain in people with recurrent canker sores.
Canker Sores and Other Health Conditions
Occasional canker sores are extremely common and rarely signal anything serious. But frequent or severe outbreaks can sometimes be a marker of an underlying condition. Iron deficiency anemia and pernicious anemia (caused by B12 deficiency) both produce mouth ulcers that look just like canker sores.
Inflammatory bowel diseases, including Crohn’s disease and ulcerative colitis, often cause oral ulcers. In some cases, mouth sores appear months to a year before any abdominal symptoms do, making them an early clue. Crohn’s disease tends to produce deep, linear ulcers with raised edges in the cheeks, while ulcerative colitis is more associated with the classic round aphthous-style sore.
Behçet’s disease is another condition where mouth ulcers are a hallmark. The sores look similar to ordinary canker sores but tend to be more numerous, last longer, and are more painful. Behçet’s also causes genital ulcers, eye inflammation, and joint pain, which helps distinguish it. Celiac disease is another condition worth considering if you get frequent canker sores, particularly if you also have digestive symptoms or unexplained fatigue. If you’re an adult getting recurrent canker sores for the first time, that pattern is more likely to reflect an underlying issue and is worth discussing with a doctor.
How to Manage Pain at Home
Most minor canker sores don’t need prescription treatment. The goal is to reduce pain and avoid irritating the sore while it heals on its own.
A simple saltwater or baking soda rinse is one of the most effective home measures. Memorial Sloan Kettering recommends mixing 1 teaspoon of salt and 1 teaspoon of baking soda into 1 quart (4 cups) of water, then rinsing every 4 to 6 hours. You can also use salt alone or baking soda alone at the same ratio. This helps keep the area clean and can temporarily ease the sting.
Over-the-counter numbing gels or patches containing benzocaine can dull pain before meals. Avoiding acidic foods, spicy dishes, and rough or crunchy textures during an outbreak helps prevent further irritation. If your toothpaste contains SLS (check the ingredients for “sodium lauryl sulfate”), switching to an SLS-free brand may reduce how often sores come back.
Prescription Treatments
For canker sores that are large, unusually painful, or recur frequently, doctors and dentists have several options. Topical corticosteroid pastes are the standard first-line prescription. They work by dampening the local immune response and reducing inflammation at the ulcer site. A large network analysis of clinical trials found that these topical treatments significantly reduced ulcer size and pain compared to placebo.
Anti-inflammatory pastes designed specifically for mouth ulcers are another option, offering both pain relief and faster healing. In some cases, antibiotic mouth rinses are prescribed, not because canker sores are infections, but because certain antibiotics can inhibit the tissue-destroying enzymes involved in ulcer formation. Honey also performed well in clinical trials for speeding healing time, which supports the folk remedy many people already use.
Reducing How Often They Come Back
If you get canker sores repeatedly, a few evidence-based strategies can lower the frequency. Making sure you’re getting enough B12, folate, and iron through your diet (or supplements, if a deficiency is confirmed) is one of the most practical steps. Good dietary sources include leafy greens, legumes, eggs, meat, and fortified cereals.
Keeping a food diary can help you identify personal triggers. The foods most commonly associated with outbreaks are citrus fruits, tomatoes, cheese, chocolate, nuts, and anything particularly spicy. Some people find that eliminating a suspect food for a few weeks and then reintroducing it makes the connection clear.
Switching to an SLS-free toothpaste, using a soft-bristled toothbrush, and being careful not to bite the inside of your cheeks or lips during eating are small changes that can make a noticeable difference over time. Stress management matters too, since emotional stress is one of the most consistent triggers people report.