Canker sores are small, painful ulcers that form inside the mouth, typically on the inner cheeks, lips, tongue, or soft palate. They are not contagious, not caused by a virus, and in most cases heal on their own within one to two weeks. Nearly everyone gets one at some point, and for some people they come back regularly, a condition called recurrent aphthous stomatitis.
What Canker Sores Look and Feel Like
A canker sore usually starts as a small red spot that quickly develops into a round or oval ulcer with a yellowish-gray center and a bright red border. Before the sore appears, you may notice a tingling or burning sensation in the spot where it’s forming. The pain tends to peak in the first few days, making eating, drinking, and even talking uncomfortable, especially if the sore is on the tongue or near the gum line.
Most canker sores are the minor type, measuring under 10 mm across. These heal within 7 to 14 days without leaving a scar. Major canker sores are larger, deeper, and can take weeks or even months to fully heal, sometimes leaving scar tissue behind. A third, less common variety called herpetiform canker sores appears as clusters of very tiny ulcers, each only 1 to 3 mm across. Despite the name, herpetiform canker sores have nothing to do with the herpes virus. They tend to merge into larger irregular sores and typically heal within 7 to 10 days.
Why They Happen
The exact cause isn’t fully understood, but the process involves an immune response that attacks the lining of your mouth. Immune cells flood into the tissue, and the body overproduces inflammatory signaling molecules. This inflammatory cascade breaks down the surface layer of the oral mucosa, creating the open ulcer you see and feel. Researchers have also found a three-fold increase in certain inflammatory cells (mast cells) inside canker sore tissue compared to other types of mouth ulcers, which helps explain why they can be disproportionately painful for their size.
Several triggers can set this process in motion:
- Minor mouth injuries: Biting the inside of your cheek, aggressive tooth brushing, dental work, or rough-edged braces can all trigger a sore at the injury site.
- Stress and fatigue: Emotional or physical stress is one of the most commonly reported triggers for recurrent sores.
- Certain foods: Acidic or spicy foods, including citrus fruits, tomatoes, and chocolate, can irritate the mouth lining enough to provoke an outbreak.
- Hormonal shifts: Some people notice sores appearing in a predictable pattern tied to their menstrual cycle.
- Nutritional gaps: Deficiencies in vitamin B12, iron, and folate are found in roughly 14 to 18 percent of people with recurrent canker sores. Correcting these deficiencies can reduce how often sores return.
- Toothpaste ingredients: Sodium lauryl sulfate (SLS), a foaming agent in many toothpastes, is a well-documented irritant. A systematic review of clinical trials found that switching to an SLS-free toothpaste significantly reduced the number of ulcers, the duration of each episode, the number of recurrences, and the level of pain.
Canker Sores vs. Cold Sores
People often confuse these two, but they are completely different conditions. Canker sores form inside the mouth and are not contagious. Cold sores (fever blisters) are caused by the herpes simplex virus, appear outside the mouth (usually around the border of the lips), and spread easily through direct contact. Cold sores start as fluid-filled blisters that eventually crust over. Canker sores never blister; they’re open ulcers from the start. If your sore is inside your mouth and there’s no blister stage, it’s almost certainly a canker sore.
How to Manage the Pain
Most minor canker sores don’t require treatment and resolve on their own. But because they can make a week or two genuinely miserable, several options can cut down on pain and speed things along.
Rinsing your mouth with warm salt water a few times a day is one of the simplest and most effective home remedies. A rinse made with a teaspoon of baking soda dissolved in warm water also works by neutralizing acids in the mouth that irritate the ulcer. Avoid spicy, acidic, and crunchy foods while a sore is active, since these aggravate the exposed tissue.
Over-the-counter numbing gels and pastes containing benzocaine (sold under brand names like Anbesol and Zilactin-B) can provide temporary pain relief when applied directly to the sore. Antiseptic rinses containing hydrogen peroxide can also help keep the area clean and reduce irritation. For best results, apply these products as soon as you feel a sore forming.
For more severe or persistent sores, a doctor or dentist may prescribe a steroid-containing mouth rinse to reduce inflammation or a topical solution that chemically cauterizes the sore, potentially reducing healing time to about a week. Silver nitrate, another cauterizing option, can relieve pain but hasn’t been shown to speed healing itself.
Reducing How Often They Come Back
If you get canker sores frequently, the most practical first step is switching to an SLS-free toothpaste. This is a low-effort change backed by solid clinical evidence. Check the ingredients list on your toothpaste for “sodium lauryl sulfate.” Several major brands sell SLS-free versions.
It’s also worth paying attention to whether specific foods consistently precede your outbreaks. Keeping a simple food diary for a few weeks can reveal patterns. If you suspect a nutritional deficiency, a blood test can check your B12, iron, and folate levels. Supplementation, when a deficiency is confirmed, can meaningfully reduce recurrence rates.
Managing stress is easier said than done, but people with recurrent sores often notice a clear connection between high-stress periods and new outbreaks. Anything that reduces your baseline stress, whether that’s sleep, exercise, or something else, may translate to fewer sores.
When a Canker Sore May Signal Something Else
A single canker sore that heals within two weeks is rarely a sign of anything serious. But certain patterns deserve attention. Any mouth ulcer lasting longer than two weeks should be evaluated, as prolonged ulcers can sometimes indicate other conditions that require testing.
Canker sores can be an early sign of inflammatory bowel diseases like Crohn’s disease or ulcerative colitis. In some cases, mouth ulcers appear months or even a year before any abdominal symptoms develop. The oral sores associated with Crohn’s disease tend to be deep, linear ulcers with raised edges, often in the cheek area, which look different from typical round canker sores.
Behcet’s disease, a rare inflammatory condition, also causes mouth ulcers that look similar to canker sores but tend to be more numerous, longer-lasting, and more painful. The distinguishing feature is that Behcet’s also causes genital ulcers, eye inflammation, and joint pain. Iron deficiency anemia and pernicious anemia (from severe B12 deficiency) can both cause small, shallow ulcers that mimic canker sores as well.
Patterns worth flagging to a healthcare provider include sores that are unusually large, sores that recur very frequently without obvious triggers, sores accompanied by fever or fatigue, and ulcers that appear alongside symptoms in other parts of the body like the eyes, skin, or digestive tract.