Canker sores don’t have a single, definitive cause. They result from an immune response where certain white blood cells attack the cells lining your mouth, creating small, painful ulcers. What triggers that response varies widely from person to person, but researchers have identified a consistent set of factors: minor mouth injuries, stress, nutritional deficiencies, hormonal shifts, certain foods, and even your toothpaste.
How Canker Sores Actually Form
Unlike cold sores, which are caused by the herpes virus, canker sores are not an infection and are not contagious. They form entirely inside the mouth, on the inner cheeks, lips, tongue, or soft palate. Cold sores, by contrast, appear outside the mouth around the lip border.
The leading theory is that canker sores are a T-cell mediated immune response. Your immune system essentially overreacts to some trigger, whether that’s bacteria already in your mouth, a bit of tissue damage, or a food protein, and attacks the lining of your mouth in the process. This creates the characteristic shallow, white or yellow crater ringed by red, inflamed tissue. Researchers at McGill University describe it as a kind of “heightened antigenic sensitivity,” where the immune system is too aggressive in responding to everyday exposures.
The Most Common Triggers
Minor Mouth Injuries
Biting the inside of your cheek, poking your gums with a chip, or getting scraped by braces or dental appliances can all set off a canker sore in someone who’s susceptible. Hard, sharp, or abrasive foods like toast, pretzels, raw vegetables, and potato chips are common culprits. Even aggressive tooth brushing can do it. The sores tend to form on soft, non-toughened tissue rather than the harder surfaces of the gums or roof of the mouth.
Stress
Emotional stress is one of the most reliable triggers. Outbreaks spike during high-pressure periods like exams, and researchers list stress as a standalone predisposing factor. The connection likely runs through the immune system: stress hormones can shift immune activity in ways that make the overreaction more likely.
Certain Foods
Several food categories are associated with canker sore flare-ups. Acidic fruits like pineapples, oranges, lemons, and especially strawberries are frequent offenders. Chocolate contains an alkaloid called theobromine that can trigger sores in some people. Nuts, particularly walnuts, peanuts, cashews, and almonds, contain an amino acid called L-arginine that’s been linked to outbreaks. Coffee and alcohol are both acidic enough to irritate susceptible tissue. Some people also react to dairy proteins in milk, cheese, and yogurt, and there’s evidence that cow’s milk sensitivity in particular may play a role. Spicy foods like hot sauce, jalapeños, and curries can also provoke sores, though they more often aggravate existing ones than cause new ones from scratch.
Hormonal Changes
Some women develop canker sores on a predictable schedule tied to their menstrual cycle. The sores tend to appear during the luteal phase, when progesterone levels drop. Women who are prone to them may notice outbreaks during every cycle. Interestingly, canker sores often improve or disappear entirely during pregnancy, when progesterone stays elevated.
Quitting Smoking
This one surprises people. Stopping smoking can actually trigger or worsen canker sores. Nicotine appears to have a mild protective effect on the oral lining, so when it’s removed, some people become more vulnerable to outbreaks. This isn’t a reason to keep smoking, but it does explain why some people notice new mouth sores after quitting.
Your Toothpaste May Be Making It Worse
Sodium lauryl sulfate (SLS), a foaming agent in most toothpastes, is a well-documented canker sore trigger. A systematic review published through the American Dental Association found that switching to an SLS-free toothpaste resulted in fewer ulcers, shorter healing times, fewer overall episodes, and less pain. In the pooled data, people using SLS-free toothpaste developed roughly one fewer ulcer per outbreak on average. If you get canker sores regularly, switching toothpaste is one of the simplest changes you can make.
Nutritional Deficiencies
Chronic or recurring canker sores can signal that your body is low on specific nutrients. The deficiencies most strongly linked to recurrent outbreaks are iron, vitamin B12, and folate. Deficiencies in vitamin B3 (niacin) and vitamin C are also associated with mouth ulcers, though these are less common in developed countries. If your sores keep coming back and you can’t identify an obvious trigger, a blood test checking these levels is a reasonable step. In some cases, supplementing the missing nutrient resolves the problem entirely.
Underlying Health Conditions
Most canker sores are just canker sores. But frequent, severe, or unusually persistent ulcers can sometimes be a sign of an underlying condition. The diseases most commonly associated with recurrent mouth ulcers include celiac disease, Crohn’s disease, and Behçet’s syndrome (an inflammatory condition that causes ulcers in multiple parts of the body). Iron deficiency anemia, HIV, and other immune deficiencies can also present with chronic mouth sores. In children, a condition called PFAPA syndrome causes recurring episodes of fever, mouth sores, sore throat, and swollen neck glands on a predictable cycle.
The key distinction is pattern and severity. An occasional canker sore after biting your cheek is normal. Sores that recur constantly, are unusually large, come with other symptoms, or don’t heal within three weeks warrant investigation.
Three Types of Canker Sores
Not all canker sores are the same size or severity. Minor aphthous ulcers, which account for the vast majority of cases, are less than 5 mm across and heal within two weeks without scarring. Major aphthous ulcers are larger than 10 mm, can last 5 to 10 weeks, and often leave scars. Herpetiform ulcers (despite the name, they’re not caused by herpes) appear as clusters of tiny sores, each just 2 to 3 mm, that typically heal within one to two weeks.
If a mouth ulcer hasn’t healed after three weeks, that’s the threshold at which guidelines recommend further evaluation. Persistent, unexplained ulcers, especially combined with a lump in the mouth or neck, red or white patches, or pain that doesn’t improve, should be assessed to rule out more serious conditions including oral cancer.