Canker sores form when your immune system mistakenly attacks the thin lining inside your mouth. Between 5 and 25% of people get them repeatedly, and in some populations the rate climbs as high as 60%. They’re one of the most common oral complaints, yet the exact chain of events that sets them off involves a surprisingly complex mix of genetics, immune signaling, and everyday triggers.
What Happens Inside Your Mouth
The tissue lining your cheeks, lips, and the floor of your mouth is delicate compared to the tougher, keratinized gums around your teeth. When something goes wrong, specific immune cells called T cells begin attacking this soft tissue directly. They release inflammatory signals that recruit more immune cells to the area, creating a feedback loop of tissue destruction. The result is that familiar shallow, white or yellowish crater ringed with red, inflamed tissue.
People who get frequent canker sores show a measurably different immune profile than those who don’t. They carry a higher proportion of a particular type of T cell (gamma-delta T cells) during active outbreaks, and their blood contains elevated levels of inflammatory markers even between episodes. There also appears to be an abnormality in how their immune system’s early-warning sensors respond to stimuli in the mouth, pushing the inflammatory response toward an exaggerated, tissue-damaging reaction rather than a proportionate one.
Genetics and Family History
If your parents get canker sores, you’re significantly more likely to get them too. Research consistently finds a high frequency of family history among people with recurrent outbreaks. Specific variations in immune-related genes, particularly genes in the HLA system that help your body distinguish its own cells from invaders, have been linked to susceptibility. Certain HLA gene variants appear to predispose people to canker sores, while other variants may actually protect against them. This genetic component helps explain why some people are plagued by canker sores their entire lives while others never get a single one.
The peak age of onset is the second decade of life, with many people experiencing their first outbreak during childhood or adolescence. For most, the frequency of episodes gradually decreases with age.
Common Triggers
Even in people who are genetically predisposed, canker sores don’t appear randomly. They’re typically set off by one or more identifiable triggers.
Physical trauma is by far the most frequent. Biting your cheek, scraping your gum with a chip, or getting poked by an orthodontic wire can all start the process. Studies estimate that 60 to 80% of all oral ulcers are traumatic in origin. Braces are a well-known culprit, as the brackets and wires create repeated friction against the inner cheeks and lips. Aggressive tooth brushing can do the same thing.
Toothpaste ingredients play a role many people don’t realize. Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, strips protective proteins from the mucosal lining. It denatures the proteins that hold epithelial layers together, causing the tissue to peel and leaving it more vulnerable to ulceration. Switching to an SLS-free toothpaste has been shown to reduce canker sore frequency in susceptible people and may speed healing of existing sores.
Stress and sleep deprivation are classic triggers. Emotional or physical stress shifts immune activity in ways that can tip the balance toward the overactive inflammatory response that causes canker sores. Many people notice outbreaks during exam periods, after a move, or during illness.
Certain foods trigger outbreaks in some people. Acidic fruits like citrus and tomatoes, as well as spicy foods, chocolate, and coffee are commonly reported culprits. These foods may irritate the mucosal lining directly or provoke an immune reaction in sensitive individuals.
Nutritional Deficiencies
Low levels of iron, zinc, folic acid, and B vitamins (especially B12) are consistently associated with recurrent canker sores. These nutrients are essential for maintaining healthy mucosal tissue and regulating immune function. In some cases, correcting a deficiency through diet or supplementation reduces or eliminates outbreaks entirely. If you get frequent canker sores with no obvious trigger, a nutritional deficiency is one of the first things worth investigating.
When Canker Sores Signal Something Else
Occasional canker sores are almost always harmless. But frequent or severe outbreaks sometimes point to an underlying condition. Celiac disease, Crohn’s disease, and other inflammatory bowel conditions can produce mouth ulcers as one of their earliest symptoms, sometimes before any digestive problems appear. Behçet syndrome, a systemic inflammatory disorder, uses recurrent oral ulcers as one of its defining diagnostic criteria. Lupus and reactive arthritis can also cause oral lesions that look similar to canker sores.
If your canker sores are unusually large, appear in clusters, recur constantly, or come with other symptoms like joint pain, digestive issues, skin rashes, or unexplained fatigue, there may be more going on than a simple mouth sore.
Three Types and How They Differ
Not all canker sores are the same. The vast majority, around 80%, are minor aphthous ulcers. These are less than 5 millimeters across (roughly the size of a pencil eraser) and heal on their own within 10 to 14 days without scarring. They appear on the soft, non-keratinized surfaces: inner cheeks, inner lips, the floor of the mouth, and the soft palate.
Major aphthous ulcers exceed 1 centimeter in diameter and are considerably more painful. They can take up to six weeks to heal and often leave scars. These are the ones that make eating and talking genuinely difficult.
Herpetiform ulcers are the rarest type. Despite the name, they have nothing to do with the herpes virus. They start as many tiny ulcers, sometimes up to 100 at once, that merge together into large, irregularly shaped sores. They tend to appear later in life than the other types.
Reducing Outbreaks
Because canker sores arise from an immune response you can’t fully control, you can’t guarantee prevention. But you can reduce the frequency and severity by addressing known triggers. Switching to SLS-free toothpaste is one of the simplest changes with the best evidence behind it. Using orthodontic wax to cover sharp brackets, eating with care to avoid biting your cheek, and using a soft-bristled toothbrush all reduce the mechanical trauma that starts many outbreaks.
Keeping a food diary can help you identify personal dietary triggers. Managing stress through sleep, exercise, or whatever works for you removes another common catalyst. And if you suspect a nutritional gap, particularly if you follow a restricted diet, addressing it can make a measurable difference. Most minor canker sores heal fine on their own, but over-the-counter topical gels or rinses containing a numbing agent can reduce pain and protect the sore while it heals.