Canker sores on the gums are caused by an overactive immune response that attacks the thin lining of your mouth. Unlike cold sores, which are viral infections that typically appear on or around the lips, canker sores form exclusively inside the mouth, on soft tissues like the gums, inner cheeks, tongue, and soft palate. The exact trigger varies from person to person, but the underlying process involves immune cells destroying small patches of oral tissue, creating those painful, shallow craters.
How the Immune System Creates the Ulcer
Canker sores are not infections. They form when your immune system mistakenly targets the cells lining your mouth. Specific immune cells, including a type called gamma-delta T cells, attack the oral tissue directly while releasing inflammatory signals that keep the damage going. People with active canker sores have a higher proportion of these aggressive immune cells compared to people without sores.
One leading theory is that oral bacteria naturally present in your mouth share structural similarities with proteins in your own tissue. Your immune system mounts a response against the bacteria but ends up damaging the gum lining in the process. People prone to canker sores also appear to have an abnormal inflammatory pathway that skews the immune response toward tissue destruction rather than a measured defense.
Nutritional Deficiencies
Low levels of certain vitamins and minerals are consistently linked to recurring canker sores. In a study comparing 35 people with frequent sores to 26 healthy controls, vitamin B12 deficiency stood out the most: eight of the 35 canker sore patients had B12 levels below 200 pg/mL, a statistically significant difference from the control group. Iron, ferritin, and folic acid were also low in some patients, though those differences were less pronounced.
If you get canker sores repeatedly, especially more than three times a year, a simple blood test can check for these deficiencies. B12 is particularly worth investigating because it’s common in vegetarians, vegans, and older adults whose absorption declines with age. Correcting a deficiency often reduces how frequently sores return.
Physical Damage to the Gums
Mechanical trauma is one of the most straightforward triggers. Biting your cheek, brushing too hard, or jabbing your gums with a sharp chip can break the tissue just enough for a canker sore to take hold. Orthodontic hardware is a major culprit: in a study of 100 patients with fixed braces, 31% developed oral ulcerations, most of them from arch wires pressing against the inner cheeks and lips. While most of those were classified as traumatic ulcers, braces also increase the frequency of true canker sores in some patients.
Ill-fitting dentures, sharp tooth edges, and even aggressive flossing can do the same thing. The gum tissue is thinner and more delicate than the roof of your mouth or the outer surface of your lips, making it especially vulnerable to small injuries that spiral into full sores.
Toothpaste Ingredient: Sodium Lauryl Sulfate
Sodium lauryl sulfate (SLS) is a foaming agent found in most commercial toothpastes, and it’s a well-documented canker sore trigger. SLS strips away the mucin layer, a protective coating of proteins that shields your gum tissue from irritants. Once that barrier is compromised, the exposed tissue becomes more permeable to food acids, bacteria, and even the SLS itself, which can penetrate into deeper layers and cause further damage.
SLS doesn’t directly cause canker sores in everyone, but for people already prone to them, it removes the protective buffer that would normally prevent a minor irritation from becoming a full ulcer. Switching to an SLS-free toothpaste is one of the simplest changes you can make if you get frequent sores. Several widely available brands market themselves as SLS-free for exactly this reason.
Food Triggers
Certain foods irritate the gum lining enough to provoke a sore, especially if your mucin barrier is already weakened. The most common triggers fall into a few categories:
- Acidic foods: citrus fruits, tomatoes, tomato sauce, strawberries, and coffee. The acid directly irritates soft gum tissue.
- Spicy foods: hot peppers, curry, and salsa can inflame sensitive oral tissue and worsen existing sores.
- Sharp or rough-textured foods: almonds, walnuts, sunflower seeds, chips, and pretzels create tiny abrasions that can become canker sores. The salt on many snack foods adds a second layer of irritation.
- Chocolate: contains theobromine, a compound that triggers oral irritation in some people.
- Carbonated drinks: soda and sparkling water are acidic enough to irritate the mouth lining over time.
Not everyone reacts to the same foods. If you notice a pattern between eating something specific and getting a sore a day or two later, that’s a meaningful signal worth acting on. Some people also react to dairy proteins or food preservatives, though these triggers are less common.
Stress, Anxiety, and Depression
Psychological stress is one of the most frequently reported triggers, though the biological mechanism is more nuanced than “stress weakens your immune system.” A study published in Contemporary Clinical Dentistry measured salivary cortisol (the body’s primary stress hormone) in canker sore patients and found no significant difference compared to healthy controls. Cortisol levels didn’t correlate with sore severity either.
What the study did find was a strong association between canker sores and anxiety and depression. This suggests the connection between stress and canker sores may work through behavioral pathways (disrupted sleep, poor diet, teeth clenching, neglected oral care) rather than a simple hormonal surge. The practical takeaway is the same: periods of high emotional strain tend to bring on sores, even if cortisol isn’t the direct cause.
Hormonal Changes
Some women get canker sores at predictable points in their menstrual cycle, typically in the luteal phase when progesterone and estrogen levels shift. A study in women with cycle-associated sores found that those who took supplemental ovarian hormones (through hormone replacement therapy or oral contraceptives) experienced complete or partial remission at a statistically significant rate. Women whose sores were unrelated to their cycle saw no benefit or even a worsening of symptoms.
This pattern suggests that hormonal fluctuations can directly influence the oral immune environment, at least in a subset of women. If your canker sores reliably appear in the week before your period, the hormonal connection is likely playing a role.
Underlying Health Conditions
Recurrent canker sores can sometimes signal a systemic condition rather than a standalone problem. Celiac disease is one of the better-known links: the intestinal damage from gluten intolerance impairs nutrient absorption, leading to the same B12, iron, and folate deficiencies that trigger sores. Some people with celiac disease develop canker sores as their first noticeable symptom, well before digestive issues appear.
Inflammatory bowel diseases like Crohn’s disease and ulcerative colitis are also associated with oral ulcers. Behcet’s disease, a rarer inflammatory condition, lists painful mouth sores that look identical to canker sores as its most common symptom. In Behcet’s, these sores begin as raised round lesions that quickly ulcerate, typically healing within one to three weeks but returning frequently. The distinguishing feature is that Behcet’s also causes ulcers in other areas (genitals, skin, eyes), so mouth sores alone don’t indicate the condition.
Types and Healing Timelines
Not all canker sores behave the same way. They’re classified into three types based on size and pattern:
- Minor canker sores are the most common, measuring less than 1 cm across. They’re shallow, oval-shaped, and typically heal on their own within one to two weeks without scarring.
- Major canker sores are larger and penetrate deeper into the tissue. These can take weeks or even over a month to heal and sometimes leave scars. They’re significantly more painful and can interfere with eating and speaking.
- Herpetiform canker sores appear as clusters of many tiny sores that can merge together. Despite the name, they have nothing to do with the herpes virus. They tend to recur frequently.
If a sore on your gums hasn’t healed after three weeks, is unusually large, or keeps coming back in the same spot, that’s worth a closer look from a dentist or doctor, particularly to rule out the systemic conditions mentioned above or other causes of persistent oral ulceration.