Canker sores happen when your immune system mistakenly attacks the thin lining inside your mouth, creating a small, painful ulcer. The exact reason this occurs varies from person to person, but it usually comes down to a combination of genetics, nutrient gaps, physical irritation, or stress. Most canker sores are harmless and heal on their own within one to two weeks.
What’s Actually Happening Inside Your Mouth
A canker sore isn’t an infection. It’s an immune system overreaction. In people prone to these ulcers, the body releases an inflammatory signal that ramps up immune activity in the mouth’s soft tissue. Certain immune cells then begin targeting and destroying the cells lining the inside of your cheeks, lips, or tongue. The result is that familiar round, white or yellowish crater with a red border.
Because this is an immune-driven process rather than a viral one, canker sores are not contagious. You can’t give someone a canker sore by sharing a drink or kissing. This is one of the key differences between canker sores and cold sores (fever blisters), which are caused by the herpes simplex virus, appear outside the mouth along the lip border, and look like clusters of small fluid-filled blisters.
Genetics Play a Major Role
If your parents get canker sores, you’re far more likely to get them too. When both parents have a history of recurrent canker sores, their child has roughly a 90% chance of developing them. When neither parent is affected, that number drops to about 20%. So if you’ve always been prone to mouth ulcers and your mom or dad was too, genetics is likely the biggest factor.
Nutrient Deficiencies That Trigger Outbreaks
Low levels of certain vitamins and minerals are strongly linked to recurring canker sores. In one study of patients with recurrent oral ulcers, about half were deficient in vitamin B12, nearly 46% had low folate levels, and roughly 11% were low in iron. These nutrients are essential for maintaining healthy tissue inside the mouth, and when levels drop, the lining becomes more vulnerable to breakdown.
You don’t need to be severely anemic or malnourished for this to matter. Even mild, subclinical deficiencies can tip the balance in someone who’s already genetically predisposed. If you notice canker sores coming back frequently, it’s worth having your B12, folate, and iron levels checked through a simple blood test.
Common Triggers You Might Recognize
Beyond the underlying causes, specific triggers often set off individual outbreaks:
- Physical trauma: Biting the inside of your cheek, brushing too aggressively, or irritation from braces or dental work is one of the most common triggers. The damaged tissue gives the immune response a place to start.
- Certain foods: Chocolate, peanuts, and eggs have been reported as triggers in some people, though clinical studies haven’t confirmed a direct causal link. Acidic foods like citrus fruits and tomatoes can also irritate the mouth lining enough to provoke an ulcer.
- Hormonal shifts: Some people notice canker sores appearing at specific points in their menstrual cycle, suggesting that hormonal changes play a role.
- Stress: Many people swear their canker sores appear during stressful periods. The research here is mixed. One study found no measurable difference in cortisol levels between people with and without active canker sores, but stress affects the immune system in complex ways that a single cortisol reading may not capture. Clinically, the pattern is reported often enough that stress remains a widely recognized trigger.
Does Your Toothpaste Matter?
You may have heard that sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, can cause canker sores. The idea is that SLS strips away the protective layer of mucus inside the mouth, leaving tissue more exposed. Some small studies have found that switching to an SLS-free toothpaste reduced ulcer frequency, but a broader review concluded there isn’t enough evidence to say this definitively works. A double-blind study found no significant change in ulcer patterns after removing SLS.
That said, if you’re getting frequent canker sores and haven’t tried switching toothpaste, it’s a low-risk experiment. SLS-free options are widely available and won’t compromise your dental hygiene.
When Canker Sores Signal Something Else
Occasional canker sores are extremely common and rarely a sign of anything serious. But frequent or severe outbreaks can sometimes point to an underlying condition. Celiac disease, Crohn’s disease, and Behçet’s syndrome all include recurrent mouth ulcers as a symptom. Immune deficiencies and certain blood disorders can also cause persistent oral ulceration.
A few patterns are worth paying attention to. Canker sores that don’t heal within two to three weeks, ulcers that are unusually large (bigger than a centimeter across), sores that keep coming back in rapid succession, or ulcers accompanied by other symptoms like joint pain, skin rashes, digestive problems, or fever may warrant further evaluation. Most small canker sores heal without any treatment, but ones that are deeper or larger can take significantly longer and sometimes leave scars.
How to Help Them Heal Faster
Most minor canker sores, the kind smaller than a centimeter, resolve on their own within 7 to 14 days without scarring. You can speed things along and reduce pain with a few simple approaches. Over-the-counter topical pastes containing benzocaine or other numbing agents help with pain. Rinsing with warm salt water several times a day can reduce inflammation and keep the area clean. Avoiding spicy, acidic, or crunchy foods while you have an active sore prevents further irritation.
If you’re dealing with recurrent canker sores, the most productive long-term strategy is identifying your personal triggers. Track whether outbreaks correlate with specific foods, your menstrual cycle, stressful periods, or dental work. Get your B12, folate, and iron checked. Try an SLS-free toothpaste for a few months and see if the frequency changes. For most people, one or two of these adjustments makes a noticeable difference.