If you keep getting canker sores, the short answer is that your immune system overreacts to minor triggers inside your mouth, and a combination of genetics, nutritional gaps, stress, and sometimes an underlying health condition determines how often that happens. Recurrent canker sores affect anywhere from 5% to 66% of the population depending on the group studied, making them one of the most common oral complaints. But “common” doesn’t mean random. There are specific, identifiable reasons some people get them repeatedly while others never do.
Your Immune System Treats Minor Damage as a Major Threat
Canker sores aren’t infections. They form when your immune system launches a disproportionate inflammatory response to something relatively harmless, like biting your cheek, eating rough food, or brushing too hard. In people prone to canker sores, certain white blood cells flood the tissue of the mouth lining, releasing inflammatory signals that break down the surface and create a painful open ulcer.
Stress hormones, particularly cortisol, play a direct role in this process. When you’re under psychological stress, your body ramps up cortisol production, which in turn shifts the balance of immune cells and inflammatory chemicals in your body. Research published in the Journal of Oral Medicine and Oral Surgery found that stress changes nervous system markers, hormonal markers, and immune system activity, all of which can initiate and worsen canker sore episodes. Stressful events are particularly likely to trigger new outbreaks in people who already have a history of them. If you notice your sores cluster around deadlines, poor sleep, or emotionally difficult periods, that connection is well established.
Nutrient Deficiencies Are Surprisingly Common
One of the most actionable reasons people get frequent canker sores is a shortage of specific vitamins and minerals. A study comparing 57 patients with recurrent canker sores to 57 healthy controls found striking deficiency rates in the canker sore group: about half (50.1%) were deficient in vitamin B12, nearly half (45.6%) had low folate levels, and roughly 1 in 10 (10.5%) had low iron stores.
These nutrients all play roles in maintaining healthy mucosal tissue, the delicate lining inside your mouth. When levels drop, the tissue becomes more fragile and slower to repair, making it easier for ulcers to form and harder for them to heal. The good news is that these deficiencies are detectable through a standard blood test and correctable through diet or supplements. Foods rich in B12 include meat, fish, eggs, and dairy. Folate is found in leafy greens, legumes, and fortified grains. Iron comes from red meat, lentils, and spinach.
Gut Conditions You Might Not Know About
Frequent canker sores can be an early or overlooked sign of celiac disease or inflammatory bowel conditions like Crohn’s disease. A study in the journal Digestive and Liver Disease compared 269 children with confirmed celiac disease to 575 healthy controls and found that mouth ulcers appeared in 22.7% of the celiac group versus just 7.1% of controls. That makes someone with celiac disease more than four times as likely to develop canker sores.
This matters because celiac disease often goes undiagnosed for years, and mouth ulcers may be the most visible symptom. If you get canker sores frequently and also experience bloating, fatigue, unexplained weight changes, or irregular digestion, it’s worth getting screened. In many celiac patients, canker sores improve or resolve entirely once gluten is removed from the diet.
Genetics and Family History
If your parents got canker sores, you’re significantly more likely to get them too. The tendency runs in families, and researchers believe it involves inherited variations in how the immune system regulates inflammation in mucosal tissue. You can’t change your genetic predisposition, but knowing it exists helps explain why some people get sores from triggers that wouldn’t bother someone else, and it shifts the focus toward managing the modifiable factors like nutrition, stress, and oral care.
What About Toothpaste?
You may have heard that sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, triggers canker sores. This idea has been around for years, and many people swear by SLS-free toothpaste. However, the evidence is weak. A 2019 review found there was not enough data to determine whether SLS-free toothpastes reduced ulcer duration, frequency, pain, or number. A double-blind study found that switching to SLS-free toothpaste produced no significant change in ulcer patterns. Allergens in toothpaste also did not appear to trigger canker sores.
That said, some individuals do notice a difference. If you want to try switching, it’s low-risk and inexpensive. Just don’t expect it to be a cure if your underlying triggers are nutritional or immune-related.
Other Common Triggers
Beyond the major causes, several everyday factors can set off an episode in someone who’s already prone:
- Physical trauma: Biting your cheek, aggressive brushing, dental work, or braces rubbing against tissue
- Acidic or spicy foods: Citrus fruits, tomatoes, and hot peppers irritate the mouth lining and can trigger or worsen sores
- Hormonal shifts: Some women notice sores correlate with their menstrual cycle, suggesting hormonal fluctuations play a role
- Sleep deprivation: Poor sleep raises cortisol and suppresses immune regulation, creating conditions for outbreaks
How to Reduce Outbreaks
Since canker sores result from multiple overlapping causes, the most effective approach addresses several at once. Start with a blood test to check your B12, folate, and iron levels, especially if you eat a limited diet or follow a vegetarian or vegan lifestyle. Correcting a deficiency can dramatically reduce how often sores appear.
Stress management matters more than most people realize. Regular sleep, exercise, and whatever coping strategies work for you aren’t just general wellness advice; they directly influence the hormonal and immune pathways that trigger canker sores. If your outbreaks clearly follow stressful periods, this is one of the most effective interventions available.
For treating sores once they appear, prescription topical pastes applied directly to the dried ulcer several times daily can reduce pain and speed healing. Over-the-counter numbing gels provide temporary relief. Most minor canker sores heal on their own within one to two weeks.
When a Sore Isn’t Just a Canker Sore
Most canker sores are harmless, but certain features warrant attention. According to Cleveland Clinic, you should have a mouth sore evaluated if it lasts longer than two weeks, bleeds persistently, or is getting worse rather than better. Red, white, or mottled patches in the mouth, a hard bump under the sore, or an area that becomes permanently discolored are all signs that something other than a canker sore may be involved. A sore that doesn’t heal is the key distinction, since canker sores, even stubborn ones, resolve within a few weeks.