TL;DR
Canker sores are not caused by a virus. They result from the immune system mistakenly attacking the lining of the mouth. Multiple triggers can set this off in people who are genetically susceptible to it. The most evidence-backed triggers are: nutritional deficiencies (B12, iron, folate, zinc), SLS in toothpaste, psychological stress, oral trauma, and hormonal fluctuations. This page breaks down the mechanism behind each.
The Core Mechanism: Immune Dysregulation
Before listing triggers, it's important to understand why triggers cause ulcers — because the same trigger (e.g., stress) can affect everyone differently. In canker sore sufferers, the immune system overreacts to minor irritation in the mouth that most people's bodies would simply ignore.
Current evidence points to:
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Inflammatory proteins spike at the ulcer site: Two proteins that signal tissue destruction — TNF-α and IL-6 — are significantly elevated in canker sore tissue compared to the surrounding healthy mouth lining. TNF-α appears to be the key driver of tissue damage (Natah et al., 2004 — PMID: 14988753).
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Immune cells attack the mouth lining: Early ulcers show one type of immune cell (CD4+ T-cells) flooding the area; established ulcers show a different, more destructive type (CD8+ cytotoxic T-cells) taking over. This is the same kind of cell-mediated attack the immune system uses against viruses — but here it's targeting healthy tissue.
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The immune system's "off switch" isn't working: Healthy immune responses have a built-in brake — regulatory T-cells that call off the attack once a threat is resolved. In chronic canker sore sufferers, these brakes are less effective, so the inflammatory response keeps going longer than it should.
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Early warning cells fire before the ulcer appears: Immune cells called mast cells activate and release inflammatory chemicals 1–2 days before the ulcer becomes visible — which is why you often feel a burning or tingling sensation before anything shows up.
This is not a simple infection or a textbook autoimmune disease. It's a misfiring immune response that, so far, has no single identified cause.
Nutritional Deficiencies
This is the most useful category to investigate first — deficiencies show up on a standard blood test and can be corrected.
Vitamin B12 Deficiency
Vitamin B12 deficiency is significantly overrepresented in RAS patients. One study found deficiency in 28% of RAS patients versus 3% of controls (Volkov et al., 2005 — PMID: 15705112). The mechanism likely involves B12's role in maintaining the mucosal epithelium and modulating immune function — B12 deficiency impairs DNA synthesis in rapidly dividing epithelial cells, increasing mucosal fragility.
More compelling: supplementation works. An RCT (Volkov et al., 2009 — PMID: 20012098) found that 1000mcg sublingual B12 nightly reduced ulcer frequency significantly versus placebo, regardless of baseline serum B12 levels. This is one of the stronger RCTs in this space.
Worth knowing: B12 deficiency is often an absorption problem, not a dietary one. If you eat animal products but your B12 stays low, the culprit may be intrinsic factor — see Intrinsic Factor, B12 Absorption, and Canker Sores.
Iron Deficiency
Iron deficiency disrupts mucosal integrity via impaired oxidative metabolism in epithelial cells. Studies have found serum ferritin levels significantly lower in RAS patients (Rogers & Hutton, 1986 — PMID: 3531785). Iron deficiency anemia should be screened with ferritin, not just hemoglobin — ferritin depletion precedes anemia.
Folate Deficiency
Folate (vitamin B9) is required for DNA synthesis and cell proliferation. Mucosal cells turn over rapidly; folate deficiency creates a vulnerability. Associated with RAS in several case series (Wray et al., 1975 — PMID: 1165839), particularly in patients with celiac disease (which impairs folate absorption) and Crohn's disease (where sulfasalazine depletes folate). Note: roughly 40% of the population carries an MTHFR gene variant that impairs conversion of dietary folic acid into the bioactive form — methylfolate supplementation bypasses this.
Vitamin D Deficiency
Vitamin D deficiency is found at elevated rates in RAS patients across multiple independent studies — mean 25(OH)D levels consistently run 8–10 ng/mL lower in RAS patients than in controls (Esen et al., 2020 — PMID: 31849023). The mechanism is direct: the Vitamin D receptor (VDR) on T-lymphocytes suppresses Th1 immune activity — the same IFN-γ and TNF-α pathway that drives mucosal destruction in aphthous ulcers. Deficiency removes this brake. Vitamin D also upregulates cathelicidin, an antimicrobial peptide supporting mucosal barrier integrity. Deficiency is common (~41% of US adults below the 20 ng/mL threshold); testing and correcting it is a low-threshold intervention.
Zinc Deficiency
Zinc is essential for epithelial repair and T-cell immune regulation. Zinc deficiency both weakens the mucosal barrier and dysregulates the immune response — a dual mechanism. Serum zinc has been found lower in RAS patients versus controls in multiple studies (Orbak et al., 2003 — PMID: 12756553).
See the full supplement guide: Best Supplements for Canker Sore Prevention →
Sodium Lauryl Sulfate (SLS) in Toothpaste
SLS is a detergent used in most commercial toothpastes. It disrupts the oral mucin layer — the protective protein film over the epithelium. Without this layer, the mucosa is more vulnerable to physical trauma and immune-mediated damage.
The evidence is reasonably consistent:
- Herlofson & Barkvoll (1994 — PMID: 8088761): Switching from SLS-containing to SLS-free toothpaste reduced aphthous ulcer frequency by 64% in a double-blind crossover study.
- Multiple subsequent studies have replicated this association.
Verdict: Strong evidence. If you use SLS-containing toothpaste and suffer from RAS, switching to SLS-free is a low-risk, evidence-supported intervention. See our review: Do Home Remedies for Canker Sores Actually Work? →
Psychological Stress
Stress is consistently reported as a trigger, and the mechanism is well-characterized.
Stress triggers a hormone cascade that raises cortisol, which suppresses a protective protein (secretory IgA) in your saliva that normally helps guard the mouth lining. With less of it, the mucosa becomes more vulnerable to the immune triggers that kick off an ulcer. Stress hormones also disrupt the behavior of the immune cells already prone to misfiring in canker sore sufferers.
Multiple studies have confirmed the stress-canker sore association, though it's not perfectly predictable — stress seems to lower the threshold for an outbreak rather than directly cause one. High stress doesn't guarantee a sore, but it makes one more likely.
Oral Trauma
Minor trauma — biting the cheek, dental procedures, ill-fitting dentures, sharp food edges — is a well-established trigger, particularly in susceptible individuals. In people who don't have RAS, the same trauma heals without ulceration. In RAS sufferers, trauma can initiate the immune cascade that produces ulcers.
This likely explains why ulcers preferentially form at specific sites: the movable mucosa (inner cheeks, tongue), which is subject to more mechanical stress.
Two of the most common trauma triggers have dedicated guides: Canker Sores From Braces and Canker Sores After Dental Work.
Hormonal Fluctuations
Many women report canker sores in the premenstrual phase. Progesterone fluctuations are thought to affect mucosal immune function. Studies have documented a significant association between RAS outbreaks and the hormonal cycle (luteal phase of the menstrual cycle), and some patients report remission during pregnancy — when progesterone is elevated and stable (Ship, 1965 — PMID: 14282450).
This is not universal, but the pattern is notable enough to track if you menstruate.
Genetic Predisposition
RAS has a strong familial component. If both parents have RAS, their offspring have a ~90% chance of developing it — a pattern that confirms strong genetic predisposition. HLA typing studies have identified several associated alleles, though no single HLA type is necessary or sufficient.
Genetics establishes the susceptibility — environmental triggers determine whether and when ulcers occur. This is why identical twins with the same HLA profile can differ in outbreak frequency.
Celiac Disease and IBD
Celiac disease and inflammatory bowel disease (Crohn's and ulcerative colitis) are both significantly associated with RAS. The association is likely multifactorial:
- Malabsorption of B12, iron, folate, and zinc → nutritional deficiency mechanism
- Systemic immune dysregulation → generalized mucosal vulnerability
- In Crohn's disease specifically: oral manifestations including aphthous ulcers are a recognized feature
If you have frequent, severe, or atypical RAS that doesn't respond to nutritional interventions, screening for celiac and IBD is clinically indicated. For a deeper look at the gluten connection — and the critical reason to get tested before going gluten-free — see Does a Gluten-Free Diet Help Canker Sores?
What Does NOT Cause Canker Sores
- Herpes simplex virus: Despite the common confusion with cold sores, HSV is not involved in RAS. They are mechanistically distinct.
- Poor dental hygiene: No evidence supports this. Some antiseptic mouthwashes may actually irritate the mucosa and worsen outbreaks.
- Citrus / acidic foods: These are irritants that worsen existing ulcers, not causes of new ones. For the full breakdown of which foods genuinely trigger outbreaks vs. which just aggravate active ulcers, see Foods That Cause Canker Sores.