Canker Sores: Why They Keep Coming Back and What Actually Helps

A canker sore is small. The pain is not. Up to 25% of people experience recurrent aphthous stomatitis — a condition where oral ulcers keep coming back, months or years on end. The exact cause remains unknown. It is not herpes, not contagious, not caused by poor hygiene. It is a chronic immune-mediated condition with identifiable triggers and no single cure.

Not a Cold Sore: Getting the Basics Right

Canker sores (aphthous ulcers) and cold sores are routinely confused, and the confusion leads to wrong treatment.

Cold sores are caused by herpes simplex virus (HSV-1), appear on the lip border and surrounding skin, and are contagious. Canker sores appear on the non-keratinised mucosa inside the mouth — inner lips, cheeks, tongue, floor of the mouth — are not viral, and cannot be transmitted.

Recurrent aphthous stomatitis (RAS) comes in three clinical types. Minor aphthae — under 10 mm, heal in 7–14 days, account for roughly 80% of cases. Major aphthae — over 10 mm, may take 6 weeks to heal and leave scarring. Herpetiform — clusters of tiny ulcers that look like herpes lesions under a microscope but are not viral.

Getting the type right matters because it determines what management approach makes sense and when to refer.

Triggers: What the Evidence Points To

The aetiology of RAS is described in the literature as multifactorial. That is not a hedge — it reflects genuine complexity. No single cause has been identified, and likely none will be, because different patients have different dominant mechanisms.

Mucosal trauma is the most reproducible trigger. Biting the cheek, a sharp food edge, an orthodontic bracket rubbing the mucosa — in susceptible individuals, the local immune response to minor injury produces an ulcer instead of normal healing. The injury-to-ulcer window is typically 1–3 days.

Psychological stress correlates consistently with flare frequency. A study by Gallo, Mimura and Sugaya (PMC2710437) confirmed the relationship between stress levels and RAS recurrence, though the exact neuroimmune pathway remains under investigation. The practical implication: stress management is not separate from oral health management.

Nutritional deficiencies — particularly iron, folate, vitamin B12 and zinc — occur approximately twice as often in RAS patients compared to controls. These nutrients are involved in epithelial repair and immune regulation. Correcting a deficiency does not guarantee resolution, but leaving it uncorrected leaves a known contributing factor in place.

Hormonal variation accounts for the pattern some women notice: ulcers appear in the luteal phase of the menstrual cycle and often reduce during pregnancy or with hormonal contraceptives. The mechanism is hypothetical, but the clinical observation is consistent.

Systemic disease — coeliac disease, Crohn's disease, ulcerative colitis — can present with RAS as a first or prominent symptom. In coeliac disease, oral ulcers sometimes appear before gastrointestinal symptoms become obvious. When RAS is severe, frequent, or treatment-resistant, systemic investigation is warranted.

SLS in Toothpaste: The Data Is Uncomfortable

Sodium lauryl sulfate (SLS) is a surfactant added to most toothpastes to generate foam and improve distribution. It works well for that purpose. For people with RAS, it may be a problem.

A 2019 systematic review by Alli et al. (PMID 30839136) found that SLS-free toothpaste statistically significantly reduced ulcer count, duration, episode frequency, and pain compared to SLS-containing formulas across multiple randomised controlled trials.

A separate RCT (PMID 22435470) found a more nuanced result: the number of ulcers and episodes did not differ significantly between groups, but ulcer duration and mean pain score were significantly lower with SLS-free toothpaste.

The practical implication is simple: switching to an SLS-free toothpaste is a low-risk, evidence-backed change for anyone with recurrent canker sores. SLS-free toothpaste cleans teeth equally well.

What Actually Helps: Evidence by Category

There is no treatment that eliminates RAS. There are interventions that reduce severity, shorten duration, and manage pain.

Antiseptic rinses. Chlorhexidine is the most studied. A Cochrane-referenced review (PMC4356175) concluded that chlorhexidine reduces ulcer severity and pain and shortens healing time, but does not reduce the frequency of new episodes. This distinction matters: chlorhexidine works on the current ulcer, not the next one. Prolonged use causes tooth staining and taste alteration.

Topical corticosteroids. Triamcinolone acetonide in adhesive paste is widely used as first-line clinical management. A 2022 network meta-analysis (PMC9227309) covering 72 RCTs and 5,272 participants confirmed that triamcinolone, amlexanox, and glycyrrhiza outperform placebo for pain reduction and healing speed. Application is local and short-course; systemic absorption at recommended doses is minimal.

Local anaesthetics. Lidocaine or benzocaine gels provide fast, short-lived pain relief. Their purpose is functional: allowing normal eating and speech during the acute phase, not healing the ulcer.

Nutritional correction. If deficiencies are present, addressing them reduces recurrence. A randomised double-blind placebo-controlled trial by Volkov et al. (PMID 19124628) found that sublingual vitamin B12 at 1000 mcg per day for six months significantly reduced ulcer count, episode duration, and pain — regardless of baseline serum B12 levels. The last point is notable: B12 supplementation appeared to work even when blood levels were within normal range.

Chlorhexidine treats the ulcer in front of you. Vitamin B12 addresses how often new ones appear. These are different problems with different tools.

When to See a Doctor

Most minor aphthae resolve without intervention in 7–14 days. Some situations require professional assessment.

See a dentist or physician if:

• the ulcer does not heal within three weeks;
• it is larger than 1 cm;
• new ulcers appear before previous ones have healed;
• ulcers are accompanied by fever, swollen lymph nodes, skin lesions, or genital ulcers — this pattern may indicate Behcet's disease;
• a child has an unusual first episode;
• you have never had haematinic blood tests and experience frequent recurrences — serum B12, iron, ferritin, and folate are the starting point.

The Bottom Line

Recurrent aphthous stomatitis is manageable, even if it is not curable. Understanding your personal trigger pattern is the most useful starting point.

Practical first steps: switch to SLS-free toothpaste, check for nutritional deficiencies, reduce mucosal trauma where possible. For acute episodes: antiseptic rinse plus topical anaesthetic for pain, topical corticosteroid under medical supervision if needed.

Frequency and severity can be reduced substantially for most people. That is a realistic and worthwhile target.

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