The most common causes of canker sores are minor mouth trauma, stress and hormonal changes, SLS toothpaste, acidic or spicy foods, nutrient deficiencies (vitamin B12, iron, folate, zinc), underlying health conditions, and genetic predisposition. Canker sores — technically called aphthous ulcers — rarely have a single cause. They result from a breakdown of the mouth's delicate mucous membrane, usually triggered by several of these factors overlapping. Figuring out which triggers apply to you is the most effective way to reduce how often you get them.
One important distinction first: canker sores are not caused by the herpes simplex virus. That's cold sores — a different condition entirely, caused by HSV-1 and appearing on or outside the lips. Canker sores appear inside the mouth, on the soft tissue, and are not contagious. They are, however, very common — affecting roughly one in five people.
Canker Sore Causes at a Glance
| Cause category | How it triggers a canker sore | What to do |
|---|---|---|
| Physical trauma | Minor injury to the soft tissue inside the mouth — biting your cheek, a hard toothbrush, or a rubbing dental appliance. | Use a soft-bristle toothbrush, be mindful of hard or sharp foods, and have dental appliances checked if they rub. |
| Stress & hormonal changes | High stress raises cortisol, which suppresses immune function; shifting hormone levels before menstruation follow a predictable pattern for many women. | Manage stress actively through exercise, adequate sleep, and stress-reduction techniques. |
| SLS toothpaste | Sodium Lauryl Sulfate, a foaming agent in most mainstream toothpastes, is associated with increased canker sore frequency. | Switch to an SLS-free toothpaste — one of the most commonly reported improvements. |
| Dietary triggers | Acidic foods (citrus, tomatoes) and spicy foods can irritate or break down the mucous-membrane lining. | Keep a short food diary during a period of frequent outbreaks to identify and limit your personal triggers. See how diet and food triggers affect mouth ulcers. |
| Nutrient deficiency | Low levels of vitamin B12, iron, folate, or zinc can impair tissue repair and immune function, contributing to ulcers. (Summary — full detail in the diet guide.) | Ask your doctor for a simple blood test to check B12, iron, folate, and zinc. Read the full nutrient-deficiency guide. |
| Underlying health conditions | Recurrent ulcers can be an oral sign of celiac disease, inflammatory bowel disease (Crohn's, ulcerative colitis), Behçet's disease, or immune-suppressing conditions. | See your doctor if ulcers are frequent, severe, or accompanied by other symptoms, so an underlying condition can be ruled out. |
1. Physical Trauma
The most common and direct trigger for canker sores is minor physical injury to the soft tissue inside the mouth. The mucous membrane lining your cheeks, lips, tongue, and floor of the mouth is thin and surprisingly vulnerable. Common culprits include:
- Accidentally biting your cheek or lip — a single bite is enough to start an ulcer in people who are prone to them
- Sharp, hard, or abrasive foods — crusty bread, chips, and popcorn hulls can cause micro-lacerations that develop into ulcers
- Dental appliances — ill-fitting dentures, braces brackets, or retainers that rub against the tissue can cause recurring sores in the same location
- Aggressive tooth brushing — scrubbing with a hard-bristled brush is a frequently overlooked source of minor oral trauma
- Dental procedures — injections, impressions, and cleanings can occasionally cause sores in the days following an appointment
Getting canker sores in the same spot repeatedly? Look for a mechanical cause first. A small adjustment — a soft-bristle toothbrush, an appliance refit, or more care with hard foods — can sometimes eliminate recurring sores entirely.
2. Stress and Hormonal Changes
The relationship between psychological stress and canker sores is well documented. During periods of high stress, cortisol levels rise, which suppresses aspects of immune function — including the localized immune response that keeps the mucous membrane healthy and resilient. That makes the tissue more vulnerable to breakdown from minor irritants it would normally handle without incident.
The timing often seems cruel. Canker sores tend to appear during exam periods, work deadlines, or emotionally difficult stretches — exactly when you least want mouth pain.
Hormonal fluctuations are a related but distinct trigger, particularly for women. Many women notice a predictable pattern of canker sores in the days before their menstrual cycle, when progesterone and estrogen levels shift and immune sensitivity changes. Some also report sores during pregnancy or when starting or stopping hormonal contraception. If your outbreaks follow a hormonal pattern, tracking your cycle alongside them can help confirm the connection.
3. SLS Toothpaste
Sodium Lauryl Sulfate (SLS) — the foaming agent in most mainstream toothpastes — has been associated with increased canker sore frequency (Cleveland Clinic). SLS can disrupt the mucin layer that protects the oral mucosa, leaving the tissue more exposed to irritation.
Because this trigger sits in your bathroom rather than your kitchen, it's easy to overlook — and easy to fix. Switching to an SLS-free toothpaste is one of the most commonly reported improvements among people who get recurring sores, and it's a simple, low-risk change to try first. Most pharmacies and health food stores carry SLS-free options.
4. Diet: Food Triggers and Nutrient Deficiencies
Diet contributes to canker sores in two distinct ways. First, certain foods — acidic ones like citrus and tomatoes, and spicy dishes — can directly irritate or break down the mucous-membrane lining in people who are prone to ulcers. Gluten can also be a factor for those with celiac disease or gluten sensitivity.
Second, deficiencies in vitamin B12, iron, folate, and zinc are each linked to more frequent or slower-healing ulcers, because these nutrients support tissue repair and immune function (StatPearls; Cleveland Clinic). A simple blood test can identify a deficiency, and correcting one can sometimes dramatically reduce outbreak frequency.
We cover both in depth — which foods to watch, which nutrients matter, and how to correct each gap — in our dedicated guide to how diet and nutrient deficiencies trigger mouth ulcers.
5. Underlying Health Conditions
Occasional canker sores are a normal part of life for many people. But if you're experiencing frequent, large, or unusually slow-healing ulcers, it's worth considering whether an underlying condition may be contributing:
- Celiac disease: Recurrent mouth ulcers are one of the known oral manifestations of celiac disease. In some cases, they appear even before gastrointestinal symptoms do.
- Inflammatory bowel disease (IBD): Both Crohn's disease and ulcerative colitis are associated with recurrent aphthous ulcers. The mouth and digestive tract share the same mucosal lining, and systemic inflammation affects both.
- Behçet's disease: A rare autoimmune condition characterized by recurrent oral ulcers, genital ulcers, and eye inflammation. If you have recurrent severe ulcers in multiple locations, see a physician.
- Immunodeficiency conditions: Any condition that suppresses immune function — whether from disease or medication — can increase susceptibility to oral ulcers.
If your canker sores are large (greater than 1 cm), taking longer than 3 weeks to heal, accompanied by fever, or appearing in clusters regularly, consult a physician. Most canker sores are benign and self-resolving, but persistent or unusual presentations warrant investigation.
6. Genetic Predisposition
There's a meaningful hereditary component to canker sore susceptibility. Studies of recurrent aphthous stomatitis report a positive family history in roughly 24 to 46 percent of people affected (StatPearls). If a parent gets canker sores regularly, you're more likely to as well. The specific genetic mechanisms aren't fully understood, but the pattern is consistent enough that family history is considered a real risk factor. This doesn't make canker sores inevitable — it means your baseline threshold is lower, so managing the other triggers matters more for you.
How Can You Reduce How Often You Get Canker Sores?
Knowing your triggers is the foundation. Here are the most practical, evidence-informed steps:
- Switch to an SLS-free toothpaste. One of the most commonly reported improvements, and a low-risk change to try first.
- Manage stress actively. Exercise, adequate sleep, and stress-reduction techniques have documented effects on immune function. They won't eliminate stress as a trigger, but they can raise your threshold.
- Identify and limit food triggers. Keep a simple food diary during a period of frequent outbreaks and look for patterns. Our diet and mouth ulcers guide covers the most common culprits.
- Get your B12, iron, folate, and zinc checked. A simple blood test can identify deficiencies. Supplementing a confirmed deficiency is often more effective than any topical treatment.
- Protect your oral tissue. Use a soft-bristle toothbrush, be mindful of hard foods, and have dental appliances checked if they're rubbing.
What Should You Do When You Already Have One?
Prevention matters enormously, but even with all the right habits, a canker sore will occasionally break through. Untreated, most heal on their own within about two weeks (Cleveland Clinic). When one appears, you can either manage the pain while you wait — or treat the ulcer surface itself.
ORALMEDIC uses chemical cautery to seal the ulcer surface in a single application. Unlike numbing gels or protective pastes, which mask pain while the ulcer heals on its own, it treats the ulcer surface directly. According to the manufacturer, this stops pain quickly and reduces healing time to 3 to 5 days, compared with the roughly two weeks an untreated sore typically takes.
Understanding your triggers reduces how often canker sores appear. For the ones that still break through, a single-application treatment means you're not reapplying numbing gel for a week.
Frequently Asked Questions
What causes canker sores?
Canker sores usually stem from several overlapping triggers rather than one cause. The most common are minor mouth trauma, stress and hormonal changes, SLS toothpaste, acidic or spicy foods, nutrient deficiencies, an underlying health condition, and a genetic predisposition. Identifying your personal triggers helps reduce how often they appear.
What deficiency causes canker sores?
Deficiencies in vitamin B12, iron, folate, and zinc are the ones most often linked to recurrent mouth ulcers, because each supports tissue repair or immune function (StatPearls). A simple blood test can identify them. See our diet and mouth ulcers guide for the full nutrient breakdown and how to correct each one.
Can SLS toothpaste cause canker sores?
Sodium Lauryl Sulfate (SLS), a foaming agent in most mainstream toothpastes, is associated with increased canker sore frequency (Cleveland Clinic). Switching to an SLS-free toothpaste is one of the most commonly reported improvements for people who get recurring sores, and it's a simple, low-risk change to try first.
Are canker sores hereditary?
There is a genetic component. Studies of recurrent aphthous stomatitis find a positive family history in roughly 24 to 46 percent of people affected (StatPearls), so if a parent got frequent canker sores you may be more prone to them too. Genetics is one factor among trauma, stress, diet, and deficiencies.
This article is for general information only and is not a substitute for professional dental or medical advice. If a mouth ulcer lasts longer than three weeks, is unusually large or painful, or keeps recurring, see a dentist or physician. ORALMEDIC is a Class I medical device; always follow the product instructions.