Mouth sores most commonly develop from minor tissue injury, viral infections, or immune responses that aren’t fully understood. The most frequent type, canker sores, affects anywhere from 5% to 66% of the population depending on the group studied, and they tend to appear more often in younger people, becoming less frequent with age. While most mouth sores heal on their own within one to two weeks, understanding what triggers them can help you reduce how often they come back.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re completely different problems. The easiest way to tell them apart is location. Canker sores form inside the mouth, on the inner cheeks, lips, or tongue. They typically appear as a single round white or yellow sore with a red border. Cold sores show up outside the mouth, usually around the border of the lips, as a cluster of small fluid-filled blisters.
The causes are also distinct. Cold sores come from herpes simplex virus (usually type 1). Once you’re infected, the virus stays in your body and can reactivate during stress, illness, or sun exposure. Canker sores, on the other hand, have no confirmed single cause. They seem to result from a combination of triggers, including injury to the mouth lining, stress, smoking, and nutritional deficiencies in iron, folate, or vitamin B12.
Physical Injury to the Mouth Lining
One of the most straightforward reasons you develop a mouth sore is mechanical damage. Biting the inside of your cheek, scraping your gums with a sharp chip, or burning the roof of your mouth on hot food can all create a small wound that turns into a painful ulcer. Braces are a well-known culprit. Orthodontists routinely recommend wax to cover brackets and wires that rub against the cheeks, especially after adjustments when the hardware shifts position. Ill-fitting dentures cause the same kind of repeated friction.
These injury-related sores usually heal within a week or two as long as the source of irritation is removed or managed. The problem comes when the irritation is ongoing, like a rough tooth edge or a wire that keeps poking the same spot, because the tissue never gets a chance to fully repair.
Nutritional Deficiencies
If you get canker sores frequently without an obvious physical trigger, a nutritional gap may be involved. Low levels of iron, folate, and vitamin B12 are all linked to recurrent oral ulcers. These nutrients play key roles in maintaining healthy mucous membranes and supporting the rapid cell turnover that happens inside your mouth. When levels drop, the lining becomes more fragile and slower to heal.
This connection is worth paying attention to if your diet is restrictive, you follow a vegan or vegetarian eating pattern (B12 is found almost exclusively in animal products), or you have a condition that impairs nutrient absorption. A simple blood test can check these levels.
Stress and Hormonal Changes
Stress is one of the most commonly reported triggers for canker sores, though the exact mechanism isn’t fully mapped out. The link likely involves stress hormones suppressing parts of the immune system that protect the mouth lining, making it easier for sores to develop.
Hormonal fluctuations matter too. Some women who are prone to canker sores notice a pattern where outbreaks recur with every menstrual cycle. The shifts in estrogen and progesterone that happen throughout the cycle appear to influence the mouth’s vulnerability to ulceration, though this pattern doesn’t affect everyone equally.
Viral Infections
Several viruses cause mouth sores as a primary symptom. Cold sores from herpes simplex are the most recognized, but hand, foot, and mouth disease is another common one, particularly in children. It’s caused by coxsackievirus and produces painful sores that start as small red spots on the tongue and inside the mouth, then blister. Children with hand, foot, and mouth disease often stop eating or drinking, drool more than usual, or will only accept cold fluids because swallowing hurts.
Other viral infections, including HIV, can cause persistent or unusually severe oral ulcers. In these cases, the sores are a result of the immune system being compromised rather than the virus directly attacking the mouth tissue.
Underlying Health Conditions
Recurring mouth sores can sometimes signal a broader health issue. Celiac disease, an autoimmune reaction to gluten, frequently causes oral ulcers, and for some people these sores are the first noticeable symptom before any digestive problems appear. Crohn’s disease, an inflammatory bowel condition, also produces mouth ulcers in a significant number of patients.
Behçet syndrome is a rarer condition that causes ulcers in multiple locations, including the mouth, genitals, and skin. A pattern called PFAPA syndrome affects children specifically, causing recurring episodes of fever, mouth sores, sore throat, and swollen neck glands on a predictable cycle. Immune deficiencies and low white blood cell counts can also make the mouth lining more susceptible to ulceration.
If you get frequent mouth sores alongside other symptoms like joint pain, skin rashes, digestive issues, or ulcers elsewhere on your body, the sores may be pointing to one of these systemic conditions rather than simple canker sores.
Toothpaste and Chemical Irritants
You may have heard that sodium lauryl sulfate (SLS), a foaming agent in most toothpastes, triggers canker sores and that switching to an SLS-free brand will help. The evidence for this is weak. A 2019 review found there wasn’t enough data to determine whether SLS-free toothpastes actually reduced how often sores appeared, how long they lasted, or how much they hurt. A double-blind study specifically testing the question found no significant change in ulcer patterns when people switched away from SLS. It’s a low-risk thing to try if you’re desperate, but don’t expect dramatic results.
Other chemical irritants are more clearly problematic. Acidic foods like citrus, tomatoes, and vinegar-based dressings can aggravate existing sores or trigger new ones in people who are susceptible. Alcohol-based mouthwashes can also irritate the mouth lining.
When Mouth Sores Need Attention
Most mouth sores are harmless and resolve without treatment. But a sore that lasts longer than three weeks needs to be evaluated by a dentist or doctor. The same goes for a sore that looks different from ones you’ve had before, is unusually large, appears near the back of your throat, or becomes increasingly painful, red, or starts bleeding, which can indicate infection. Oral cancer can also present as a persistent sore that doesn’t heal, which is the main reason the three-week guideline exists.
If mouth ulcers appear alongside sores on other parts of your body, or with painful, red, or swollen joints, that combination suggests something systemic is going on and warrants a medical evaluation beyond just treating the sore itself.