Mouth ulcers are caused by a mix of physical injury, stress, nutritional gaps, medications, and sometimes underlying health conditions. Most are minor canker sores (aphthous ulcers), which affect anywhere from 5% to 66% of the population depending on the study. The wide range reflects how common and varied the triggers are. Most heal on their own within 10 to 14 days, but understanding what’s behind them can help you reduce how often they come back.
Physical Injury and Irritation
The most straightforward cause is mechanical damage to the soft tissue inside your mouth. Accidentally biting your cheek, burning your mouth on hot food, or jabbing your gums with a toothbrush can all create a small wound that develops into an ulcer. These usually appear within a day or two of the injury and heal without any special treatment.
Ongoing irritation is a bigger problem than a one-time accident. Braces, ill-fitting dentures, and broken or misaligned teeth can rub against the same spot repeatedly, preventing healing and creating ulcers that stick around. If you notice ulcers forming in the same location over and over, a sharp tooth edge or poorly adjusted dental appliance is likely the culprit. A dentist can smooth the surface or adjust the fit to stop the cycle.
Stress and Your Immune System
Stressful periods are one of the most reliable triggers for recurring mouth ulcers, and the connection is biological, not just coincidental. When your brain registers stress, it activates a hormonal chain reaction that ends with your adrenal glands pumping out cortisol. Cortisol suppresses parts of your immune system: it reduces antibody production, limits the movement of white blood cells, and dials down inflammation in ways that sound helpful but actually leave your mouth’s lining more vulnerable.
At the same time, stress activates the sympathetic nervous system, which triggers the release of pro-inflammatory signals. This creates a contradictory situation where some immune defenses are weakened while inflammation ramps up in the wrong places. The result is that immune cells, particularly a type called cytotoxic T cells, begin attacking the cells lining your mouth. Inflammatory molecules then cause those lining cells to die off, and an ulcer forms. This is why canker sores tend to cluster during exam weeks, major life changes, or periods of poor sleep.
Vitamin and Mineral Deficiencies
Recurring mouth ulcers are often linked to low levels of vitamin B12, folate (vitamin B9), and iron. These nutrients fuel cell repair and tissue regeneration. When your body doesn’t have enough of them, the mucous membrane inside your mouth becomes thinner and more sensitive to everyday irritation. A minor scratch that would normally heal overnight can instead turn into a full ulcer.
Zinc deficiency can also play a role, since zinc supports immune function and wound healing. If you’re getting frequent ulcers and can’t pinpoint an obvious trigger like stress or injury, a blood test checking these levels is a reasonable step. Correcting the deficiency, whether through diet or supplements, often reduces the frequency of outbreaks significantly.
Medications That Cause Mouth Ulcers
Several common medications list mouth ulcers as a side effect. Anti-inflammatory painkillers like aspirin and naproxen can irritate or chemically burn the mouth’s lining, especially if tablets are chewed or dissolve against the cheek before swallowing. Some blood pressure medications, including certain ACE inhibitors and angiotensin-receptor blockers, can cause similar chemical irritation to oral tissues.
In rarer cases, certain anti-seizure medications, gout medications like allopurinol, and some antibiotics can trigger a severe allergic reaction called Stevens-Johnson syndrome, which causes widespread and rapidly developing ulceration of the mouth and skin. This is a medical emergency, not a routine side effect. If you notice new mouth ulcers appearing shortly after starting a medication, it’s worth flagging with whoever prescribed it.
Underlying Health Conditions
When mouth ulcers are persistent, unusually severe, or accompanied by other symptoms, they can signal a systemic condition. Several diseases are well known for producing oral ulcers, sometimes as the very first sign.
Crohn’s Disease
Oral ulcers from Crohn’s disease can actually appear before any gut symptoms develop. They tend to show up as deep, linear ulcers along the inside of the cheeks, often with swollen, firm tissue around them. The swelling typically affects the lips, cheeks, and surrounding soft tissue. Unlike standard canker sores, these don’t necessarily correlate with how active the intestinal disease is.
Behçet’s Syndrome
This inflammatory condition causes painful ulcers that look similar to canker sores but tend to be more numerous and frequently appear on the soft palate and the back of the throat. Oral ulcers are the most common feature of Behçet’s syndrome and are often the first symptom people notice, sometimes years before other signs emerge.
Lupus
Mouth ulcers in lupus typically look different from ordinary canker sores. The classic presentation is a well-defined area of redness or ulceration surrounded by white, radiating lines. Other variations include silvery white scarred patches or irregularly shaped ulcers. Small bruise-like spots (petechiae) inside the mouth can also occur.
Celiac Disease and Pemphigus Vulgaris
Celiac disease can cause recurrent mouth ulcers because the autoimmune response and resulting nutrient malabsorption (particularly iron, folate, and B12) both contribute to breakdown of the oral lining. Pemphigus vulgaris, a rarer autoimmune condition, causes painful, widespread oral ulceration that results from blisters forming and immediately rupturing. In 50% to 80% of people with pemphigus vulgaris, mouth ulcers are the first sign, sometimes appearing a full year or more before any skin involvement.
Minor vs. Major Ulcers
Most mouth ulcers are minor, meaning they’re small, shallow, and heal within 10 to 14 days without scarring. They show up on soft, non-keratinized surfaces like the inner cheeks, inner lips, and the floor of the mouth.
Major aphthous ulcers are larger, exceeding 1 centimeter in diameter, and can appear on tougher surfaces inside the mouth. These take up to six weeks to heal and often leave a scar. If you’re dealing with ulcers that are unusually large, take more than three weeks to heal, or keep appearing before old ones have resolved, those patterns deserve medical attention. Painless ulcers that don’t heal are also a red flag, since a mouth ulcer that persists beyond three weeks without improvement can sometimes be a sign of oral cancer.
Other Common Triggers
Beyond the major categories above, a handful of everyday factors can set off ulcers in susceptible people. Sodium lauryl sulfate, a foaming agent in many toothpastes, irritates the mouth lining in some individuals, and switching to an SLS-free toothpaste can reduce outbreaks. Acidic foods like tomatoes, citrus fruits, and pineapple don’t cause ulcers directly but can aggravate vulnerable tissue or trigger pain in spots where a ulcer is forming. Hormonal shifts, particularly around menstruation, are a commonly reported trigger, though the exact mechanism is less well understood than the stress pathway. Quitting smoking, paradoxically, can temporarily increase mouth ulcers in some people, likely because nicotine has a mild suppressive effect on certain immune responses in the mouth.