Mouth ulcers are caused by a combination of factors, and in most cases, no single trigger is responsible. About 25% of the global population gets recurrent mouth ulcers, making them one of the most common oral conditions. The causes range from minor physical trauma and nutritional gaps to stress, hormonal shifts, and underlying health conditions.
Physical Damage and Irritation
The most straightforward cause is mechanical injury to the soft tissue inside your mouth. Biting your cheek, scraping your gums with a sharp chip, burning your palate on hot food, or irritation from braces and ill-fitting dentures can all trigger an ulcer. These usually appear within a day or two of the injury and heal on their own within one to two weeks.
A less obvious physical trigger is your toothpaste. Many standard toothpastes contain a foaming agent called sodium lauryl sulfate (SLS), which can irritate the lining of the mouth and make ulcers more likely. In one clinical study, people who switched to an SLS-free toothpaste saw their ulcer count drop by roughly 60 to 70%. If you get ulcers frequently, switching toothpaste is one of the simplest things to try first.
Nutritional Deficiencies
Your body needs certain vitamins and minerals to maintain healthy oral tissue, and running low on them is a well-established ulcer trigger. A study of people with recurrent mouth ulcers found that about half were deficient in vitamin B12, nearly 46% had low folate levels, and around 10% had low iron stores. These deficiencies don’t just slightly raise your risk. They’re present in a large share of people who get ulcers repeatedly.
You don’t need to be severely anemic for this to matter. Even borderline-low levels of B12, folate, or iron can compromise how quickly the cells lining your mouth repair themselves. If your ulcers keep coming back without an obvious cause, a blood test checking these levels is a reasonable step. Correcting a deficiency often reduces how frequently ulcers appear.
Stress and the Cortisol Connection
Most people who get recurrent ulcers notice they flare during stressful periods, and the biology backs this up. Researchers measuring stress hormones in saliva found that people with active mouth ulcers had significantly higher levels of cortisol (the body’s primary stress hormone) compared to people without ulcers. These elevated levels persisted even after the ulcer healed, suggesting that ongoing stress keeps the body in a state that favors ulcer formation rather than just triggering a single episode.
The mechanism likely involves cortisol’s effect on the immune system. Chronic stress suppresses the immune response in some areas while ramping up inflammation in others, and the delicate tissue inside the mouth is particularly vulnerable to this imbalance.
Hormonal Changes in Women
Some women notice ulcers appearing like clockwork in the days before their period. This pattern is linked to the rise in progesterone that occurs in the second half of the menstrual cycle. In susceptible women, this hormonal surge can trigger painful oral ulcers that build in intensity through menstruation and then fade once the period ends. The pattern tends to repeat every cycle, which is the clearest clue that hormones are involved. Pregnancy, menopause, and hormonal contraceptives can also shift ulcer patterns for the same reason.
Immune System Overreaction
At a cellular level, a mouth ulcer is an inflammatory response. Your immune system attacks a small area of the oral lining, breaking down the surface tissue and creating the characteristic painful crater. In many cases, this response is disproportionate to the trigger. A minor scrape that should heal uneventfully instead becomes an open sore that takes a week or more to resolve.
Why the immune system overreacts in some people and not others isn’t fully understood, but genetics play a role. If one or both of your parents got recurrent ulcers, you’re more likely to get them too. Researchers have investigated specific immune-related genes as potential culprits, but so far no single genetic marker has been reliably linked to ordinary recurrent ulcers in the general population.
Food Sensitivities
Certain foods trigger ulcers in susceptible people, though the specific culprits vary from person to person. The most commonly reported triggers include citrus fruits, tomatoes, chocolate, coffee, cheese, nuts, and spicy foods. These are not allergies in the traditional sense. They don’t cause hives or breathing problems. Instead, they provoke a localized inflammatory response in the mouth that leads to ulcer formation, usually within 24 to 48 hours of eating the trigger food.
Keeping a food diary when ulcers flare can help you identify patterns. If you notice the same food appearing before each episode, eliminating it for a few weeks is a straightforward test.
Underlying Health Conditions
Recurrent mouth ulcers can be a symptom of a broader medical condition. Celiac disease, Crohn’s disease, and ulcerative colitis are all associated with oral ulcers, sometimes before any gut symptoms appear. In celiac disease, the ulcers may result from nutrient malabsorption (especially iron and folate) rather than a direct effect on the mouth.
Behçet’s disease is a rarer condition where mouth ulcers are the hallmark symptom, appearing in 97% of patients. The ulcers in Behçet’s look similar to ordinary ones but tend to be more numerous, larger, more painful, and more frequent. If you’re getting clusters of severe ulcers along with other symptoms like genital sores, eye inflammation, or skin lesions, Behçet’s is worth investigating.
HIV and other conditions that suppress the immune system also increase ulcer frequency and severity, as does taking certain medications, particularly some anti-inflammatory drugs and blood pressure medications.
Three Types of Mouth Ulcers
Not all mouth ulcers are the same, and the type you get affects how long it lasts and how much it hurts.
- Minor ulcers are the most common type, measuring less than 5 mm across. They heal within one to two weeks and don’t leave scars.
- Major ulcers are larger, often exceeding 10 mm, and can take weeks or even months to heal. These are deeper, more painful, and sometimes leave scarring.
- Herpetiform ulcers appear as clusters of tiny pinpoint sores that can merge together. Despite the name, they have nothing to do with the herpes virus. They typically heal within a month.
When an Ulcer Might Be Something Else
Most mouth ulcers are harmless and self-limiting, but an ulcer that doesn’t heal within three weeks deserves medical attention. Oral cancer can look similar to a common ulcer in its early stages, though there are key differences. Ordinary ulcers are usually painful, flat, and surrounded by red, inflamed tissue. Oral cancers, by contrast, are often painless in early stages and may have a small lump or bump beneath the surface that you can feel with your tongue or finger.
Other warning signs include a small spot that grows larger over time, a white patch that turns red, or a lesion that starts bleeding when it previously didn’t. These changes don’t automatically mean cancer, but they do warrant a professional evaluation rather than watchful waiting.