Mouth ulcers are most commonly caused by minor trauma, stress, nutritional deficiencies, or immune system responses rather than infections. Between 5% and 25% of the general population experiences recurrent mouth ulcers, with some studies estimating rates as high as 60% depending on the population studied. Most are harmless canker sores that heal on their own, but understanding the triggers can help you reduce how often they appear.
Physical Trauma and Irritation
The simplest and most frequent cause is mechanical damage to the soft tissue inside your mouth. Biting your cheek, jabbing yourself with a toothbrush, or scraping the inside of your lip on a sharp piece of food can all break the delicate lining and create an ulcer. Dental appliances like braces, retainers, and ill-fitting dentures are especially common culprits because they create repeated friction against the same spot.
People who are genetically prone to mouth ulcers are more likely to develop one from these minor injuries. For others, the same bite or scrape would heal without ever forming a visible sore.
Stress and Hormonal Shifts
Psychological stress is one of the most well-documented triggers. Ulcers tend to flare during high-pressure periods, with outbreaks notably increasing around school and university exams. The exact pathway isn’t fully mapped, but stress suppresses parts of the immune response that keep the mouth lining intact.
Hormonal changes play a role too. Some women notice ulcers appearing in the days before their period, which tracks with the natural drop in progesterone during the second half of the menstrual cycle. This hormonal dip appears to make the oral lining more vulnerable to breakdown.
Nutritional Deficiencies
Running low on certain vitamins and minerals makes your mouth lining thinner and slower to repair itself. In a study of 273 people with recurrent mouth ulcers, about 20% had iron deficiency and another 20% had anemia. Vitamin B12 deficiency showed up in roughly 5% to 7% of patients across multiple studies, and folic acid deficiency appeared in about 2.6%.
If you get mouth ulcers frequently and can’t pinpoint an obvious trigger like trauma or stress, a simple blood test can check for these deficiencies. Correcting the underlying shortage often reduces or eliminates the ulcers entirely.
Your Toothpaste May Be a Factor
A foaming agent called sodium lauryl sulfate (SLS), found in most mainstream toothpastes, can strip away the protective outer layers of the mouth’s lining. For people already prone to ulcers, SLS-containing toothpaste can significantly increase how often sores return. One clinical study found four times fewer soft tissue lesions after using an SLS-free toothpaste compared to one containing SLS.
Switching to an SLS-free toothpaste is a low-effort change worth trying if you deal with frequent ulcers. Several brands now market SLS-free formulas specifically for sensitive mouths.
Food Sensitivities and Dietary Triggers
Certain foods trigger ulcers in susceptible people through allergic or sensitivity reactions rather than direct injury. Common offenders include citrus fruits, tomatoes, chocolate, coffee, cheese, nuts, and wheat-based products. The reaction isn’t the same as a food allergy that causes hives or throat swelling. Instead, it’s a localized immune response in the mouth lining that leads to tissue breakdown over the following day or two.
Tracking which foods precede your outbreaks can help you identify personal triggers, since these vary widely from person to person.
Viral and Bacterial Infections
Not all mouth ulcers are canker sores. Infections can produce similar-looking lesions with different underlying causes. Herpes simplex virus type 1 (HSV-1), the same virus responsible for cold sores, is the most common infectious cause. It produces clusters of small, painful blisters that rupture into shallow ulcers, typically on the gums and the roof of the mouth.
Coxsackieviruses, which spread through contaminated hands and surfaces, cause the mouth sores seen in hand, foot, and mouth disease. These tend to appear alongside a rash on the palms and soles. Bacterial infections involving streptococcus can also produce oral ulcers, particularly in children, when the bacteria triggers widespread gum and mouth inflammation known as gingivostomatitis.
Medications That Cause Mouth Ulcers
Several common medications list mouth ulcers or oral tissue damage as side effects. Nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin and naproxen can cause chemical burns to the mouth lining, producing white or yellowish lesions and tissue sloughing. Some blood pressure medications, including certain angiotensin-receptor blockers, can cause similar reactions. Even albuterol inhalers carry a small risk of tongue ulcers.
If your ulcers started or worsened after beginning a new medication, that timing is worth mentioning to your prescriber. The ulcers typically resolve after switching to an alternative.
Autoimmune and Systemic Diseases
Chronic, severe, or unusually frequent mouth ulcers can sometimes signal an underlying systemic condition. Behçet’s disease, a rare autoimmune disorder that causes blood vessel inflammation throughout the body, produces painful mouth sores as its most common and often earliest symptom. These sores look like ordinary canker sores but tend to be larger, more frequent, and accompanied by other symptoms like genital ulcers or eye inflammation.
Celiac disease and Crohn’s disease are also linked to recurrent mouth ulcers. In celiac disease, the connection may partly run through nutritional deficiencies caused by poor absorption in the gut. In Crohn’s, the same inflammatory process that damages the intestines can directly affect the mouth lining. Large, slow-healing ulcers can also be associated with HIV infection, particularly when the immune system is significantly weakened.
Three Types of Canker Sores
Canker sores (the non-infectious kind) fall into three categories based on size and behavior. Minor ulcers are the most common type: small, shallow sores less than 1 cm across that appear on the inner lips, cheeks, soft palate, or floor of the mouth. They heal on their own within one to two weeks without scarring.
Major ulcers are larger, deeper, and more painful. They take longer to heal and can leave scars. Herpetiform ulcers, despite the name, have nothing to do with herpes. They appear as clusters of many tiny, blister-like sores that can merge into larger irregular ulcers. The peak age of onset for all three types is the teenage years, with first episodes typically appearing in childhood or adolescence.
When a Mouth Ulcer Could Be Something Serious
Most mouth ulcers are painful but harmless. The key warning sign is an ulcer that doesn’t heal within two weeks. Oral cancer can appear as a persistent sore on the lip or inside the mouth, sometimes accompanied by a white or reddish patch, a lump or growth, loose teeth, ear pain, or difficulty swallowing and chewing. Any sore that lingers beyond two weeks warrants a medical evaluation, even if it doesn’t seem dramatic. Catching oral cancer early makes a significant difference in outcomes.
Quitting smoking, interestingly, can temporarily increase mouth ulcers. Nicotine has a mild protective effect on the oral lining, and when that effect disappears, some people experience a wave of ulcers in the weeks and months after stopping. This is temporary and not a reason to keep smoking.