Mouth sores have dozens of possible causes, ranging from biting your cheek to underlying health conditions. The most common type, canker sores (recurrent aphthous stomatitis), affects roughly 25% of the world’s population at some point. Understanding what triggers your sores is the first step toward preventing them or recognizing when something more serious is going on.
Canker Sores: The Most Common Culprit
Canker sores are small, shallow ulcers that form inside the mouth, on the tongue, or along the gums. They’re not contagious, and most heal on their own within one to two weeks. But they tend to come back, and certain triggers make recurrence more likely.
The best-documented triggers include local trauma (like biting the inside of your cheek, aggressive brushing, or irritation from braces), emotional stress, and nutritional gaps. If you’re prone to canker sores, even minor injuries to the soft tissue inside your mouth can set off a new round of ulcers. Quitting smoking is another recognized trigger, which catches many people off guard. Heredity plays a role too: if your parents dealt with frequent canker sores, you’re more likely to get them.
Your Toothpaste May Be Making It Worse
A foaming agent called sodium lauryl sulfate (SLS), found in most commercial toothpastes, has been linked to more frequent and more painful canker sores. In clinical trials, people who switched to SLS-free toothpaste experienced fewer ulcers, shorter healing times, fewer episodes overall, and less daily pain compared to those using standard SLS-containing products. SLS irritates the oral mucosa and can slow wound healing. If you get canker sores regularly, switching to an SLS-free toothpaste is one of the simplest changes you can make.
Cold Sores From Herpes Simplex Virus
Cold sores look and behave differently from canker sores. They’re caused by herpes simplex virus type 1 (HSV-1), which spreads through direct contact with infected saliva or skin. After the initial infection, the virus travels along nerve fibers to a cluster of nerve cells near the base of the skull, where it stays dormant. It can reactivate at any time, often triggered by stress, illness, sun exposure, or a weakened immune system.
The first outbreak tends to be the worst. Symptoms appear three days to one week after exposure and often start with fever, fatigue, swollen lymph nodes, and localized burning or tingling. Painful, fluid-filled blisters then develop on the mouth and lips, sometimes progressing to open sores before crusting over. Full healing takes two to six weeks. Recurrent outbreaks are typically milder, with a 24-hour warning phase of tingling, burning, or itching before blisters appear, usually along the lip border rather than inside the mouth.
Hand, Foot, and Mouth Disease
This viral infection, caused by a coxsackievirus, is most common in children under five but can affect anyone. Mouth sores from this illness start as small red spots on the tongue and the insides of the mouth, then blister and become painful. They typically show up alongside a fever and a distinctive rash on the palms, soles of the feet, and sometimes the buttocks, legs, or arms. The fluid inside the blisters is contagious.
Children with these sores often drool more than usual, refuse food and drink, or will only accept cold fluids because swallowing hurts. Symptoms are generally mild and clear up within 7 to 10 days without specific treatment.
Oral Thrush and Fungal Infections
An overgrowth of Candida yeast, a fungus that normally lives in the mouth in small amounts, can cause several types of oral lesions. The most recognizable form produces creamy white patches that can be wiped away, leaving red or raw tissue underneath. Patients often notice a foul taste or mild tingling.
A less obvious form, erythematous candidiasis, shows up as painful red patches rather than white ones, often on the tongue where the normal surface texture disappears. This is the only type of oral Candida infection that consistently causes pain. People who wear dentures can develop a related condition called denture stomatitis, which causes redness and soreness under the denture plate. Oral thrush is more common in people taking antibiotics, using inhaled corticosteroids, or with weakened immune systems.
Nutritional Deficiencies
Low levels of certain nutrients can directly cause or worsen mouth sores. Vitamin B12 deficiency is one of the most well-studied links. Oral changes, including recurrent ulcers, a swollen or inflamed tongue, burning sensations, cracked corners of the mouth, and altered taste, appear in 50 to 60% of patients with the type of anemia caused by B12 deficiency. Iron deficiency and low folate levels are also established triggers for recurrent canker sores.
These deficiencies don’t always show up on standard blood tests in obvious ways. B12 levels, for example, can appear normal or even elevated in 22 to 35% of people who actually have pernicious anemia, an autoimmune condition that prevents B12 absorption. If you’re getting frequent mouth sores with no clear cause, nutritional testing is worth pursuing.
Medications and Cancer Treatment
Certain medical treatments cause mouth sores as a direct side effect. Chemotherapy is the most significant offender. Drugs that interfere with how cells divide are especially likely to damage the fast-growing cells lining the mouth, leading to a condition called oral mucositis: widespread redness, swelling, and painful ulceration. Radiation therapy targeting the head and neck causes similar damage. The severity varies by drug class, but certain categories of chemotherapy agents carry particularly high rates of oral mucositis.
Outside of cancer treatment, some medications used for autoimmune conditions and organ transplants can also increase mouth sore frequency by suppressing immune function or directly irritating oral tissue.
Autoimmune and Inflammatory Diseases
Mouth sores that keep coming back, especially in combination with other symptoms, can signal an underlying autoimmune or inflammatory condition. Behçet’s disease causes painful oral ulcers alongside genital sores and eye inflammation. Diagnosing it requires the presence of at least two of these three features when other explanations have been ruled out.
Crohn’s disease, an inflammatory bowel condition, can produce oral ulcers that look identical to ordinary canker sores but occur as part of a broader pattern of digestive symptoms. Celiac disease, lupus, and HIV infection are also associated with recurrent mouth ulcers. The key difference between these and garden-variety canker sores is that they tend to be more frequent, slower to heal, and accompanied by symptoms in other parts of the body.
When a Sore Could Signal Oral Cancer
Most mouth sores are harmless, but a sore that lasts longer than three weeks without healing deserves medical attention. Early-stage mouth cancer can look exactly like an ordinary ulcer, which is why duration matters more than appearance. Other warning signs include a white or red patch inside the mouth, a lump on the lip, tongue, or neck, persistent mouth pain, difficulty swallowing or speaking, a hoarse voice that doesn’t resolve, and unexplained weight loss. None of these symptoms on their own confirm cancer, but any combination that persists beyond three weeks should be evaluated.