Mouth sores have several common causes, ranging from minor irritation to viral infections to nutritional deficiencies. Most are harmless and heal on their own within one to two weeks. The two most frequent types are canker sores (small ulcers inside the mouth) and cold sores (fluid-filled blisters caused by herpes simplex virus), but mechanical trauma, fungal overgrowth, and underlying health conditions can also produce painful lesions.
Canker Sores
Canker sores are the most common type of mouth ulcer, and despite how frequently they occur, they have no single known cause. They appear inside the mouth, typically on the inner cheeks, lips, or tongue, and look like round white or yellow sores ringed by a red border. They are not contagious.
Several factors can trigger them. Physical injury to the mouth lining, such as accidentally biting your cheek or brushing too aggressively, is one of the most reliable triggers. Stress, smoking, and hormonal shifts also play a role. Nutritional gaps are a well-documented contributor: deficiencies in vitamin B12, iron, and folate are all linked to recurrent canker sores. If you get them frequently, a blood test checking these levels may be worth pursuing.
Most canker sores resolve within 7 to 14 days without treatment. For painful ones, prescription-strength topical steroid pastes can reduce pain and shorten healing time, though they don’t prevent future outbreaks. Antiseptic mouth rinses can also reduce the severity of pain but won’t change how often sores appear. Over-the-counter protective gels that coat the sore and shield exposed nerve endings offer temporary relief as well.
Cold Sores From Herpes Simplex Virus
Cold sores (also called fever blisters) look and behave very differently from canker sores. They appear as clusters of small, fluid-filled blisters, usually along the border of the lips rather than inside the mouth. They’re caused by herpes simplex virus type 1 (HSV-1), and they are very contagious.
After the initial infection, HSV-1 doesn’t leave the body. It retreats into nerve cells and stays dormant, sometimes for months or years. When something triggers reactivation, the virus travels back along the nerves to the skin’s surface and produces a new outbreak. Common triggers include illness, physical or emotional stress, and hormonal changes. Some people notice outbreaks after sun exposure or when their immune system is otherwise compromised.
Cold sores typically crust over and heal within 7 to 10 days. Antiviral medications can shorten outbreaks and reduce their frequency if taken early.
Physical Trauma and Dental Irritation
A surprising number of mouth sores come from straightforward mechanical damage. Sharp tooth edges, rough fillings, orthodontic brackets, and ill-fitting dentures can all create ulcers by repeatedly rubbing or cutting the soft tissue inside your mouth. These traumatic ulcers are usually a single painful sore with reddened edges and a clean base, and they form at the exact spot where the irritation occurs.
Denture-related problems are especially common. About 50% of people who wear removable dentures develop some form of irritation to the palate, whether from chronic friction, unbalanced bite pressure, or secondary fungal infection under the denture. Roughly 5% of denture wearers develop ulcers from overextended or poorly balanced dentures, and about 12% develop tissue overgrowth along the denture border from chronic irritation. If you notice a sore that lines up with a denture edge or a chipped tooth, that’s likely your cause.
These sores generally heal once the source of irritation is removed or corrected, such as smoothing a rough filling or adjusting a denture.
Oral Thrush
Oral thrush is a fungal infection caused by Candida, a yeast that normally lives in the mouth in small amounts. When something disrupts the balance, Candida can overgrow and produce white, creamy patches on the tongue, inner cheeks, or roof of the mouth. These patches can be sore and may bleed slightly when scraped.
Two of the most common medication-related triggers are antibiotics and inhaled corticosteroids (used for asthma). Antibiotics kill off bacteria that normally keep Candida in check, while inhaled steroids suppress local immune defenses in the mouth and throat. Rinsing your mouth with water after using an inhaler significantly reduces this risk. People with weakened immune systems or diabetes are also more susceptible.
Foods That Worsen Mouth Sores
Spicy and acidic foods do not cause mouth sores, but they can make existing ones significantly more painful. Citrus fruits, tomatoes, vinegar-based foods, and hot peppers are common culprits. If you have an active sore, soft, bland foods and cool drinks (ideally through a straw to bypass the sore) will cause the least irritation.
Systemic Diseases That Cause Mouth Sores
When mouth sores keep coming back or don’t respond to typical treatments, they can sometimes signal a broader health condition. This is relatively uncommon, but worth knowing about if your sores are persistent or unusual.
Crohn’s disease, an inflammatory bowel condition, produces oral lesions in up to 20% of people with the disease. In some cases, mouth sores appear before any abdominal symptoms and serve as the first sign of the condition. The sores in Crohn’s can include deep linear ulcers, diffuse swelling of the lips or cheeks, and a cobblestone-like texture to the inner mouth lining. These oral changes don’t always track with how active the intestinal disease is.
Behçet syndrome is a rarer condition that causes recurrent, painful ulcers resembling canker sores, often numerous and concentrated on the soft palate and back of the throat. Oral ulcers are frequently the earliest and most common symptom of Behçet syndrome.
Celiac disease, an autoimmune reaction to gluten, is another condition associated with recurring mouth ulcers, likely tied to the nutritional malabsorption it causes.
When a Mouth Sore Needs Evaluation
Most mouth sores heal within two weeks. That two-week mark is a meaningful threshold in clinical practice: sores that persist beyond 14 days after any obvious irritant (like a sharp tooth) has been addressed are typically recommended for further evaluation, including possible biopsy. This is primarily to rule out oral cancer, which can initially look like a harmless sore that simply won’t heal.
Other signals that warrant a closer look include sores that keep recurring in the same spot, sores that grow rather than shrink, unexplained numbness, or sores accompanied by other symptoms like weight loss, fatigue, or genital ulcers (which may point to Behçet syndrome). A single canker sore that heals in a week is almost never a concern. A painless ulcer that lingers for a month is a different situation entirely.