Mouth sores develop from a wide range of causes, from minor physical irritation to viral infections, nutritional gaps, medications, and underlying health conditions. Most are harmless and heal on their own within one to three weeks, but understanding what’s behind them helps you prevent recurrences and recognize the rare sore that needs medical attention.
Canker Sores vs. Cold Sores
The two most common types of mouth sores look and behave quite differently. Canker sores (aphthous ulcers) are painful white or yellow sores that form only inside the mouth, typically on the inner cheeks, lips, or tongue. Cold sores (fever blisters) are fluid-filled blisters that appear on the outside of the mouth, usually along the border of the lips. Location is the simplest way to tell them apart: inside the mouth means canker sore, outside means cold sore.
Canker sores are not contagious and are not caused by a virus. Cold sores, on the other hand, are caused by the herpes simplex virus and spread through direct contact. These two types have different triggers, different courses, and different treatments, so identifying which one you’re dealing with is the first step.
Physical Trauma and Irritation
The most straightforward cause of a mouth sore is mechanical damage to the soft tissue inside your mouth. Biting your cheek or tongue, burning your palate on hot food, or scratching your gums with sharp foods like chips or toast can all break the delicate lining and create an ulcer. These sores are usually isolated, and you can typically trace them back to a specific moment.
Orthodontic hardware is a particularly common culprit. Braces, wires, brackets, and bands rub against the soft tissue of the cheeks and lips, creating friction that breaks down the protective lining and leads to ulceration. The sharp edges of brackets and wire ends are the worst offenders, and tightening appointments often make things worse temporarily. Orthodontic wax applied over the rough spots can reduce irritation while the tissue adapts. Ill-fitting dentures and rough dental restorations cause the same kind of friction-based sores.
Herpes Simplex Virus (Cold Sores)
Cold sores are caused by herpes simplex virus type 1 (HSV-1). The virus initially infects the skin or mucous membranes, then travels along nerve fibers to a cluster of nerve cells near the base of the skull, where it goes dormant. It can stay inactive for weeks, months, or years before reactivating and traveling back along the same nerve to produce a new outbreak on or around the lips.
Part of what makes HSV-1 so persistent is its ability to hide from the immune system. The virus interferes with the way immune cells display signals on their surface, essentially preventing them from calling for backup. This means the body can never fully clear the infection. Common triggers for reactivation include stress, fatigue, sun exposure, illness, fever, and hormonal changes, though the exact trigger isn’t always obvious.
Toothpaste and Chemical Irritants
A surprisingly common and fixable cause of recurrent canker sores is sodium lauryl sulfate (SLS), a foaming agent found in most commercial toothpastes. SLS strips away the protective mucus layer that coats the inside of your mouth, leaving the tissue underneath exposed and vulnerable. In one study, people who switched from an SLS-containing toothpaste to an SLS-free formula saw their average number of canker sores drop from about 14 to 5 over the study period. If you get frequent canker sores, switching toothpaste is one of the easiest interventions to try.
Other chemical irritants include alcohol-based mouthwashes, acidic foods like citrus and tomatoes, and spicy foods. These don’t necessarily cause sores on their own, but they can aggravate existing damage or trigger outbreaks in people who are prone to them.
Nutritional Deficiencies
Recurrent canker sores are sometimes a sign that your body is low on specific nutrients, particularly vitamin B12, iron, and folate. These nutrients play essential roles in maintaining the rapidly dividing cells that line your mouth. When levels drop, the lining becomes fragile and more prone to breaking down into ulcers.
Vitamin B12 deficiency is the best-studied link. In documented cases, patients with chronic mouth sores had B12 levels well below the normal range (around 65 pmol/L, compared to a normal range of roughly 116 to 781 pmol/L). Their sores resolved after B12 supplementation. Even levels at the low end of normal have been associated with recurrent ulcers. If you’re getting canker sores frequently and can’t identify an obvious trigger, a blood test to check B12, iron, and folate levels is a reasonable next step, especially if you follow a vegetarian or vegan diet, which tends to be lower in B12.
Medications That Cause Mouth Sores
A large number of medications can cause mouth sores as a side effect. A pharmacovigilance study identified 289 drugs with a statistically significant association with oral ulcers. The highest-risk categories include chemotherapy drugs (especially antimetabolites), immunosuppressants, cardiovascular medications, certain antibiotics, and bisphosphonates used for bone density.
Chemotherapy drugs carry the highest risk because they work by disrupting rapidly dividing cells, and the cells lining the mouth are among the fastest-dividing in the body. The drugs damage these cells directly, compromising the protective barrier. Immunosuppressants like everolimus and mycophenolate are also well-documented offenders. Less obvious culprits include inhaled asthma medications, which can dry out and irritate the mouth, and certain antifungal drugs. If mouth sores appeared shortly after starting a new medication, the timing is worth mentioning to your prescriber.
Autoimmune and Systemic Conditions
Mouth sores that keep coming back despite good oral hygiene and no obvious triggers can sometimes point to an underlying systemic condition. Behçet’s disease is one of the most recognizable examples. Painful mouth sores that look like canker sores are the most common sign of this condition. They begin as raised, round lesions that quickly become painful ulcers, typically healing within one to three weeks but returning frequently.
Celiac disease, an immune reaction to gluten, is another condition where chronic mouth sores may be one of the earliest or most prominent symptoms. The inflammation and malabsorption caused by celiac disease can deplete the nutrients needed for healthy oral tissue, creating a cycle of recurring ulcers. Crohn’s disease and other inflammatory bowel conditions can also produce oral ulcers as part of the broader inflammatory process. In these cases, treating the underlying condition is what eventually brings the mouth sores under control.
Types of Canker Sores by Severity
Not all canker sores are equal. They fall into three categories based on size and pattern:
- Minor aphthous ulcers are the most common type, measuring less than 1 cm across. They’re painful but generally heal without scarring.
- Major aphthous ulcers are larger than 1 cm and penetrate deeper into the tissue. They take longer to heal and can leave scars.
- Herpetiform ulcers are clusters of many tiny sores (each under 1 cm) that can merge into larger irregular ulcers. Despite the name, they are not caused by herpes virus.
Most people who get canker sores deal with the minor variety. If your sores are consistently larger than a centimeter or appear in clusters, that pattern is worth discussing with a dentist or doctor, as it may point to one of the systemic causes above.
Managing Pain and Healing
Most mouth sores heal on their own within one to three weeks without treatment. The goal in the meantime is pain control and avoiding further irritation. Over-the-counter topical numbing gels containing benzocaine can be applied directly to the sore up to four times a day for temporary relief. These are available as gels, sprays, and ointments, and should not be used on children under two years old.
Rinsing with warm salt water several times a day can help keep the area clean and reduce inflammation. Avoiding acidic, spicy, and crunchy foods while the sore is active prevents additional irritation. For people with frequent or severe canker sores, prescription-strength topical anti-inflammatory pastes can reduce healing time by calming the immune response at the ulcer site.
When a Mouth Sore Could Be Something Serious
Any mouth sore that hasn’t healed within two to three weeks deserves professional evaluation. That’s the general timeframe used to distinguish a normal sore from something that needs a closer look. A lesion that doesn’t respond to typical treatment or doesn’t resolve on its own should be considered potentially abnormal until proven otherwise.
Warning signs that warrant prompt attention include a persistent red or white patch, a sore that keeps growing rather than shrinking, unusual surface changes or hardened areas in the tissue, progressive swelling, or any ulcer that simply refuses to heal. These can be signs of oral cancer or precancerous changes. A biopsy is the only way to get a definitive answer, and early detection makes a significant difference in outcomes.