Recurring mouth sores are almost always canker sores, clinically called recurrent aphthous stomatitis. They affect roughly 20% of the general population and tend to come back because of a combination of triggers rather than a single cause. Understanding what sets them off is the first step toward making them less frequent.
Canker Sores vs. Herpes Sores
Before diving into causes, it helps to know which type of sore you’re dealing with, because the two most common kinds have completely different origins.
Canker sores appear on the soft, movable tissues inside your mouth: the inner cheeks, inner lips, floor of the mouth, and sides of the tongue. They look like small, shallow craters with a white or yellowish center and a smooth red border. They are not caused by a virus and are not contagious. Their exact cause is still unclear, but they’re driven by immune system activity rather than infection.
Herpes sores (cold sores) show up on harder, fixed surfaces: the roof of the mouth, the gums right along the teeth, and most commonly the outer edges of the lips. They start as a cluster of tiny blisters, each about a millimeter across, that burst and merge into a larger sore with an uneven, scalloped border. Unlike canker sores, herpes is caused by the herpes simplex virus, usually HSV-1, which lives permanently in nerve fibers and reactivates periodically. These sores are contagious throughout the outbreak, especially while blisters are visible.
If your sores keep appearing on the inner cheeks, inner lips, or tongue, you’re likely dealing with recurrent canker sores, and the triggers below apply directly to you.
Your Toothpaste May Be a Trigger
One of the most overlooked and easiest-to-fix causes is a common detergent in toothpaste called sodium lauryl sulfate (SLS). This foaming agent strips away the protective mucous layer inside your mouth, leaving the tissue more vulnerable to irritation and ulcer formation.
In a preliminary study published in Acta Odontologica Scandinavica, all 10 patients with recurrent canker sores who switched to an SLS-free toothpaste saw improvement. The average reduction in ulcer frequency was about 70% compared to their usual toothpaste, and 60% compared to a toothpaste containing 1.2% SLS. That’s a significant difference from simply changing one product. Most major toothpaste brands contain SLS, but SLS-free options are widely available at pharmacies and grocery stores. If you get canker sores regularly, this is worth trying first.
Nutritional Gaps That Weaken Oral Tissue
Your mouth lining replaces itself every one to two weeks, which means it has high nutritional demands. Deficiencies in iron, folate (vitamin B9), and vitamin B12 are all linked to recurrent canker sores because these nutrients are essential for healthy cell turnover and immune function.
A study in the Journal of the European Academy of Dermatology and Venereology measured blood levels in people with chronic canker sores and found that many fell below standard thresholds for these nutrients. The relevant normal minimums used were 200 pg/mL for B12, 3 ng/mL for folate, and iron levels of 60 µg/dL for men and 37 µg/dL for women. You don’t need to memorize those numbers, but if you’re getting frequent sores and also experiencing fatigue, pale skin, or lightheadedness, a simple blood test from your doctor can check for these deficiencies. Correction through diet or supplements often reduces sore frequency over time.
Common dietary sources that cover all three nutrients include leafy greens (folate), red meat and legumes (iron), and eggs, dairy, or fortified cereals (B12). People who follow plant-based diets are at higher risk for B12 deficiency in particular.
Stress and Hormonal Shifts
If you notice sores appearing during high-stress periods, that’s not a coincidence. Psychological stress suppresses parts of the immune system while ramping up inflammatory activity, creating conditions where the mouth lining breaks down more easily. Many people with recurrent canker sores can trace flare-ups to exam weeks, work deadlines, or emotionally difficult periods.
Hormonal changes play a similar role, particularly for women. The rise in progesterone during certain phases of the menstrual cycle alters blood flow to the gums and changes how the oral tissue responds to irritation. Some women develop canker sores, gum swelling, or bleeding gums at predictable points in their cycle, typically in the days just before their period. If you notice a monthly pattern to your sores, hormonal fluctuation is a likely contributor.
Physical Irritation and Other Common Triggers
Mechanical trauma is one of the most reliable triggers for people prone to canker sores. Biting the inside of your cheek, brushing too aggressively, sharp edges on braces or dental appliances, and even rough-textured foods like chips or crusty bread can all start the process. In someone who isn’t predisposed, a small bite heals normally. In someone with a tendency toward canker sores, that same minor injury can develop into a full ulcer.
Certain foods also trigger sores in susceptible people, though the specific culprits vary. Acidic foods like citrus, tomatoes, and pineapple are common offenders. Some people react to chocolate, coffee, or spicy foods. Keeping a simple log of what you ate in the 24 hours before a sore appears can help you identify personal triggers.
How Long They Last
Most canker sores are the minor type: less than 1 cm across, shallow, and self-healing within 7 to 14 days without treatment. They don’t leave scars.
Major canker sores are deeper, larger (often over 1 cm), and can take weeks to heal. These sometimes leave scarring because they destroy more tissue. If you regularly get sores this large, or if you get many small ones clustered together, it’s worth discussing with a healthcare provider because more aggressive management may help.
People with weakened immune systems, including those with HIV, can experience extremely slow healing, sometimes taking months for a single sore to resolve.
What Actually Helps Them Heal
No single treatment cures canker sores permanently, but several options reduce pain and speed healing. Over-the-counter topical pastes containing a mild corticosteroid reduce inflammation at the sore site, helping it heal faster. In one large study, 21% of patients using a topical treatment achieved complete healing within three days, compared to just 8% of untreated patients.
Numbing gels and rinses containing a local anesthetic provide temporary pain relief by blocking nerve signals in the area. These are especially useful before meals if eating is painful. Antimicrobial mouth rinses can also help by preventing secondary bacterial infection in the ulcer, which slows healing.
Natural options with anti-inflammatory properties include aloe vera gel (applied directly to the sore), licorice-based oral patches, and curcumin, the active compound in turmeric. These work by calming the inflammatory response and, in the case of aloe, promoting tissue repair. They tend to be gentler but may take longer to show results than corticosteroid treatments.
When a Sore Signals Something Serious
Most recurrent mouth sores are harmless canker sores, but a sore that doesn’t heal within two weeks needs professional evaluation. Cleveland Clinic specifically flags any mouth sore lasting longer than two weeks, especially one that bleeds, as a potential warning sign for oral cancer. Other red flags include a sore that keeps growing rather than shrinking, a hard or raised lump around the sore, difficulty swallowing, or numbness in part of your mouth.
Persistent mouth sores can also be a sign of an underlying condition like celiac disease, Crohn’s disease, or an autoimmune disorder like Behçet’s disease. If your sores are unusually frequent (more than three or four episodes a year), unusually large, or accompanied by other symptoms like joint pain, digestive problems, or skin rashes, those patterns are worth mentioning to your doctor. A few targeted blood tests can often rule out or confirm these possibilities.