Canker sores don’t have a single known cause, but they form when your immune system launches an inflammatory attack against the soft tissue inside your mouth. About 25% of the general population gets them, and most people who do will get them repeatedly. The triggers range from biting your cheek to running low on certain vitamins, and understanding those triggers is the most practical way to reduce how often they show up.
What Actually Happens Inside Your Mouth
A canker sore is a small ulcer, usually no larger than 5 millimeters, that forms on the softer lining of your lips, cheeks, tongue, or the floor of your mouth. It looks white or yellow with a red border, and it hurts because the surface layer of tissue has broken down, exposing the nerve-rich tissue underneath.
The process is immune-driven. Certain white blood cells, particularly a type called T cells, begin attacking and destroying the oral tissue as if it were a threat. Your body releases a cascade of inflammatory signaling molecules that sustain and intensify the damage, creating the open sore you feel. People who get canker sores frequently show higher levels of these inflammatory markers in their blood, along with an elevated ratio of certain immune cells compared to people who never get them. In short, your immune system overreacts to a trigger and damages your own tissue in the process.
Physical Injury Is the Most Common Trigger
The single most frequent trigger is mechanical trauma to the inside of your mouth. This includes biting your cheek or tongue, poking yourself with a sharp chip or cracker, getting jabbed by orthodontic braces or a rough dental appliance, brushing too aggressively, or burning yourself on hot food. The injury creates a small wound, and in susceptible people, the immune system overresponds to that wound and turns it into a full ulcer instead of letting it heal quietly.
This is why canker sores so often appear in the exact spot where you remember biting yourself a day or two earlier. The trauma itself isn’t the sore. It’s the spark that sets off the immune response.
Nutritional Deficiencies That Fuel Recurrence
If you get canker sores often, a nutritional gap may be part of the picture. The deficiencies most consistently linked to recurrent canker sores are:
- Vitamin B12: supports nerve function and healthy cell production in mucous membranes
- Iron: essential for tissue repair and healthy blood
- Folate (vitamin B9): plays a key role in cell division, which matters when your mouth lining needs to regenerate
- Zinc: strengthens immune regulation and wound healing
You don’t need to be severely deficient for this to matter. Even borderline-low levels of B12 or iron can tip the balance toward more frequent sores. Eating a diet rich in leafy greens, lean meats, eggs, legumes, and fortified cereals covers most of these bases. If your canker sores keep coming back and you suspect a dietary gap, a simple blood test can check your levels.
Stress, Hormones, and Other Internal Triggers
Emotional stress is one of the most commonly reported triggers, and it fits the immune picture. Stress alters how your immune system behaves, making inflammatory overreactions more likely. Many people notice canker sores appearing during exam periods, work deadlines, or emotionally difficult stretches, not during the stress itself but in the days just after, when the body’s stress response shifts.
Hormonal changes also play a role for some people. Women sometimes notice a pattern tied to their menstrual cycle, with sores appearing at specific points each month. Sleep deprivation and general fatigue seem to lower the threshold as well, though these often overlap with stress.
Your Toothpaste Might Be a Factor
Many commercial toothpastes contain sodium lauryl sulfate (SLS), a foaming agent that can irritate the delicate lining of the mouth. For some people, SLS appears to either trigger new canker sores or worsen existing ones. If you get sores frequently, switching to an SLS-free toothpaste is a low-risk experiment worth trying. Several widely available brands now make SLS-free versions, and some people report a noticeable drop in how often sores appear after making the switch.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re completely different conditions. Canker sores form only inside the mouth, on the cheeks, lips, tongue, or floor of the mouth. Cold sores (fever blisters) form on the outside of the mouth, typically around the border of the lips. Cold sores are caused by herpes simplex virus (usually HSV-1) and are contagious. Canker sores are not caused by a virus and are not contagious at all. If your sore is inside your mouth, white or yellow, and not fluid-filled, it’s almost certainly a canker sore.
How Long They Last
Most canker sores resolve on their own within 10 to 14 days without any treatment. The pain is usually worst in the first few days, then gradually fades as the tissue regenerates. You may develop more than one at a time, and sometimes multiple sores merge together, which makes them feel larger and more painful than a single ulcer.
Over-the-counter topical gels or rinses can dull the pain and create a protective barrier over the sore while it heals. Avoiding acidic, spicy, or rough-textured foods during an active sore helps prevent re-irritation. Rinsing with warm salt water a few times a day can also reduce discomfort.
When Canker Sores Signal Something Bigger
Occasional canker sores are normal and not a sign of disease. But frequent, severe, or unusually large sores, especially combined with other symptoms like chronic digestive issues, fatigue, or joint pain, can sometimes point to an underlying condition. Celiac disease, inflammatory bowel disease (Crohn’s and ulcerative colitis), and certain autoimmune disorders are all associated with recurrent mouth ulcers. In these cases, the canker sores are a surface-level symptom of a deeper immune dysfunction. If your sores are constant, unusually large, slow to heal, or accompanied by other persistent symptoms, it’s worth getting evaluated beyond just the mouth.