Canker sores form when your immune system overreacts to a minor irritation inside your mouth, attacking the thin tissue lining your cheeks, gums, or tongue. The result is a small, painful open ulcer that most people will experience at some point. The specific trigger varies from person to person, but it almost always falls into one of a few well-understood categories: physical injury, stress, nutritional gaps, or an underlying health condition.
What’s Actually Happening Inside Your Mouth
A canker sore isn’t an infection. It’s your own immune system turning against the soft tissue inside your mouth. When something irritates the oral lining, certain white blood cells (T cells) begin destroying the surface tissue, and the body releases inflammatory signals that keep the process going. People who get canker sores frequently tend to have elevated levels of these inflammatory signals in their blood, even between outbreaks. Their immune systems are essentially primed to overreact to triggers that wouldn’t bother someone else.
This is why canker sores tend to be a recurring problem. If your immune system has this tendency, you’ll likely keep getting them throughout your life, though outbreaks often become less frequent with age.
Physical Triggers
The most common reason for a single canker sore is minor mouth trauma. Biting the inside of your cheek, scraping your gums with a chip or cracker, brushing too hard, or getting poked during dental work can all break the surface just enough for your immune system to launch a disproportionate response. Even braces or a rough edge on a filling can create enough friction to trigger one.
Your toothpaste may also play a role. Most toothpastes contain sodium lauryl sulfate (SLS), a foaming agent that irritates the delicate lining of the mouth. It’s the same detergent used in many soaps and shampoos, and in the mouth it can strip away the protective mucous layer and leave tissue more vulnerable. If you notice canker sores appearing regularly, switching to an SLS-free toothpaste is one of the simplest changes you can try.
Stress and Immune Function
Emotional or mental stress is one of the most frequently reported triggers. Stress weakens immune regulation, making it easier for that overactive T-cell response to kick in. Many people notice canker sores appearing during exam periods, work deadlines, or emotionally difficult stretches, not during the stressful event itself but in the days just after, when the body’s defenses are depleted.
Hormonal shifts can have a similar effect. Some women notice outbreaks tied to specific points in their menstrual cycle, likely because fluctuating hormone levels influence how the immune system behaves.
Nutritional Deficiencies
If you’re getting canker sores frequently, your body may be short on certain nutrients. In one study of patients with recurrent oral ulcers, about 50% were deficient in vitamin B12, roughly 46% had low folate levels, and about 11% had low iron stores. All three nutrients play essential roles in maintaining healthy tissue and a well-functioning immune system.
You don’t need to be severely deficient to see an effect. Even borderline-low levels of B12, folate, or iron can make your mouth tissue more fragile and your immune response less balanced. A simple blood test can check these levels. If a deficiency is found, correcting it with dietary changes or supplements often reduces how frequently sores appear.
Underlying Health Conditions
Occasional canker sores are normal. But frequent or severe outbreaks can sometimes signal a deeper health issue. Several conditions are known to cause recurring mouth ulcers that look identical to ordinary canker sores:
- Celiac disease damages the gut lining and impairs nutrient absorption, which can trigger mouth sores even before digestive symptoms appear.
- Crohn’s disease and other forms of inflammatory bowel disease cause widespread inflammation that can extend to the mouth.
- Behçet syndrome, a rare inflammatory disorder, causes recurrent ulcers in the mouth and other parts of the body.
- Immune deficiencies, including HIV/AIDS and conditions that affect white blood cell production, can lead to persistent or unusually severe sores.
If your canker sores are large, take months to heal, appear in clusters, or keep coming back every few weeks, these possibilities are worth investigating with your doctor.
How Long They Last
Most canker sores follow a predictable pattern. You’ll first notice a tingling or burning sensation at the site, followed by a visible ulcer within a day or two. Pain tends to peak in the first few days, then gradually fades. Minor canker sores, which make up the majority of cases, heal within about two weeks without any treatment and leave no scarring.
Major canker sores are larger and deeper. These can take months to heal and sometimes leave scars. A third, less common type called herpetiform canker sores appears as clusters of tiny ulcers that merge together, though these also tend to heal within about two weeks.
Managing Pain and Speeding Recovery
Canker sores heal on their own, but you can reduce the pain and potentially shorten the process. Over-the-counter products containing benzocaine numb the area on contact. Hydrogen peroxide rinses help keep the sore clean and may prevent secondary irritation.
For more severe or persistent sores, prescription options include steroid mouth rinses that reduce inflammation and topical treatments that chemically seal the ulcer surface, which can cut healing time to about a week. Silver nitrate, another cauterizing option, doesn’t speed healing but can provide significant pain relief.
Beyond medication, practical steps help: avoid acidic or spicy foods that sting the sore, use a soft-bristled toothbrush, and switch to SLS-free toothpaste. Rinsing with warm salt water several times a day can soothe irritation and keep the area clean. If stress is a consistent trigger, the sores are essentially a signal that your body is running on empty, and addressing that pattern will do more than any topical treatment.