Canker sores happen when your immune system overreacts and attacks the thin tissue lining the inside of your mouth. Unlike cold sores, which are caused by a virus and appear on the outside of your lips, canker sores are not infections. They’re an inflammatory response, and they affect anywhere from 5% to over 60% of people depending on the population studied. The wide range reflects how common they are and how many different factors can set them off.
What’s Actually Happening Inside Your Mouth
The lining of your mouth is one of the most delicate tissues in your body, and it turns over rapidly. When something triggers your immune system to misfire, immune cells flood into a small area of that lining and destroy healthy tissue, leaving behind the shallow, painful crater you recognize as a canker sore. Blood tests of people with recurrent canker sores show significantly elevated levels of inflammatory signaling molecules compared to people who rarely get them. This suggests the immune system in canker sore-prone individuals runs hotter than normal, at least in the mouth.
The result is a sore that’s white or yellowish in the center with a red border. It hurts because the tissue destruction exposes nerve endings that are normally protected. Eating, drinking, and even talking can irritate these exposed nerves, which is why a sore the size of a pencil eraser can dominate your entire day.
The Three Types
Most canker sores are the minor type: small, relatively shallow, and gone within one to two weeks without scarring. These are the ones nearly everyone has experienced at some point.
Major canker sores are larger and deeper. They can linger for up to six weeks and sometimes leave permanent scars in the mouth tissue. These are far less common but significantly more disruptive.
Herpetiform canker sores are the rarest type. Despite the name, they have nothing to do with herpes. They appear as clusters of pinpoint-sized ulcers that can merge into larger irregular sores. Like the minor type, they typically heal within one to two weeks without scarring.
Common Triggers
No single cause explains every canker sore, but several well-established triggers account for most outbreaks:
- Physical injury: Biting your cheek, brushing too hard, dental work, braces, or even a sharp chip can damage the lining enough to spark an immune response.
- Stress: Elevated stress hormones suppress some parts of your immune system while ramping up others, creating conditions where your mouth’s lining becomes more vulnerable. Students during exam periods and adults during high-pressure work stretches commonly report flare-ups.
- Hormonal shifts: The rise in progesterone during certain phases of the menstrual cycle can trigger canker sores, swollen gums, or both. Some women notice a predictable pattern tied to their cycle.
- Acidic or irritating foods: Citrus fruits, tomatoes, pineapple, vinegar, and spicy foods don’t cause canker sores on their own, but they can irritate already-vulnerable tissue or worsen an emerging sore.
- Sleep deprivation and illness: Anything that taxes your immune system can lower the threshold for an outbreak.
Nutritional Deficiencies
Recurring canker sores can be a sign that your body is low on specific nutrients. Vitamin B12 deficiency is one of the most recognized culprits, and mouth ulcers are listed among its hallmark symptoms. Iron and folate deficiencies also increase your risk. These nutrients play essential roles in maintaining the fast-turnover cells that line your mouth, so when levels drop, that tissue becomes fragile and more prone to breakdown.
If you get canker sores frequently (several times a year or more), it’s worth having your levels checked through a simple blood test. In many cases, correcting the deficiency reduces or eliminates the outbreaks entirely.
Toothpaste and Oral Care Products
Sodium lauryl sulfate, the foaming agent in most toothpastes, has long been suspected of triggering canker sores. The clinical evidence is nuanced. A controlled crossover trial found that switching to an SLS-free toothpaste did not reduce the number of canker sores or outbreaks. However, it did significantly reduce how long each sore lasted and how much pain people experienced day to day.
If you’re prone to canker sores, switching to an SLS-free toothpaste probably won’t prevent them, but it may make each episode shorter and less painful. That’s a meaningful difference for people who deal with sores regularly.
When Canker Sores Signal Something Bigger
Occasional canker sores are normal and not a sign of disease. But frequent, severe, or unusually persistent sores can sometimes point to an underlying condition.
Celiac disease, an autoimmune reaction to gluten, lists recurrent mouth ulcers as one of its symptoms. Some people discover their celiac diagnosis after investigating why they keep getting canker sores. Crohn’s disease and other inflammatory bowel conditions can also cause oral ulcers, sometimes before any digestive symptoms appear.
Behçet’s disease is a rarer condition in which the immune system attacks blood vessels throughout the body. Painful mouth sores that look identical to canker sores are its most common symptom. These sores typically heal within one to three weeks but keep returning. Behçet’s also causes sores in other areas and can affect the eyes, joints, and skin.
The pattern matters more than any single sore. Large sores that take weeks to heal, sores that come back before the last one has healed, or sores accompanied by other symptoms like joint pain, genital ulcers, or persistent digestive problems are worth investigating with a healthcare provider. In some cases, testing for other health conditions is appropriate, especially when sores are severe and ongoing.
The Genetics Question
People with canker sores often notice the problem runs in their family, and there does appear to be a hereditary component. However, researchers have struggled to pin down the exact genetic mechanism. One large study looked at a specific immune system gene variant called HLA-B*51, which is strongly linked to Behçet’s disease, to see if it also predicted canker sores. It didn’t. The gene appeared in canker sore patients at the same rate as healthy controls (about 46% in both groups).
This doesn’t mean genetics play no role. It means the genetic predisposition to canker sores is likely spread across multiple genes affecting immune regulation, tissue repair, and inflammatory responses rather than sitting in one identifiable spot. If one or both of your parents dealt with frequent canker sores, your odds of dealing with them are higher, even if science can’t yet explain exactly why.
Managing an Active Sore
Minor canker sores heal on their own within one to two weeks, and most people don’t need any treatment. To reduce pain during that window, avoid acidic foods, vinegar, and hot spices like chili powder, all of which directly irritate the exposed tissue. Over-the-counter oral gels that coat the sore can create a temporary barrier against irritation while eating.
Rinsing with warm salt water several times a day can help keep the area clean and may speed healing slightly. For sores that are unusually large, deep, or painful, prescription options exist that reduce inflammation and promote faster tissue repair. These are typically reserved for major canker sores or for people whose sores interfere significantly with eating and speaking.