Canker sores are one of those frustrating little problems that seem to appear out of nowhere and make even the simplest things—like eating or talking—suddenly painful. These small, round ulcers typically show up inside your mouth, often on the soft tissues like the inner cheeks, the underside of your tongue, or the inside of your lips. They’re not contagious like cold sores and aren’t caused by the herpes virus, but they can certainly disrupt your day.
Most people will get a canker sore at some point in their lives. For some, they happen occasionally and heal quickly. For others, they can be a recurring issue that’s both painful and disruptive. If you’re in the latter group, you’ve probably found yourself searching the internet, wondering: What causes canker sores, and how can I stop them from coming back? In this post, we’ll take a close look at the most common canker sore causes, how to recognize them, and what you can do to find relief and prevention.
Understanding What a Canker Sore Is
Before diving into what causes canker sores, it’s helpful to understand exactly what they are. A canker sore—known medically as an aphthous ulcer—is a small, shallow lesion that appears on the soft tissues inside the mouth. It’s not related to any infection, and unlike cold sores, it doesn’t form on the outside of the lips or skin. You won’t catch a canker sore from kissing someone, sharing food, or being around someone who has one. They are a non-contagious, inflammatory condition that typically resolves on its own within a week or two.
The Role of Minor Trauma
One of the most common canker sore causes is minor trauma to the mouth. This might sound surprising, but something as simple as accidentally biting your cheek or brushing your teeth too vigorously can create the right environment for a sore to develop. Even eating hard, crunchy foods like chips or crusty bread can irritate the mouth’s delicate tissues enough to lead to an ulcer. People who wear braces or dental appliances may also be more susceptible, especially if a wire or bracket rubs against the inside of the mouth repeatedly. While these types of sores may seem random, they’re often a result of a small, unnoticed injury that kicks off an inflammatory response.
Stress as a Trigger
Another major contributor to canker sore development is emotional or physical stress. When the body is under pressure—whether from a demanding job, lack of sleep, or emotional overwhelm—your immune system can take a hit. For some individuals, that immune suppression shows up in the form of a sore inside the mouth. Stress-related canker sores are especially common in teenagers and young adults, likely due to the combination of academic, social, and hormonal stress. It’s a frustrating cycle: stress weakens the body’s defenses, and the resulting sore causes even more discomfort, which may in turn increase stress.
The Impact of Nutritional Deficiencies
Diet plays a powerful role in overall health, and your mouth is no exception. Deficiencies in certain key vitamins and minerals have been linked to canker sore causes. In particular, low levels of vitamin B12, iron, folate (or folic acid), and zinc can make your mouth more vulnerable to ulcers. These nutrients are crucial for cell repair and immune function, and when your body doesn’t have enough of them, the tissues in your mouth may become inflamed more easily. People who follow restrictive diets, have absorption disorders like celiac or Crohn’s disease, or experience chronic fatigue may be more likely to struggle with this type of deficiency. When the body lacks the tools it needs to heal and protect itself, canker sores are more likely to form and linger.
Food Sensitivities and Diet-Related Triggers
While nutrition is important, certain foods can also act as direct triggers for mouth ulcers. Acidic and spicy foods are often mentioned in connection with canker sore causes. Things like tomatoes, citrus fruits, pineapple, vinegar, and hot peppers can irritate the inside of the mouth and trigger a sore in someone who’s already sensitive. Interestingly, some people also report issues with gluten or dairy. In those cases, it may not be the food itself, but an underlying sensitivity or autoimmune condition, like celiac disease, that’s to blame. Everyone is different, and what causes a flare-up for one person might have no effect on another. That’s why tracking your diet when you notice a sore forming can be an effective way to identify your personal triggers.
Sodium Lauryl Sulfate (SLS) in Toothpaste
You may not expect your toothpaste to be a problem, but for some people, it absolutely is. Sodium lauryl sulfate, or SLS, is a common foaming agent in many commercial toothpaste brands. While it makes your toothpaste bubble and foam nicely, it can also irritate the lining of your mouth. Studies have found a link between SLS and increased canker sore frequency in certain individuals. If you’re someone who constantly seems to have mouth sores and you use a toothpaste containing SLS, making a simple switch to an SLS-free brand could make a big difference. This is one of the more overlooked canker sore causes, but for sensitive people, it’s a major one.
Hormonal Changes and Women’s Health
Hormonal fluctuations are another potential cause of canker sores, particularly in women. Many women report that their sores tend to show up around their menstrual cycle, especially in the days leading up to their period. Others find that pregnancy or starting a new form of hormonal birth control can trigger mouth ulcers. While the exact mechanism isn’t completely understood, researchers believe that hormone-related changes in the immune system or inflammation levels may play a role. These hormonal shifts can create an internal environment where the soft tissues in the mouth become more susceptible to ulcers.
Underlying Health Conditions
In some cases, frequent or severe canker sores may be a sign of a more serious underlying health condition. Diseases that affect the immune system—such as Crohn’s disease, ulcerative colitis, celiac disease, lupus, HIV/AIDS, or Behçet’s disease—can cause or worsen mouth ulcers. These conditions often involve chronic inflammation or immune dysregulation, which makes the body more prone to creating sores in response to even minor irritation. If you experience other symptoms alongside canker sores, such as digestive problems, unexplained fatigue, joint pain, or skin rashes, it’s important to talk to a healthcare provider. Canker sores on their own are usually harmless, but when paired with other systemic issues, they can be a clue to a larger health puzzle.
The Genetics Factor
Sometimes, the answer to why you get canker sores lies in your DNA. Research shows that canker sores can run in families, meaning if your parents or siblings get them often, you may have a higher chance of developing them yourself. Genetic predisposition doesn’t mean you’re doomed to suffer with them forever, but it does mean your body might react more intensely to the usual triggers like stress, injury, or certain foods. Knowing that you have a genetic tendency can help you be more proactive in avoiding known canker sore causes and supporting your body with the nutrients and care it needs.
How to Prevent Canker Sores
Although there’s no guaranteed way to completely avoid canker sores, there are several effective strategies for prevention. Maintaining good oral hygiene is one of the simplest and most effective. Brushing gently with a soft-bristled brush and avoiding harsh ingredients in toothpaste can help protect the soft tissues in your mouth. Managing stress through regular sleep, exercise, and mindfulness practices can reduce one of the most common triggers. Eating a well-balanced diet rich in vitamins and minerals supports your body’s natural healing processes, and eliminating or reducing exposure to your personal food triggers can help you avoid future outbreaks. For people who get frequent or sores, when you feel one coming on (or are experiencing a known trigger, like stress) you can take Cankies to prevent the onset of canker sores (and to quickly zap existing ones!). Available here!
When Should You See a Doctor?
While most canker sores heal on their own, there are times when it’s best to seek medical advice. If a sore lasts longer than two weeks, is unusually large, or is so painful that you’re unable to eat or drink comfortably, it’s worth getting evaluated. If you experience frequent outbreaks—such as more than two or three sores per month—or if your sores are accompanied by other concerning symptoms, you may need additional testing to rule out underlying health conditions. Canker sores themselves aren’t dangerous, but persistent or severe ones can signal that something deeper is going on in your body.
Final Thoughts on Canker Sore Causes
Canker sores may be small, but they can cause big problems. Understanding what causes them is the first step toward managing them effectively. From minor mouth trauma and stress to vitamin deficiencies, hormonal changes, and underlying immune issues, the list of canker sore causes is long and varied. Everyone’s body is different, which means the key to finding relief is identifying the specific triggers that apply to you. With the right approach and a little trial and error, it’s absolutely possible to reduce how often you get canker sores—and how bad they feel when they do show up.
If you’ve been battling recurring mouth sores and nothing seems to work, don’t give up. There are real solutions out there. You can try effective remedies like Cankies™—a candy that tastes good and helps you feel better without the harsh burn of traditional treatments — not that we’re biased or anything. 😉
Cankies Canker Sore Treatment 
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