Mouth sores most commonly fall into two categories: canker sores (small ulcers inside the mouth) and cold sores (fluid-filled blisters on or around the lips). Canker sores are by far the more common reason people notice painful spots inside their mouth, and they don’t have a single known cause. Instead, they’re triggered by a combination of factors ranging from minor injuries and stress to nutritional gaps and irritating ingredients in everyday products like toothpaste.
Canker Sores vs. Cold Sores
These two types of mouth sores look different, appear in different places, and have completely different causes. Knowing which one you have changes what you should do about it.
Canker sores show up inside the mouth, usually on the inner cheeks, lips, or tongue. They look like a single round white or yellow sore with a red border. They are not caused by a virus and are not contagious. Cold sores, on the other hand, appear on the outside of the mouth around the lips as a cluster of small, fluid-filled blisters. They’re caused by the herpes simplex virus (HSV-1) and are very contagious, especially when blisters are open or weeping.
If your sores are inside your mouth and aren’t blisters, you’re almost certainly dealing with canker sores. The rest of this article focuses on why those keep happening.
Common Triggers for Canker Sores
Canker sores don’t have one identifiable cause, which can be frustrating when you’re trying to figure out why they keep coming back. But several well-established triggers can set them off, and most people find that more than one applies to them.
Physical injury: Biting your cheek, brushing too hard, eating sharp or crunchy foods, or wearing braces can all break down the delicate lining of your mouth. Orthodontic appliances are a particularly common culprit because brackets, wires, and bands create constant friction against soft tissue. Even a small scratch can develop into a full ulcer in someone who’s prone to them.
Stress: Psychological stress affects immune function in ways that make the mouth lining more vulnerable to injury and slower to heal. Many people notice their sores cluster during high-pressure periods at work or school.
Hormonal shifts: Some people get mouth sores at predictable points in their menstrual cycle. Hormonal fluctuations can alter immune responses in the mouth’s soft tissue, lowering its ability to repair itself quickly.
Toothpaste ingredients: This one surprises most people. A foaming agent called sodium lauryl sulfate (SLS), found in the majority of commercial toothpastes, is strongly linked to recurrent canker sores. In one study, people who switched to an SLS-free toothpaste saw their number of ulcers drop by about 70%. Every participant in the study benefited from making the switch. If you’re getting frequent mouth sores, checking your toothpaste label is one of the simplest changes you can make.
Nutritional Deficiencies That Cause Mouth Sores
Low levels of certain nutrients are a well-documented trigger for recurrent mouth ulcers. The three most common deficiencies involved are iron, vitamin B12, and folate. When any of these drop low enough, the fast-turnover cells lining your mouth don’t regenerate properly, leaving you more vulnerable to ulcers. A sore, red tongue is another hallmark sign of B12 or folate deficiency.
These deficiencies can develop for straightforward reasons: a restrictive diet, heavy menstrual periods (for iron), or poor absorption due to gut conditions. If your sores keep returning despite managing other triggers, a blood test to check these levels is a reasonable next step. Correcting the deficiency often reduces or eliminates the sores entirely.
Gut Conditions and Autoimmune Disease
Recurring mouth sores can sometimes be an early or overlooked sign of a digestive or autoimmune condition. Celiac disease is a notable example. Research shows that about 23% of people with celiac disease develop mouth ulcers, compared to roughly 7% of the general population, making them more than four times as likely to be affected. In some cases, mouth sores appear before any obvious digestive symptoms do.
Inflammatory bowel diseases like Crohn’s disease can also produce oral ulcers. The inflammation that affects the intestines in these conditions isn’t always limited to the gut. It can show up in the mouth as well, particularly during flare-ups. Behçet’s disease, lupus, and other autoimmune conditions are less common but worth considering if sores are severe, frequent, or accompanied by other symptoms like joint pain, fatigue, or unexplained weight loss.
How Long Mouth Sores Should Take to Heal
The lining of your mouth replaces itself roughly every ten days, which means most canker sores heal within one to two weeks without any treatment. Minor sores (under a centimeter) typically resolve on their own. Larger or deeper ulcers can take longer and may be more painful.
Over-the-counter treatments can help manage pain and may shorten healing time slightly. Topical pastes containing mild anti-inflammatory ingredients reduce discomfort and can help protect the sore from further irritation while it heals. Medicated mouth rinses serve a similar purpose. These treatments work best when applied early, as soon as you notice the sore forming.
When a Mouth Sore Needs Medical Attention
Because the mouth’s lining turns over so quickly, any ulcer that hasn’t healed within two weeks is considered abnormal and should be evaluated by a doctor or dentist. This is the standard threshold used to distinguish a harmless sore from something that requires further investigation, including biopsy.
Pay attention to the appearance of the sore, not just its duration. White patches on the oral lining that can’t be scraped off (a condition called leukoplakia) need to be biopsied to rule out precancerous changes. Red patches are an even greater concern. Studies show that roughly 90% of red, velvety patches in the mouth turn out to be high-grade precancerous changes or cancer when biopsied. Any unexplained red or white patch that persists should be evaluated promptly.
Other signs that a mouth sore warrants professional evaluation include sores that keep coming back in the same spot, sores accompanied by fever or swollen lymph nodes, difficulty swallowing, and sores that are unusually large or appear in clusters of many at once.
Practical Steps to Reduce Recurrence
If you’re dealing with frequent canker sores, a few targeted changes can make a real difference. Switch to an SLS-free toothpaste. This single change reduced ulcer frequency by 60 to 70% in research participants, and SLS-free options are widely available at most drugstores. Use a soft-bristled toothbrush and avoid aggressive brushing.
Track your sores alongside your diet, stress levels, and menstrual cycle if applicable. Patterns often emerge that point to your personal triggers. Consider whether you’re getting enough iron, B12, and folate through your diet, especially if you eat little meat or follow a plant-based diet. If sores persist despite these changes, testing for celiac disease or other underlying conditions is a practical next step, particularly if you also experience bloating, fatigue, or changes in bowel habits.