Most mouth ulcers are caused by your immune system overreacting to minor damage inside your mouth. A sharp bite to the cheek, a scratch from a chip, or even vigorous toothbrushing can trigger an inflammatory chain reaction where your body’s own immune cells destroy a small patch of the tissue lining your mouth. The result is that painful, shallow crater you’re dealing with now.
These ulcers, called aphthous ulcers or canker sores, affect roughly 20% of the population at some point. They’re not contagious, they’re not cold sores (which appear on the outside of your lips), and most heal on their own within 10 to 14 days. But understanding what’s behind yours can help you prevent the next one.
What’s Actually Happening Inside Your Mouth
A mouth ulcer forms when your immune system sends inflammatory signals to a spot on your oral tissue. Specifically, immune cells release a protein that triggers a rush of white blood cells to the area, creating intense local inflammation. Those immune cells then attack and destroy the surface cells of your mouth lining, leaving behind an open sore. It’s essentially friendly fire: your body responding to a minor irritation with a disproportionate immune attack.
This is why some people get ulcers constantly while others almost never do. The difference isn’t the trigger itself but how aggressively your immune system responds to it.
The Most Common Triggers
Physical trauma is the number one cause. Biting your cheek or tongue, scratching your gums with a tortilla chip or crusty bread, irritation from braces or a rough dental filling, a toothbrush slip, even a dentist’s needle during a local anesthetic injection can all set off an ulcer. Stress plays a role too, partly because stressed people tend to bite or chew the inside of their cheeks and lips without realizing it.
Your toothpaste may also be a factor. A foaming agent called sodium lauryl sulfate (SLS), found in most mainstream toothpastes, has been shown to increase ulcer frequency. A systematic review of clinical trials found that people who switched to SLS-free toothpaste had fewer ulcers, shorter episodes, and less pain compared to those using regular toothpaste. If you’re getting ulcers often, this is one of the easiest changes to try.
Other common triggers include hormonal shifts (many women notice ulcers around their period), lack of sleep, and certain foods. Acidic or spicy foods don’t cause ulcers directly but can irritate existing damage enough to start the immune cascade.
Nutritional Deficiencies That Cause Ulcers
If your ulcers keep coming back, your diet could be part of the problem. Screening studies have found nutritional deficiencies in roughly 14% to 18% of people with recurrent mouth ulcers. The key nutrients involved are vitamin B12, iron, and folate. All three play critical roles in maintaining the health of your oral tissue, and when levels drop, the lining of your mouth becomes more fragile and slower to repair.
You don’t need to be severely deficient. Even borderline-low levels of these nutrients can make your mouth lining more vulnerable to damage. If you’re vegetarian or vegan (B12 is found mainly in animal products), have heavy periods (a common cause of iron depletion), or eat few leafy greens and legumes (major folate sources), a simple blood test can check whether a deficiency is driving your ulcers.
Three Types of Mouth Ulcers
Not all mouth ulcers are the same size or severity. Knowing which type you’re dealing with helps you understand what to expect.
- Minor aphthous ulcers are the most common type. They’re round, less than 10 mm across, pale in the center with a red border, and appear on the inner cheeks, lips, tongue edges, or floor of the mouth. They start healing within about 5 days, with the wound shrinking, redness fading, and pain easing. Full healing takes 10 to 14 days, and they leave no scar.
- Major aphthous ulcers exceed 10 mm in diameter and extend deeper into the tissue. They’re significantly more painful, take more than 14 days to heal (sometimes 20 to 30 days), and can leave scarring.
- Herpetiform ulcers are tiny, less than 1 mm each, but appear in clusters of anywhere from 1 to 100 at a time. Neighboring ulcers often merge into larger irregular sores. Despite the name, they have nothing to do with the herpes virus.
When Ulcers Signal Something Bigger
Recurrent mouth ulcers can sometimes be an early sign of a systemic condition. Celiac disease, Crohn’s disease, iron deficiency anemia, lupus, and HIV can all cause ulcers that look identical to ordinary canker sores. In Behçet’s disease, an inflammatory condition affecting blood vessels, mouth ulcers appear in 97% to 100% of patients and are often the very first symptom.
This doesn’t mean your ulcers are caused by one of these conditions. For most people, the explanation is much simpler. But if you’re getting ulcers frequently (more than a few times a year), if they’re unusually large or slow to heal, or if you have other symptoms like joint pain, digestive problems, skin rashes, or unexplained fatigue, it’s worth mentioning the pattern to your doctor. A few blood tests can rule out most of these causes quickly.
How to Tell an Ulcer From Something Serious
Ordinary canker sores are small, round, white or yellowish in the center with a red border, and painful from the start. They heal within one to two weeks. Oral cancer lesions look different: they tend to be persistent sores, lumps, or red and white patches that don’t heal. They can appear anywhere in the mouth, including on the gums, throat, and lips.
One key difference is pain. Canker sores hurt. Oral cancer often doesn’t, at least not early on. Many people with mouth cancer experience no pain at all. Other warning signs of oral cancer include thickening in the mouth or throat, difficulty swallowing or chewing, voice changes, and unexplained weight loss. The critical red flag is timing: any mouth sore that hasn’t healed after two weeks, is unusually large and painful, or comes with fever or swollen lymph nodes needs professional evaluation.
What Helps Them Heal Faster
Most minor ulcers heal fine on their own, but treatment can cut the pain and shorten the process. Over-the-counter numbing gels containing benzocaine provide immediate pain relief by blocking nerve signals at the sore. They won’t speed healing, but they make eating and talking much more comfortable.
For faster healing, prescription-strength options work on the underlying inflammation. Steroid pastes applied directly to the ulcer block the immune attack on your tissue, reducing the size, redness, and pain significantly within about a week. Another prescription option works by blocking the release of inflammatory chemicals from immune cells, which both reduces pain and shortens healing time. In clinical trials, both of these approaches outperformed numbing agents and placebos at reducing ulcer size and pain by day 8 to 10 of treatment.
Simple home strategies also make a real difference. Rinsing with warm salt water several times a day helps keep the area clean. Avoiding acidic foods and drinks (citrus, tomatoes, coffee) reduces irritation while the ulcer is open. And switching to a soft-bristled toothbrush prevents you from re-injuring the area while it heals.
Preventing the Next One
If you get ulcers regularly, prevention matters more than treatment. Switch to an SLS-free toothpaste. Pay attention to whether certain foods seem to precede your outbreaks. If you wear braces or have a sharp tooth edge that keeps catching your cheek, getting that fixed removes a constant source of tissue damage.
Address any nutritional gaps, particularly B12, iron, and folate. Manage stress where you can, and watch for unconscious habits like cheek biting or lip chewing, especially during tense moments. None of these changes will guarantee you’ll never get another ulcer, but together they can dramatically reduce how often they show up and how severe they are when they do.