Mouth sores have dozens of possible causes, ranging from something as simple as biting your cheek to signs of a deeper health condition. The most common culprit is the canker sore, which affects roughly 20% of people at some point and typically heals on its own within two to three weeks. But infections, nutritional gaps, medications, and even your toothpaste can all trigger painful lesions inside your mouth.
Canker Sores
Canker sores (aphthous ulcers) are the most frequent type of mouth sore, and their exact cause remains unknown. They show up on soft tissue that isn’t attached to bone: the inside of your cheeks, your lips, the underside of your tongue, and the floor of your mouth. They look like shallow, round ulcers with a whitish or yellowish center and an inflamed red border.
There are three types, and they differ mainly in size and healing time. Minor canker sores, the most common kind, measure less than one centimeter across and heal within a few weeks without scarring. Major canker sores are larger than one centimeter, extremely painful, and can take months to heal, often leaving scars behind. Herpetiform canker sores are rare and appear as clusters of tiny pinpoint ulcers that heal in about two weeks.
Triggers vary from person to person. Stress, hormonal changes, minor mouth injuries (like biting your cheek or brushing too hard), and certain acidic or spicy foods are all common culprits. Some people notice flare-ups during periods of poor sleep or illness, when the immune system is already under strain.
Cold Sores
Cold sores are caused by herpes simplex virus (usually HSV-1) and behave very differently from canker sores. They most often appear on the lips or just outside the mouth, though they can also show up on the hard palate inside the mouth. About 60% of people experience a warning stage of itching, burning, or tingling before the sore appears. Within 24 hours, small fluid-filled blisters form, then crust over into painful ulcers that may bleed.
A first infection tends to be the worst, sometimes producing numerous blisters throughout the mouth. After that, the virus stays dormant in your nerve cells and can reactivate during stress, sun exposure, illness, or immune suppression. Cold sores are contagious, especially when blisters are open, which is a key distinction from canker sores (which are not).
Your Toothpaste May Be a Factor
A surprisingly common contributor to recurring canker sores is sodium lauryl sulfate, or SLS, the foaming agent found in most commercial toothpastes. SLS strips away the protective mucus layer that coats the inside of your mouth, leaving the underlying tissue exposed to irritation, bacteria, and inflammation.
The clinical evidence is compelling. In one study, patients with recurrent canker sores averaged 14.3 ulcers over three months while using an SLS toothpaste, then just 5.1 ulcers after switching to an SLS-free version. That’s a 64% reduction from a single change. A 2019 systematic review of four clinical trials involving 124 participants found consistent results: SLS-free toothpaste reduced the number of ulcers, the duration of each episode, and the pain associated with them. If you get canker sores frequently, switching to an SLS-free toothpaste is one of the easiest things you can try.
Nutritional Deficiencies
Low levels of certain nutrients are directly linked to mouth sores. Iron, vitamin B12, and folate deficiencies can all cause a sore, red tongue along with recurrent mouth ulcers. These deficiencies are especially common in people with restricted diets, heavy menstrual periods, or digestive conditions that interfere with nutrient absorption. If your sores keep coming back and you can’t pinpoint a trigger, a blood test checking these levels is a reasonable step.
Medications That Cause Mouth Sores
Several classes of medication can trigger oral sores as a side effect. Common pain relievers like aspirin and naproxen can cause chemical burns on the inner lining of the mouth, producing white or yellowish lesions and tissue sloughing. Blood pressure medications, including both ACE inhibitors and angiotensin-receptor blockers, are also known offenders. ACE inhibitors can cause localized swelling inside the mouth by interfering with how the body breaks down certain signaling molecules.
Certain anti-seizure medications (such as lamotrigine, carbamazepine, and phenytoin) carry a risk of triggering severe immune-mediated reactions that affect both the skin and the mouth. Even some inhaled or topical medications, including steroid inhalers and antiseptic mouth rinses containing chlorhexidine, can provoke allergic reactions in the oral tissue. If you notice mouth sores appearing shortly after starting a new medication, that connection is worth raising with your prescriber.
Underlying Health Conditions
Persistent or unusual mouth sores sometimes point to a systemic disease. Crohn’s disease can cause deep, linear ulcers in the mouth along with diffuse mucosal swelling and a distinctive “cobblestone” appearance of the tissue. Behçet’s syndrome produces recurrent, painful ulcers that often cluster on the soft palate and back of the throat. Lupus can cause irregularly shaped ulcers, red patches, and small hemorrhagic spots inside the mouth.
Autoimmune skin conditions that affect mucous membranes can also show up in the mouth first. Pemphigus vulgaris causes diffuse, painful oral ulcers, sometimes before any skin lesions appear. Oral lichen planus produces red, erosive patches with a lacy white border. Leukemia can manifest as mucosal bleeding, ulceration, and swollen gums. These conditions are far less common than a simple canker sore, but they’re worth knowing about if your mouth sores are severe, recurrent, or accompanied by other symptoms.
When a Sore Could Be Something Serious
Most mouth sores are harmless and heal within two to three weeks. The ones that don’t deserve attention. A sore that lingers beyond three weeks without healing is the single most important warning sign for oral cancer. There are a few other differences worth knowing. Canker sores are usually flat and painful from the start, with red, inflamed edges. Oral cancers often have a small lump or bump beneath the surface that you can feel with your tongue or finger, and they tend to be painless in their earliest stages.
Other changes to watch for: a small spot that grows larger over time, a white patch that turns red, or a lesion that starts bleeding when it previously didn’t. These shifts in appearance suggest something more than a routine sore and warrant a professional evaluation.
Easing Pain and Speeding Healing
For ordinary canker sores, a salt water or baking soda rinse is a simple first step. Dissolve one teaspoon of baking soda in half a cup of warm water and swish gently. Dabbing milk of magnesia directly on the sore a few times a day can also soothe irritation.
Over-the-counter topical products containing benzocaine or hydrogen peroxide can numb pain and help protect the sore while it heals. Applying these as soon as you notice a sore forming tends to work better than waiting. For more severe or persistent canker sores, prescription options include steroid rinses to reduce inflammation and topical numbing agents. A chemical cauterizing solution can shorten healing time to about a week in some cases.
Beyond treating individual sores, reducing how often they come back matters more for people who get them frequently. Switching to SLS-free toothpaste, addressing any nutritional deficiencies, managing stress, and avoiding foods that seem to trigger flare-ups are all practical strategies that can make a real difference in how often sores appear.