Mouth blisters are most commonly caused by canker sores, cold sores, or minor injuries to the tissue inside your mouth. Less often, they signal a nutritional deficiency, an autoimmune condition, or a viral infection. The cause usually determines where in your mouth the blisters appear, how they look, and how long they last.
Canker Sores: The Most Common Cause
Canker sores (aphthous ulcers) are the single most frequent reason people develop blisters or open sores inside their mouth. They show up on the inner cheeks, lips, tongue, or soft palate as round white or yellow sores with a red border. Unlike cold sores, they are not contagious and don’t appear on the outer lips.
The exact cause isn’t fully understood, but it likely involves abnormal immune function combined with environmental triggers. Common triggers include biting the inside of your cheek, stress, and certain foods. Chocolate, peanuts, and eggs are known to worsen symptoms in some people. College students, for example, are notorious for developing canker sores during final exams.
Minor canker sores, the most common type, are smaller than a pea and heal within a few weeks without scarring. Major canker sores are larger than one centimeter, intensely painful, and can take months to heal, sometimes leaving scars. A rare third type, called herpetiform canker sores, appears as clusters of tiny pinpoint sores that heal in about two weeks.
Cold Sores Are a Different Problem
Cold sores (fever blisters) are caused by herpes simplex virus type 1 and typically appear on or around the outer lips, not inside the mouth. They start as a collection of small, fluid-filled blisters that eventually break open, ooze, and crust over. Most people feel a burning or tingling sensation at the site before the blisters appear.
A first outbreak can come with fever, body aches, sore throat, and swollen lymph nodes. Later outbreaks tend to be milder. The virus stays in your body permanently and can reactivate during stress, illness, or sun exposure. Many people carry the virus without knowing it, since most infections produce no symptoms or only mild ones. The virus spreads through contact with sores, saliva, or skin around the mouth, and transmission can happen even when no visible sores are present.
How to Tell Canker Sores and Cold Sores Apart
- Location: Canker sores form inside the mouth. Cold sores form on or around the outside of the lips.
- Appearance: Canker sores are typically a single round sore with a white or yellow center and red border. Cold sores are clusters of small, fluid-filled blisters.
- Cause: Canker sores are not caused by a virus and are not contagious. Cold sores are caused by herpes simplex virus and can spread to others.
- Warning signs: Both can produce a burning or tingling sensation before the sore appears.
Your Toothpaste Might Be Making It Worse
Sodium lauryl sulfate (SLS), a foaming agent in most toothpastes, can strip away the delicate protective lining inside your mouth. Research published in the British Dental Journal found that SLS can cause oral tissue peeling and increase the frequency of recurring mouth ulcers. One clinical study found four times fewer soft tissue lesions when participants used an SLS-free toothpaste compared to one containing SLS. If you get canker sores frequently, switching to an SLS-free toothpaste is one of the simplest changes you can try.
Nutritional Deficiencies That Cause Mouth Sores
Low levels of vitamin B12, iron, and folate are linked to recurring mouth ulcers. In one study comparing people with recurrent oral ulcers to healthy controls, over half the patients with ulcers had low vitamin B12 levels, while none of the healthy participants did. The connection was striking enough to be statistically significant.
If you get mouth blisters repeatedly and can’t pin down a trigger, a blood test checking these levels is worth pursuing. Deficiencies in B12 and iron are especially common in people who follow restrictive diets, have heavy menstrual periods, or have conditions that affect nutrient absorption.
Viral Infections Beyond Cold Sores
Hand, foot, and mouth disease is a common viral illness, particularly in young children, that causes painful blister-like sores in the front of the mouth, on the tongue, gums, and inner cheeks. It’s caused by coxsackievirus A16 and related enteroviruses. Sores typically appear one to two days after a fever begins. A related illness called herpangina causes similar sores concentrated in the back of the mouth and throat. Both are contagious and spread through close contact.
Underlying Health Conditions
Recurring mouth ulcers can be a symptom of a systemic disease rather than a standalone problem. Celiac disease, Crohn’s disease, and Behçet’s syndrome all cause mouth ulcers as part of a broader pattern of inflammation. Lupus is another possibility. In these cases, the mouth sores tend to come back frequently and may not respond to the usual treatments. If your mouth blisters recur for months alongside other symptoms like digestive trouble, joint pain, or skin rashes, the ulcers may be pointing toward something deeper.
Treatments That Help
Most minor canker sores heal on their own within two to three weeks. Over-the-counter numbing gels and pastes containing benzocaine can reduce pain if applied as soon as a sore appears. Antiseptic mouth rinses with hydrogen peroxide also help. For more severe or persistent sores, a doctor may prescribe a steroid mouth rinse to reduce inflammation, or a topical solution that chemically cauterizes the sore and can cut healing time to about a week.
For cold sores, antiviral medications can shorten outbreaks and reduce their severity, especially when started at the first tingling sensation. Over-the-counter creams can ease discomfort but won’t eliminate the virus.
Simple strategies reduce how often sores come back: avoid foods that trigger them, manage stress, switch to SLS-free toothpaste, and address any nutritional gaps.
Signs a Mouth Sore Needs Attention
A mouth sore that hasn’t healed after three weeks is worth getting checked. Oral cancers can look similar to canker sores in early stages, but there are differences. Cancerous lesions often have a small lump or bump beneath them that you can feel with your tongue or finger. Early oral cancer is usually painless, which is the opposite of a canker sore. Other warning signs include a small spot that keeps growing, a white patch that turns red, or a sore that starts bleeding when it previously didn’t.