Mouth blisters are most often canker sores, which affect roughly 1 in 5 people at some point in their lives. They can also result from viral infections, physical irritation, nutritional gaps, or less commonly, autoimmune conditions. The cause usually determines where the blisters appear, how they look, and how long they last.
Canker Sores: The Most Common Culprit
Canker sores (aphthous ulcers) are small, shallow ulcers that form on the soft tissues inside your mouth: the inner cheeks, tongue, gums, or soft palate. They’re not contagious and typically heal on their own within one to two weeks. You’ll usually notice a round or oval sore with a white or yellowish center and a red border. They can sting, especially when you eat or drink something acidic.
The triggers are surprisingly mundane. Biting your cheek, scraping your gums with a chip, or getting dental work done can all set one off. Stress, lack of sleep, hormonal shifts during your period, and eating lots of acidic fruits like oranges, pineapples, or strawberries are also common triggers. Some people get canker sores repeatedly, while others might only deal with one a handful of times in their life.
Viral Infections That Cause Mouth Blisters
Two viral infections are responsible for most blister-like sores in the mouth: herpes simplex virus (HSV) and the virus behind hand, foot, and mouth disease.
Cold Sores From Herpes Simplex
HSV-1 causes clusters of small, fluid-filled blisters that typically appear on or around the lips, though they can also show up on the gums, tongue, palate, and inner cheeks. A first outbreak often comes with fever, swollen lymph nodes, and a general feeling of being unwell. The blisters eventually burst, form shallow ulcers, and crust over before healing. Unlike canker sores, cold sores are contagious and tend to recur in the same area, often triggered by stress, illness, or sun exposure.
Hand, Foot, and Mouth Disease
This illness, caused by coxsackievirus, is most common in young children but can affect adults too. It produces painful ulcers toward the back of the mouth, particularly the soft palate, though lesions also appear on the tongue and inner cheeks. The telltale sign is a bumpy rash on the hands and soles of the feet alongside those mouth sores. A low-grade fever usually comes first.
Nutritional Deficiencies
If you’re getting mouth sores repeatedly without an obvious trigger, a nutritional gap could be involved. Low levels of vitamin B12, folate, or iron can all cause mouth ulcers and a sore, reddened tongue. These deficiencies don’t always produce dramatic symptoms at first. You might just notice that you’re getting sores more often than usual, feel more tired, or look paler than normal. A simple blood test can identify whether you’re running low on any of these nutrients, and supplementation or dietary changes typically resolve the problem.
Irritation From Products You Use Daily
Your toothpaste could be playing a role. Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes worldwide, is known to irritate the mucous membranes inside the mouth. Some people with recurring canker sores switch to SLS-free toothpaste hoping for relief, and there’s a logical basis for trying it. That said, the clinical evidence is mixed. A 2019 review found there wasn’t enough data to confirm that SLS-free toothpastes reduce the frequency, duration, or pain of canker sores. Allergy testing also shows that toothpaste ingredients don’t appear to directly trigger canker sore formation. Still, if you suspect your toothpaste is irritating your mouth, trying an SLS-free option for a few weeks is a low-risk experiment.
Braces, retainers, and ill-fitting dentures are another common source of irritation. Repeated rubbing against the same spot creates ulcers that won’t fully heal until the source of friction is addressed.
Fungal Infections
Oral thrush looks different from most mouth blisters. It appears as creamy white patches on the tongue, inner cheeks, or roof of the mouth, sometimes with redness or soreness underneath. It’s caused by an overgrowth of yeast that normally lives in your mouth in small amounts. Thrush commonly develops after a course of antibiotics, which can disrupt the balance of organisms in your mouth. People with weakened immune systems, diabetes, or those using inhaled corticosteroids for asthma are also more prone to it.
Autoimmune and Chronic Conditions
Mouth blisters that keep coming back or that don’t respond to typical treatments can sometimes signal an underlying condition. Oral lichen planus, an immune-mediated condition, is one example. It most commonly appears on the inside of the cheeks and can take two main forms: white, lacy patches that may cause no discomfort, or red, swollen areas with open sores that burn and bleed. The erosive form causes sensitivity to hot, acidic, and spicy foods and can make brushing your teeth painful.
Several other systemic conditions are associated with recurring mouth ulcers, including celiac disease, Crohn’s disease, Behçet’s disease, and lupus. In these cases, mouth sores are typically one symptom among several, not the only thing going on.
When Mouth Blisters Need Attention
Most mouth blisters are harmless and heal within a couple of weeks. But certain patterns warrant a closer look. A sore that hasn’t healed after three weeks is the most important red flag. Early signs of oral cancer can mimic benign conditions like canker sores, making them easy to dismiss. Persistent ulcers, unexplained lumps, difficulty opening your mouth, or numbness in your lip or tongue are all signals that something more serious could be going on.
Blisters that keep returning in the same spot, spread rapidly, come with high fever, or make it too painful to eat or drink also deserve professional evaluation. This is especially true if you have a weakened immune system, since infections can progress more quickly.
Managing the Pain at Home
For ordinary canker sores, the goal is comfort while you wait for healing. Rinsing with warm salt water several times a day helps keep the area clean and can reduce inflammation. Over-the-counter numbing gels containing lidocaine provide temporary relief, especially before meals. Protective pastes that coat the sore and shield it from further irritation are another option available at most pharmacies.
Avoiding foods that aggravate the sore makes a noticeable difference. Citrus fruits, tomatoes, spicy dishes, and crunchy or sharp-edged foods like chips and crackers are the usual offenders. Switching to softer, cooler foods until the sore heals can cut days off your discomfort. For people who get frequent canker sores, keeping a log of what you ate, your stress levels, and your sleep in the days before each outbreak can help you identify your personal triggers and reduce how often they occur.