Mouth blisters have several common causes, ranging from minor injuries and viral infections to nutritional deficiencies and autoimmune conditions. Most are harmless and heal on their own within one to two weeks, but recurring or long-lasting sores sometimes point to something that needs attention.
Canker Sores
Canker sores (aphthous ulcers) are the most frequent type of mouth blister, affecting roughly 20 percent of the population. Unlike cold sores, they form inside the mouth on soft tissue like the inner cheeks, tongue, or gums, and they aren’t contagious. Most clear up within a week or two without treatment.
The exact cause isn’t fully understood, but a long list of triggers can set them off. Physical injury is one of the most common: accidentally biting the inside of your cheek, brushing too hard, or constant rubbing from braces, dentures, or a sharp tooth edge. Burns from hot food or irritation from strong mouthwash can also cause them. Stress, illness, and extreme fatigue tend to make outbreaks worse or more frequent.
One overlooked trigger is your toothpaste. Most toothpastes contain sodium lauryl sulfate (SLS), a foaming agent that irritates the lining of the mouth. A systematic review published by the American Dental Association found that switching to an SLS-free toothpaste led to significantly fewer ulcers, shorter healing times, and less pain. If you get canker sores regularly, trying an SLS-free toothpaste is one of the simplest changes you can make.
Cold Sores From Herpes Simplex Virus
Cold sores are clusters of tiny, fluid-filled blisters caused by herpes simplex virus type 1 (HSV-1). They most often appear along the border of the lips, but they can also show up around the nose, on the cheeks, or inside the mouth. In children under five, they’re especially likely to form inside the mouth rather than on the lips.
A cold sore typically starts as a small, hard, painful spot. Within a day or two, a cluster of blisters forms. These blisters eventually merge, burst, and leave behind shallow open sores that ooze and crust over before healing. The whole cycle usually takes about 7 to 10 days.
Once you’ve been infected, the virus hides in nerve cells in the skin and can reactivate later, often in the same spot. Triggers for reactivation include sun exposure, stress, fatigue, illness, and hormonal changes. Cold sores are contagious, especially when the blisters are open and oozing.
Hand, Foot, and Mouth Disease
Hand, foot, and mouth disease is a viral infection most common in children under seven, though adults can catch it too. It causes painful sores inside the mouth along with a skin rash on the hands and feet, sometimes accompanied by a fever. Adults who get it generally experience milder symptoms than children do. The infection is caused by a coxsackievirus and typically runs its course within a week to ten days.
Nutritional Deficiencies
Recurring mouth ulcers that don’t seem tied to any obvious injury or illness can be a sign that your body is low on certain nutrients. Deficiencies in vitamin B12, folate (vitamin B9), and iron are the most commonly linked to mouth sores. These deficiencies can also cause a sore, red tongue alongside the ulcers.
Underlying digestive conditions like celiac disease or Crohn’s disease sometimes drive these deficiencies by impairing nutrient absorption. If you’re getting mouth ulcers frequently and can’t pinpoint a trigger, a blood test checking these levels is a reasonable next step.
Mucoceles From Salivary Gland Injury
A mucocele is a fluid-filled bump that forms when a salivary gland or its duct gets damaged or blocked. Biting your lower lip is one of the most common causes. The injury disrupts the normal flow of saliva, which builds up under the tissue and forms a soft, painless, bluish cyst. Mucoceles most often appear on the inside of the lower lip, though they can develop elsewhere in the mouth. Most are harmless, but larger or persistent ones may need to be drained or removed.
Autoimmune Conditions
Less commonly, mouth blisters are caused by autoimmune disorders. Pemphigus vulgaris is one of the more serious examples. It causes blisters on the skin and mucous membranes, and it often begins with painful sores inside the mouth before appearing anywhere else. The blisters break easily, leaving open wounds that can make eating, drinking, and talking difficult. Pemphigus requires medical treatment and is diagnosed through a skin biopsy.
Other autoimmune-related conditions that can cause oral sores include lichen planus, which creates white, lacy patches or red, inflamed areas inside the mouth. These conditions tend to be persistent or recurring rather than one-off events.
Oral Thrush
Oral thrush is a yeast infection caused by an overgrowth of Candida in the mouth. It doesn’t produce typical fluid-filled blisters. Instead, it creates slightly raised, creamy white patches that look a bit like cottage cheese, usually on the tongue, inner cheeks, roof of the mouth, or gums. Scraping or rubbing these patches can cause slight bleeding. Thrush is more common in people with weakened immune systems, those taking antibiotics, and people who use inhaled corticosteroids for asthma.
When a Mouth Sore Could Be Something Serious
Oral cancer can look like a common mouth sore at first. The key difference is persistence. A sore that doesn’t heal within two weeks, a white or reddish patch on the inside of the mouth, a lump or growth, unexplained mouth pain, ear pain, or difficulty swallowing or chewing are all potential warning signs. A single mouth ulcer that lasts more than two weeks warrants a visit to a doctor or dentist, who can rule out more common causes like infection before investigating further.