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Mouth sores, also called canker sores or aphthous ulcers, are painful lesions of the oral mucosa that present as small, rounded ulcers with a whitish base surrounded by a red halo, which heal spontaneously in 7-14 days without scarring.
Although mouth sores are not serious in most cases, their impact on quality of life can be significant: pain when eating, speaking, or swallowing interferes with daily activities for days. When canker sores are frequent, very painful, large, or do not heal in the usual timeframe, they can be a sign of a nutritional deficiency, a systemic disease, or a lesion requiring a biopsy.
At Asensio Dental Clinic, Dr. Lucía Asensio Romero (Registration No. 46002287), a specialist in oral medicine in Valencia, treats cases of recurrent canker sores and mucosal lesions that do not respond to standard care. The first visit is completely free.
Why do mouth sores appear?
The causes of canker sores are multifactorial, and in most cases, several triggering factors combine. Nutritional deficiencies are the most frequent and easily correctable cause: deficiency in vitamin B12, folic acid, iron, and zinc is consistently associated with the appearance of recurrent canker sores. In people following vegetarian or vegan diets without adequate supplementation, B12 deficiency is particularly common as a cause of repeating sores.
Hormonal changes explain why canker sores are more frequent in women and appear with higher incidence during certain phases of the menstrual cycle, during pregnancy, or perimenopause. Emotional stress acts as a trigger through neuroendocrine mechanisms that alter the local immune response of the mucosa. Microtrauma —accidentally biting the cheek, friction from a sharp filling, or using braces— are common local causes that produce isolated sores at the site of injury.
Sensitivity to sodium lauryl sulfate —the foaming agent in many toothpastes— is a little-known but relevant cause: in susceptible individuals, this compound directly irritates the mucosa and promotes the appearance of canker sores. Switching to a toothpaste without sodium lauryl sulfate can significantly reduce the frequency of episodes in these patients. In some cases, canker sores are the oral manifestation of systemic diseases such as Crohn’s disease, celiac disease, lupus erythematosus, or Behçet’s disease — in these situations, the pattern of the lesions is usually more extensive, more frequent, and resistant to standard treatment. If you are pregnant and suffer from frequent canker sores, also check our page on oral health during pregnancy and lactation.
Treatment for mouth sores
The treatment of canker sores has two objectives: to alleviate pain during the episode and to reduce the frequency of recurrences by acting on the predisposing factor. Most minor canker sores heal on their own in a week without the need for treatment, but there are measures that significantly accelerate healing and reduce pain.
Gentle antiseptic rinses with salt and water or water with baking soda, applied several times a day, reduce the bacterial load on the ulcer and promote tissue healing. A 0.9% saline solution is especially well-tolerated and can be applied as many times as desired. Hydrogen peroxide diluted in half with water, applied directly to the sore with a cotton swab, has a debriding effect that cleans the ulcer’s surface.
Aloe vera gel applied to the sore after brushing —waiting at least 15 minutes after brushing so as not to dilute it— has anti-inflammatory, antimicrobial, and healing properties that reduce pain and shorten healing time. Topical corticosteroids in the form of a gel or rinse —triamcinolone, betamethasone, or clobetasol depending on severity— are the first-choice pharmacological treatment for very painful or large canker sores: they quickly reduce inflammation and shorten the episode’s duration. Low-intensity dental laser is the most effective option for immediate pain relief: applied directly to the ulcer, it produces immediate local analgesia and accelerates healing without the need for systemic medication.
Foods that help prevent and relieve sores
Diet plays an important role in both preventing canker sores and the speed of recovery during episodes. Nutrients with the most evidence in prevention are Vitamin B12 —found in meats, fish, seafood, and dairy; supplementation is essential for strict vegetarians—, folic acid —abundant in lentils, spinach, broad beans, and peas—, iron —in red meats, legumes, and nuts— and zinc —in pumpkin seeds, beef, and legumes.
Vitamin B2 or riboflavina, found in milk, yogurt, eggs, and whole grain bread, and Vitamin B3 or niacina, in meats, fish, and cereals, also play a part in maintaining the integrity of the oral mucosa. Vitamin C —in citrus fruits, kiwi, peppers, and leafy green vegetables— is essential for collagen synthesis and tissue healing; its deficiency is associated with inflamed gums and slow healing. Smokers have Vitamin C needs up to three times higher than non-smokers.
While the canker sore lasts, it is advisable to avoid very hot, hard, or crunchy foods —which mechanically traumatize the ulcer—, very acidic foods like large amounts of citrus or tomato, and very spicy or seasoned foods. Alcohol and tobacco directly irritate the mucosa and delay healing.
When to see a dentist for mouth sores
Most canker sores do not require a visit to the dentist. However, there are situations that justify a specialized consultation: when the ulcer has not healed in two weeks, when episodes are very frequent —more than three a year—, when sores are very large —more than one centimeter in diameter— or very numerous, when accompanied by fever, swollen lymph nodes, or other systemic manifestations, or when they appear in people with oncological risk factors —smoking, alcohol consumption, HPV infection.
An ulcer that does not heal in two weeks is not a simple canker sore: it could be a malignant lesion requiring an urgent biopsy. This is the 14-day rule that every patient should know. The differential diagnosis between a benign canker sore and a potentially malignant lesion can only be made by a specialist through direct examination of the lesion.
Frequently asked questions about mouth sores
Are mouth sores contagious?
No. Canker sores are not contagious because they do not have an infectious origin: they are ulcers of multifactorial cause —immunological, nutritional, hormonal— without the involvement of contagious viruses or bacteria. Unlike cold sores, which are contagious and caused by the herpes simplex virus, canker sores are not transmitted by direct contact.
Why do I always get sores in the same place?
Canker sores tend to appear in the same spots on the mucosa because those areas are more susceptible due to anatomical reasons —greater friction with teeth, thinner mucosa— or repeated microtrauma. If they always appear in the same place and are frequent, it is worth checking if there is a local factor —a sharp filling edge, friction from a bracket, tooth malposition— that is triggering them.
When can a mouth sore be dangerous?
An oral ulcer should be evaluated by a specialist when it does not heal in two weeks, when it is painless —malignant lesions often do not hurt—, when it grows progressively, when it has irregular or hardened edges, or when it appears in a person with oral cancer risk factors. The rule is clear: an ulcer of more than 14 days that does not heal needs a biopsy.