Mouth sores most commonly develop from minor immune reactions, stress, nutritional gaps, or viral infections. The majority are canker sores (small ulcers inside the mouth) that heal on their own within one to three weeks. But recurring or unusually persistent sores can signal something worth investigating, from a vitamin deficiency to an underlying health condition.
Canker Sores: The Most Common Culprit
Canker sores are round white or yellow ulcers with a red border that appear inside the mouth, usually on the inner cheeks, lips, or tongue. They’re not contagious and they’re not caused by a virus. The exact cause isn’t fully understood, but it likely involves a combination of immune system dysfunction, genetic predisposition, and environmental triggers.
Common triggers include biting your cheek, brushing too hard, stress (college students famously get them during finals week), and certain foods. Chocolate, peanuts, and eggs are among the most frequently reported dietary triggers. Acidic fruits like citrus and tomatoes can also irritate the lining of your mouth enough to start an ulcer.
Most canker sores heal within one to three weeks without treatment. If you get them repeatedly, it’s worth looking at the other causes below, because something else may be driving them.
Nutritional Deficiencies That Cause Sores
Low levels of vitamin B12, iron, and folate are all linked to recurring mouth ulcers. Your body needs these nutrients to maintain the fast-turnover tissue lining your mouth. When levels drop, that tissue becomes fragile and more prone to breaking down into sores.
This is one of the most actionable causes to investigate. A simple blood test can check all three. If you eat a limited diet, follow a vegan or vegetarian diet without supplementation, or have heavy menstrual periods (which deplete iron), deficiency is a realistic possibility. Correcting the deficiency often reduces or eliminates the sores entirely.
Stress and Its Ripple Effects
Stress is a trigger for both canker sores and cold sores, though through different pathways. For canker sores, stress appears to dysregulate the immune response in the mouth’s mucous membranes. For cold sores (caused by herpes simplex virus), stress triggers the release of cortisol, which can reactivate the virus from its dormant state in nerve cells. UV light exposure and heat stress raise cortisol through similar pathways, which is why cold sores often appear after a day in the sun.
If your sores reliably show up during high-pressure periods, stress management isn’t just general wellness advice. It’s directly relevant to reducing outbreaks.
Cold Sores vs. Canker Sores
These are completely different conditions, but people often confuse them. The simplest way to tell them apart: location and appearance.
- Cold sores appear outside the mouth, typically around the border of the lips. They look like clusters of small fluid-filled blisters.
- Canker sores appear inside the mouth. They’re single round ulcers, white or yellow with a red border.
Cold sores are caused by herpes simplex virus type 1 (HSV-1), which most people contract in childhood. Once you have it, the virus lives permanently in nerve cells and can reactivate throughout your life. It’s contagious during an active outbreak. Canker sores involve no virus and can’t be spread to anyone.
Your Toothpaste May Be Making It Worse
Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes, irritates the mouth’s lining in some people and can trigger or worsen canker sores. In a clinical study of people with recurrent mouth ulcers, switching to SLS-free toothpaste for at least three months led to fewer attacks in about 54% of patients, with nearly half also reporting less pain from the sores they did get.
SLS-free toothpastes are widely available and inexpensive. If you get frequent canker sores, switching brands is one of the easiest things to try first.
Medications That Cause Mouth Sores
Several common medications can damage the mouth’s lining as a side effect. NSAIDs like aspirin and naproxen can cause white or yellowish lesions and tissue sloughing. Some blood pressure medications, particularly certain angiotensin-receptor blockers, are also associated with chemical burns to the oral tissue. Even albuterol inhalers can occasionally cause tongue ulcers.
If your mouth sores started around the same time as a new medication, that connection is worth raising with your prescriber. In many cases, an alternative drug in the same class won’t cause the same problem.
Underlying Health Conditions
Recurring mouth sores that don’t respond to the usual fixes can sometimes be the first visible sign of a systemic condition. Behçet’s disease, a rare inflammatory disorder affecting blood vessels, often presents initially as painful mouth ulcers that look identical to canker sores. These sores begin as raised round lesions, quickly become painful ulcers, and tend to heal within one to three weeks before returning.
Inflammatory bowel diseases like Crohn’s disease can also produce mouth ulcers, sometimes before any digestive symptoms appear. Celiac disease is another possibility, particularly if the sores accompany bloating, fatigue, or unexplained weight loss. In all of these cases, the mouth sores are driven by the same immune dysfunction that affects other parts of the body.
A pattern of frequent, severe sores that don’t match any obvious trigger (stress, food, injury) is a reasonable reason to ask for further evaluation.
How to Manage Sores at Home
Most canker sores don’t need medical treatment. Over-the-counter options that help with pain and healing include topical numbing gels containing lidocaine, which you apply directly to the sore as needed. Medicated pastes designed for oral ulcers create a protective barrier over the sore, reducing pain from contact with food and drink. Rinsing with a warm saltwater solution several times a day can also soothe irritation and keep the area clean.
For sores that are especially large or painful, prescription steroid pastes or rinses can speed healing. These are applied directly to the dried ulcer several times daily. They work by calming the localized immune reaction that’s sustaining the ulcer.
When Mouth Sores Need Attention
A sore that doesn’t heal within two weeks after any obvious irritant (like a sharp tooth edge or braces wire) has been addressed warrants a closer look. Persistent sores are the primary trigger for a tissue biopsy, which screens for oral cancer and other conditions. This is especially true if the sore is painless (canker sores hurt, but early oral cancers often don’t), if it’s growing rather than shrinking, or if it’s accompanied by a lump in the neck.
Sores that keep returning in the same spot, that are unusually large, or that come with fever, skin rashes, or eye inflammation suggest something beyond a simple canker sore and point toward the systemic conditions described above.