Mouth sores develop from a wide range of causes, from minor injuries like biting your cheek to underlying nutritional deficiencies, stress, and certain medications. The two most common types are canker sores (small ulcers inside the mouth) and cold sores (fluid-filled blisters on or around the lips), and they have completely different origins. Most mouth sores heal on their own within two to three weeks, but understanding what triggers them can help you reduce how often they come back.
Canker Sores vs. Cold Sores
These two get confused constantly, but they’re fundamentally different. Canker sores form only inside the mouth, typically on the inner cheeks, lips, or tongue. They appear as single round white or yellow sores with a red border, and they are not contagious. Cold sores (also called fever blisters) form on the outside of the mouth, usually around the lips, and show up as patches of small fluid-filled blisters. They are caused by herpes simplex virus type 1 (HSV-1) and are highly contagious. An estimated 90% of U.S. adults have been infected with HSV-1 at some point, though many never develop visible sores.
Cold sores recur because the virus lives permanently in nerve cells, reactivating periodically and traveling to the skin surface. Canker sores have no single known cause, which is part of what makes them frustrating. Instead, they’re triggered by a combination of factors: physical injury to the mouth, emotional stress, smoking, and nutritional deficiencies.
Physical Injury and Irritation
The most straightforward way to get a mouth sore is tissue damage. Biting the inside of your cheek, brushing too aggressively, burning your mouth on hot food, or irritation from braces and ill-fitting dentures can all break the delicate lining of your mouth and create an ulcer. These sores typically heal quickly once the source of irritation is removed.
One less obvious irritant is a common ingredient in toothpaste: sodium lauryl sulfate (SLS), the foaming agent in most commercial brands. A systematic review published through the American Dental Association found that switching to an SLS-free toothpaste resulted in fewer ulcers, shorter ulcer duration, fewer recurring episodes, and less pain. If you get canker sores regularly, this is one of the simplest changes to try.
Nutritional Deficiencies
Low levels of vitamin B12, folate, and iron are strongly linked to recurrent canker sores. In one case-control study, 75% of participants with recurring oral ulcers had deficiencies in B12 or folate. The connection is biological: folate plays a direct role in how the cells lining your mouth renew themselves. When folate is low, that renewal process breaks down, leaving the tissue more vulnerable to ulceration. Patients with low folate levels develop more frequent and more severe sores compared to those with normal levels.
B12 deficiency causes its own set of oral problems, including a swollen, sensitive tongue (glossitis), redness of the mouth lining, and painful ulcers. The good news is that supplementation appears to help. Research has shown a clear reduction in ulcer frequency among patients who received B12, vitamin C, and folate supplements. If your mouth sores keep returning without an obvious trigger, a simple blood test checking these levels can be revealing.
Stress, Sleep, and Your Immune System
There’s a real biological pathway connecting stress to mouth sores, not just a vague “stress is bad for you” association. When you’re under psychological stress, your body produces a temporary spike in cortisol, the primary stress hormone. That spike disrupts your immune system by altering how immune cells distribute themselves and how they respond to inflammation. Specifically, stress increases the number of white blood cells at inflammatory sites, which is a pattern consistently observed during the formation of mouth ulcers.
Poor sleep compounds the problem. Insomnia and irregular sleep schedules disrupt the normal secretion of cortisol, growth hormone, and other hormones involved in tissue repair. So the combination of high stress and poor sleep creates a double hit: your immune system becomes dysregulated at the same time your body’s repair mechanisms slow down. Many people notice canker sores appearing during exam periods, work deadlines, or emotionally difficult stretches, and this hormonal disruption is the reason.
Foods That Trigger or Worsen Sores
Certain foods don’t cause mouth sores from scratch, but they can trigger flare-ups in people who are prone to them and make existing sores significantly more painful. Acidic foods are the biggest culprits: citrus fruits like oranges, lemons, and grapefruits; tomatoes and tomato-based sauces; and vinegar-heavy dressings or pickled foods. These create a sharp stinging sensation on contact with open sores and can slow healing. Carbonated drinks also irritate due to both their acidity and carbonation.
Some people find that specific foods seem to trigger new canker sores even before any visible irritation exists. Common reported triggers include chocolate, coffee, strawberries, nuts, and cheese, though the specific foods vary from person to person. Keeping a food diary when sores recur can help you identify your personal triggers.
Medications That Cause Mouth Sores
A surprising number of common medications can lead to mouth sores as a side effect, often through indirect pathways. One of the most widespread mechanisms is dry mouth. Saliva protects your oral lining, so when medications reduce saliva production, you become more susceptible to soreness, burning, and ulceration. The list of medications that cause dry mouth is long: antihistamines, antidepressants, blood pressure medications (diuretics), anti-anxiety medications, decongestants, pain medications, muscle relaxants, and bronchodilators used for asthma.
Some medications cause more direct damage. NSAIDs like aspirin and naproxen, along with certain blood pressure drugs, can cause chemical burns on oral tissue, producing white-to-yellow wrinkled lesions and sloughing of the mouth lining. In rare but serious cases, medications including certain anti-seizure drugs and antibiotics can trigger severe allergic reactions affecting the mouth, such as Stevens-Johnson syndrome. More than 200 medications have been associated with this risk.
Underlying Health Conditions
When mouth sores are frequent, severe, or resistant to healing, they sometimes signal a systemic condition. Celiac disease can manifest as recurrent oral ulcers, often before digestive symptoms become obvious. Crohn’s disease, an inflammatory bowel condition, also causes mouth sores in some patients. Behcet’s disease, a rarer condition involving blood vessel inflammation throughout the body, produces painful mouth sores as its most common symptom. These sores look like canker sores, begin as raised round lesions, and quickly become painful ulcers that heal in one to three weeks but keep returning.
Immune-suppressing conditions, including HIV and lupus, also increase vulnerability to oral ulcers because the immune system can no longer maintain the normal balance of bacteria and tissue repair in the mouth.
When a Mouth Sore Needs Attention
Most canker sores heal on their own within two to three weeks. If a sore lasts beyond that window, it warrants a visit to your doctor or dentist. Persistent sores that don’t heal are one of the early warning signs of oral cancer, and a professional evaluation can determine whether a biopsy is needed. Other signs worth having checked include sores that are unusually large, sores that spread or multiply rapidly, sores accompanied by high fever, and any sore that makes it difficult to eat or drink enough to stay hydrated.