Recurring mouth blisters are almost always canker sores (aphthous ulcers), and they affect roughly 20% of the population at some point. Some people get as few as one outbreak two to four times a year, while others experience near-continuous sores, with new ones forming as old ones heal. The good news is that most recurrent mouth sores are not dangerous, but figuring out what keeps triggering yours can help you break the cycle.
Canker Sores vs. Cold Sores
The first step is knowing which type of sore you’re dealing with, because the causes and treatments are completely different. Canker sores appear inside the mouth, on the inner cheeks, lips, tongue, or soft palate. They’re flat, white or yellow with a red border, and they are not filled with fluid. Cold sores (fever blisters) show up on the outside of the mouth, around the lips. They start as clusters of small, painful, fluid-filled blisters and are caused by the herpes simplex virus.
If your recurring blisters are inside your mouth and look like shallow, open sores rather than fluid-filled bumps, you’re dealing with canker sores. If they’re fluid-filled blisters on or around your lips, they’re likely cold sores, which recur because the herpes virus stays dormant in your nerve cells and reactivates periodically. The rest of this article focuses primarily on canker sores, since they’re the most common reason people search for recurring mouth blisters.
The Three Types of Canker Sores
Not all canker sores are the same, and knowing which type you tend to get helps you gauge what’s normal and what deserves medical attention.
Minor canker sores are the most common. They’re typically 2 to 3 mm across (smaller than a pencil eraser), heal within about 10 days, and don’t leave scars. Major canker sores measure over 1 cm, can take weeks to months to fully heal, and sometimes scar. Herpetiform canker sores appear as clusters of tiny sores (each under 1 cm) that merge into larger, irregular ulcers lasting about two weeks. Despite the name, herpetiform canker sores have nothing to do with herpes.
If your sores are consistently large, slow to heal, or leaving scar tissue behind, you’re likely dealing with major aphthous ulcers and should bring this up with a dentist or doctor.
Stress and Cortisol
Stress is one of the most well-documented triggers for recurrent canker sores, and the connection is hormonal. When you’re under stress, your body ramps up production of cortisol through the adrenal glands. Research published in the Journal of Oral Medicine and Oral Surgery found that people with active canker sores had significantly elevated salivary cortisol compared to people without sores. Cortisol remained elevated even after the sores healed, suggesting the stress response lingers.
The mechanism isn’t just about one hormone, though. Psychological stress shifts activity across your nervous, hormonal, and immune systems simultaneously. It alters levels of stress hormones, changes immune cell activity (including T cells, B cells, and natural killer cells), and can tip the balance toward inflammation in vulnerable tissues like the mouth lining. If your outbreaks tend to cluster around exams, deadlines, or emotionally difficult periods, stress is a strong suspect.
Your Toothpaste May Be a Trigger
A common and overlooked culprit is sodium lauryl sulfate (SLS), a foaming agent found in most commercial toothpastes. SLS is a strong detergent that strips proteins from the surface of your oral tissue. It breaks apart the structure connecting surface cells, causing layers of the mouth lining to peel away. Reported side effects include sloughing of the inner cheek tissue, ulceration, and inflammation.
If you notice that your sores tend to appear a day or two after brushing, or that the tissue inside your mouth peels in sheets, switching to an SLS-free toothpaste is a simple experiment worth trying. Several brands market themselves as SLS-free, and they’re widely available at pharmacies.
Nutritional Deficiencies
Recurring mouth sores can be your body’s signal that it’s missing key nutrients, particularly folate, iron, and vitamin B12. In one clinical study of patients with stomatitis (chronic mouth inflammation), 56% had low red cell folate levels. Folate is essential for healthy cell turnover, and the mouth lining replaces itself every one to two weeks, making it especially sensitive to shortfalls.
Iron deficiency can also weaken the oral mucosa, though it’s less straightforward to diagnose from symptoms alone. If your canker sores come alongside fatigue, pale skin, or brittle nails, it’s worth asking your doctor to check your iron, folate, and B12 levels with a simple blood test. Correcting a deficiency, whether through diet or supplements, can reduce outbreak frequency significantly.
Food Triggers and Amino Acid Balance
Certain foods are well known to provoke canker sores in susceptible people. Acidic foods like citrus fruits, tomatoes, and pineapple can irritate the mouth lining directly. Spicy foods do the same. But there’s also a subtler dietary factor at play for people who get cold sores specifically: the balance between two amino acids, lysine and arginine.
Research has shown that when blood levels of lysine exceed 165 nmol/ml, the recurrence rate of herpes-related cold sores drops significantly. When lysine falls below that threshold, outbreaks become more frequent. Lysine and arginine compete for absorption, so a diet heavy in arginine-rich foods (like nuts, chocolate, and seeds) without enough lysine-rich foods (like dairy, eggs, fish, and chicken) can tip the balance in favor of viral reactivation. In clinical trials, daily lysine supplementation at 1,000 mg reduced cold sore recurrence.
Underlying Health Conditions
When mouth sores are frequent, severe, or don’t respond to the usual triggers, they can sometimes point to a systemic condition. This doesn’t mean you should panic, but it’s worth being aware of the patterns.
Celiac disease is one of the more common hidden causes. People with undiagnosed celiac disease often develop recurrent canker sores as one of their earliest symptoms, sometimes before any digestive issues appear. If your sores come with bloating, unexplained fatigue, or weight changes, celiac screening (a blood test) is reasonable to request.
Crohn’s disease can produce oral lesions that actually precede any abdominal symptoms. These look different from typical canker sores: deep linear ulcers in the fold between the gums and cheek, firm painless swelling of the lips or inner cheeks, and a cobblestone texture to the tissue. Crohn’s-related mouth changes don’t necessarily correlate with intestinal disease activity, meaning your gut might feel fine while your mouth is affected.
Behçet’s disease is rarer but worth knowing about. It causes recurrent, painful ulcers that tend to be numerous and favor the soft palate and back of the throat. Oral ulcers are often the very first sign of the disease, sometimes appearing years before other symptoms like genital sores or eye inflammation develop.
When a Sore Could Be Something Serious
Most canker sores heal on their own within two to three weeks. If a sore lasts beyond that window, it needs medical evaluation. MD Anderson Cancer Center identifies several specific red flags that distinguish a potentially cancerous lesion from a harmless canker sore.
Canker sores are flat. Oral cancers often have a tiny lump or bump beneath the surface that you can feel with your tongue or finger. Canker sores typically have inflamed, red borders. Cancerous lesions usually don’t show that same angry red rim. You should also see a doctor if you have a small spot that grows larger over time, a white spot that turns red, or any lesion that starts bleeding when it didn’t before. These changes are uncommon, but catching them early makes a major difference in outcomes.
Reducing Outbreaks
Because canker sores have multiple overlapping triggers, the most effective approach is to address several at once. Switch to an SLS-free toothpaste, which removes one of the most common chemical irritants. Pay attention to whether outbreaks follow specific foods, particularly acidic or spicy ones, and reduce those during vulnerable periods. If stress is a clear trigger, even modest interventions like regular sleep and exercise can lower baseline cortisol enough to matter.
For sores that are already present, over-the-counter topical treatments containing a mild steroid can reduce pain and speed healing. Your dentist can prescribe stronger prescription-strength steroid pastes if your sores are large or slow to heal. Rinsing with warm salt water several times a day also helps keep sores clean and reduces discomfort.
If you’re getting canker sores more than four or five times a year, or if they’re large and take weeks to resolve, ask your doctor to check for nutritional deficiencies and screen for the systemic conditions mentioned above. In many cases, the answer turns out to be something correctable, like low folate or an irritating toothpaste, and making one or two changes can dramatically cut down how often sores appear.