Most canker sores heal on their own within one to two weeks. If yours has lingered beyond that window, something is likely interfering with the normal healing process, whether it’s the type of sore you have, a nutritional gap, ongoing irritation, or an underlying health condition. The good news is that persistent canker sores almost always have an identifiable cause, and once you address it, healing typically follows.
How Long Canker Sores Normally Take to Heal
Not all canker sores are the same, and the type you have determines how long you should expect it to stick around. Minor canker sores, the kind most people get, are small, oval, and heal without scarring in one to two weeks. These account for the vast majority of cases.
Major canker sores are a different story. These are 1 to 3 centimeters in diameter, noticeably deeper, and can take anywhere from 10 days to 6 weeks to fully heal. About 64% of them leave a scar. They make up roughly 10% of recurrent canker sores, so they’re less common but far more disruptive. If your sore is large and deep, you may simply be dealing with a major canker sore that needs more time.
A third type, called herpetiform canker sores, appears as clusters of tiny pinpoint sores that can merge into one large, irregular ulcer. Despite looking alarming, these generally heal within one to two weeks as well.
Irritants That Slow Healing
One of the most overlooked reasons a canker sore won’t go away is repeated irritation to the same spot. Braces, retainers, ill-fitting dentures, or a sharp tooth edge can create constant friction that reopens the wound before it has a chance to close. If your sore sits right where hardware meets soft tissue, that’s likely the culprit.
Your toothpaste may also be playing a role. A common detergent found in most toothpastes, sodium lauryl sulfate (SLS), has been shown in a systematic review to significantly affect canker sore frequency and duration. Switching to an SLS-free toothpaste reduced the number of ulcers, the length of each episode, and the amount of pain patients experienced. If you’re prone to canker sores or have one that won’t resolve, switching toothpaste is one of the simplest changes you can make.
Acidic or spicy foods, biting the inside of your cheek repeatedly, and even aggressive brushing can all reinjure the area and restart the healing clock.
Nutritional Deficiencies That Fuel Recurrence
Your body needs specific nutrients to repair damaged tissue in the mouth. Deficiencies in iron, vitamin B12, folic acid, vitamin B3, and vitamin C are all linked to recurrent or slow-healing canker sores. These deficiencies don’t just make sores more likely to appear. They make existing sores harder to resolve.
Iron and B12 deficiencies are especially common culprits, particularly in people who follow restrictive diets, have heavy menstrual periods, or absorb nutrients poorly. A simple blood test can reveal whether low levels are contributing to your problem. In many cases, correcting the deficiency through diet or supplements leads to a noticeable drop in canker sore frequency and faster healing when sores do occur.
What Your Immune System Has to Do With It
Canker sores are fundamentally an immune event. In the earliest stage, before you even see the sore, immune cells begin gathering beneath the surface lining of the mouth. Research shows that people who get recurrent canker sores have a measurable imbalance in their immune signaling. Their bodies produce higher levels of inflammatory signals and lower levels of anti-inflammatory ones, both during active sores and even during remission between outbreaks.
This imbalance means the inflammatory response that creates the ulcer is stronger than it should be, while the body’s natural “off switch” for that inflammation is weaker. The regulatory immune cells that are supposed to calm things down also function less effectively in people with recurrent sores. This helps explain why some people get canker sores over and over while others rarely do, and why certain sores seem to linger. Anything that further disrupts immune function (stress, poor sleep, illness) can tip the scales and keep a sore from resolving on schedule.
Underlying Health Conditions to Consider
When canker sores are frequent, unusually persistent, or severe, they can signal a systemic condition that needs its own treatment. Celiac disease is one of the more common hidden causes. People with undiagnosed celiac disease often develop mouth ulcers because their intestines aren’t absorbing nutrients properly, creating the same deficiencies described above. If you also experience bloating, fatigue, or digestive issues, celiac disease is worth investigating through blood work and potentially a biopsy.
Crohn’s disease and Behçet’s disease are two other conditions closely tied to oral ulcers. Nearly all patients with Behçet’s disease experience recurrent mouth sores, often as one of the earliest symptoms. Crohn’s disease can produce oral ulcers as part of its broader inflammatory effects throughout the digestive tract. Both conditions involve immune dysregulation that keeps ulcers cycling.
HIV infection, inflammatory bowel disease, and certain autoimmune disorders can also present with stubborn or recurring oral ulcers. If your canker sores are accompanied by other unexplained symptoms like joint pain, skin rashes, genital sores, eye inflammation, or chronic digestive problems, these possibilities are worth raising with your doctor.
When a “Canker Sore” Might Be Something Else
Most non-healing mouth sores are still canker sores with a complicating factor. But a sore that persists beyond three weeks without any sign of improvement deserves a closer look to rule out oral cancer. The two can look similar at first glance, but there are key differences.
A typical canker sore is round or oval, yellow or white in the center with a red border, smooth in texture, and located on soft tissue inside the mouth (inner cheeks, gums, tongue, inner lips, or the roof of the mouth). It does not bleed or produce discharge. It never appears on the outer lips.
Oral cancer lesions, by contrast, tend to have irregular borders, a patchy or textured surface, and may bleed. They can feel hard or thickened when you press on them. They don’t follow the typical canker sore pattern of gradually shrinking. A sore that changes shape, grows, bleeds, or simply refuses to heal over several weeks warrants a professional evaluation. A dentist or oral medicine specialist can determine whether a biopsy is needed.
Treatments for Stubborn Sores
For a canker sore that’s taking longer than expected, your first steps are removing irritants (switch to SLS-free toothpaste, avoid acidic foods, address any dental hardware issues) and supporting your body’s healing with adequate nutrition. Rinsing with warm salt water several times a day can help keep the area clean and reduce discomfort.
Over-the-counter topical gels that contain a numbing agent can manage pain while you wait for healing. If those aren’t enough, a dentist or doctor can prescribe a steroid paste that you apply directly to the sore. These work by reducing the inflammatory response at the site, which both relieves pain and helps the tissue heal faster. For particularly severe or recurrent cases, a healthcare provider may want to run blood tests to check for nutritional deficiencies or signs of an underlying condition.
Keeping a log of when your canker sores appear can also help identify personal triggers. Common patterns include outbreaks during stressful periods, after eating certain foods (citrus, tomatoes, chocolate, nuts), during hormonal shifts, or following mouth injuries. Recognizing your triggers won’t heal the current sore, but it can help you reduce future episodes significantly.