Canker sores are not caused by a virus. This is one of the most common mix-ups in oral health, largely because canker sores are frequently confused with cold sores, which are caused by the herpes simplex virus (HSV). But the two are entirely different conditions with different causes, different locations, and different treatments.
Why Canker Sores Get Confused With Cold Sores
Cold sores appear on or around the lips, are filled with fluid, and are caused by herpes simplex virus. They’re contagious and spread through direct contact. Canker sores, on the other hand, form inside the mouth on soft tissue like the inner cheeks, gums, tongue, or soft palate. They look like small, round or oval sores with a white or yellowish center and a red border. They are not contagious.
The medical name for canker sores is “recurrent aphthous stomatitis,” and despite decades of research, doctors still don’t know their exact cause. What they do know is that the immune system plays a central role. In people who get canker sores, certain immune cells attack the soft tissue lining of the mouth, creating the painful ulcers. People with active canker sores show elevated levels of inflammatory markers in their blood, suggesting the body is mounting an immune response against its own oral tissue.
What Actually Triggers Canker Sores
While no single cause has been identified, several triggers are well established. Interestingly, viral infections can be one of those triggers, which may be where the confusion starts. A cold, flu, or other viral illness can set off a canker sore episode in someone already prone to them. But the virus isn’t causing the sore directly. It’s more that the stress on your immune system creates the conditions for one to develop.
Other common triggers include:
- Minor mouth injuries from biting your cheek, aggressive brushing, or dental work
- Emotional or physical stress
- Hormonal changes, particularly around menstruation
- Certain foods, especially acidic or spicy items like citrus, tomatoes, and chocolate
- Sodium lauryl sulfate, a foaming agent found in many toothpastes
Nutritional Deficiencies and Canker Sores
Nutrient levels play a surprisingly large role. In one study of people with recurrent canker sores, about half were deficient in vitamin B12, nearly 46% had low folate levels, and 42% were anemic. Iron deficiency was less common but still present in about 1 in 10 patients. If you get canker sores frequently, a blood test checking these levels is a reasonable step. Correcting a deficiency can, in some cases, reduce how often sores come back.
When Canker Sores Signal Something Else
For most people, canker sores are a painful nuisance and nothing more. They typically heal on their own within 10 to 14 days without leaving a scar. But frequent or unusually severe canker sores can sometimes point to an underlying condition. Celiac disease, inflammatory bowel disease (Crohn’s and ulcerative colitis), Behçet’s disease, and HIV infection are all associated with recurrent mouth ulcers. A rare childhood condition called PFAPA syndrome also causes periodic mouth sores along with fevers and sore throat.
The distinguishing factor is that these conditions come with other symptoms beyond mouth sores. Celiac disease involves digestive problems triggered by gluten. Inflammatory bowel disease causes chronic abdominal pain and diarrhea. Behçet’s disease produces ulcers in multiple areas of the body, including the genitals and eyes. If you’re getting canker sores alongside other unexplained symptoms, that pattern is worth investigating.
How Canker Sores Are Treated
Since most canker sores resolve in about two weeks, treatment focuses on managing pain and shortening healing time rather than “curing” the sore. Over-the-counter options include protective oral gels that coat the sore and shield exposed nerve endings from irritation by food and drink. Antiseptic mouth rinses can reduce the severity and pain of an outbreak, though they don’t prevent future episodes.
For more persistent or painful sores, prescription topical anti-inflammatory pastes are the standard approach. These are applied directly to the sore and work by calming the immune response in that area. They can reduce the number of days a sore lasts and ease pain, but they don’t consistently prevent new sores from forming. For severe cases, stronger prescription formulations are available, all of which work through the same basic mechanism of dialing down local inflammation.
Simple home measures help too. Rinsing with warm salt water several times a day can soothe the area. Avoiding acidic, spicy, or rough-textured foods while you have an active sore reduces irritation. Switching to a toothpaste without sodium lauryl sulfate has helped some people reduce recurrences.
Three Types of Canker Sores
Not all canker sores are the same. Minor canker sores are the most common type, measuring less than about a centimeter across. These are the ones that heal within two weeks and don’t scar. Major canker sores are larger, deeper, and significantly more painful. They can take weeks or even months to heal and may leave scarring behind.
The third type, called herpetiform canker sores, is where the naming gets especially confusing. Despite the name, these sores have nothing to do with herpes virus. They’re called “herpetiform” only because they appear in clusters of tiny ulcers that can resemble a herpes outbreak visually. They tend to merge into larger irregular sores and are the least common type, occurring more often in older adults.