Canker sores are not contagious. You cannot spread them through kissing, sharing utensils, or any other form of contact. Unlike cold sores, which are caused by the herpes simplex virus, canker sores have no infectious agent behind them. They are an immune response that happens entirely inside your own mouth.
Why Canker Sores Can’t Spread
Canker sores form because your own immune system attacks the tissue lining your mouth. White blood cells called T cells target and destroy epithelial cells on the inner cheeks, lips, or tongue, creating those painful white or yellow craters. The process is driven by inflammatory signals your body produces on its own, not by a virus, bacterium, or fungus that could jump from one person to another.
This is the fundamental difference between canker sores and cold sores, and it’s where most of the confusion comes from. Cold sores are fluid-filled blisters that appear on the outside of the mouth, around the lips, and they are highly contagious because they’re caused by herpes simplex virus type 1 (HSV-1). Canker sores only form inside the mouth and have no viral component. If the sore is inside your mouth and looks like an open ulcer with a white or yellowish center, it’s almost certainly a canker sore, and the people around you are not at risk.
What Actually Triggers Them
Even though canker sores aren’t caused by an infection, they don’t appear randomly. About 25% of the world’s population gets them, and certain triggers make outbreaks more likely.
Nutritional deficiencies are one of the strongest links. People with recurrent canker sores tend to consume significantly less vitamin B12 and folate than people who don’t get them. One study found that canker sore sufferers ate about 7% less than the recommended daily intake of B12 and about 20% less folate compared to controls. Iron deficiency plays a similar role. If you get canker sores frequently, it’s worth looking at whether your diet provides enough of these nutrients or whether a supplement might help.
Other common triggers include physical injury to the mouth (biting your cheek, braces scraping tissue, aggressive brushing), emotional stress, hormonal changes, and certain foods. Acidic or spicy foods like citrus, tomatoes, and hot peppers are frequent culprits. Some people also notice outbreaks when they’re sleep-deprived or fighting off a cold, likely because their immune system is already on edge.
Toothpaste Matters More Than You Think
One surprisingly well-supported prevention strategy is switching to a toothpaste without sodium lauryl sulfate (SLS), the foaming agent in most standard toothpastes. A systematic review of clinical trials found that people who used SLS-free toothpaste had fewer ulcers, shorter episodes, less pain, and fewer total outbreaks compared to those using regular toothpaste. The effect was consistent across all four measures studied. SLS-free options are widely available at most drugstores and grocery stores, and this is one of the simplest changes you can make if canker sores are a recurring problem for you.
How Long They Last
Most canker sores are the minor type: small, round, and under a centimeter across. These heal on their own in about 10 to 14 days without treatment and don’t leave scars. They’re painful, especially during the first few days, but they resolve completely.
Major canker sores are larger (one to three centimeters), deeper, and can last up to a month. These are less common and significantly more disruptive to eating and speaking. A third type, called herpetiform ulcers, appears as clusters of tiny sores that can merge into larger irregular ulcers. Despite the name, they have nothing to do with the herpes virus. These clusters can persist anywhere from 10 to 100 days.
What Helps With Pain and Healing
Since canker sores aren’t an infection, treatment focuses on reducing inflammation and managing pain rather than killing a pathogen. Topical treatments are the first-line approach and are safer than anything systemic.
Over-the-counter options include antiseptic mouthwashes containing chlorhexidine, which may reduce the severity and pain of ulcers, and analgesic rinses that numb the area. For more persistent or painful sores, prescription topical corticosteroids are the standard treatment in oral medicine. These come as pastes, mouthwashes, or sprays and work by calming the immune response at the ulcer site. They can reduce pain, speed healing, and decrease the number of new sores that form.
Simple home measures also help. Rinsing with warm salt water several times a day can soothe irritation. Avoiding crunchy, acidic, or spicy foods while you have an active sore prevents further irritation to the exposed tissue.
When a Mouth Sore Needs Attention
A typical canker sore, while annoying, is harmless. But mouth ulcers can also be a sign of something more significant. Inflammatory bowel diseases like Crohn’s and ulcerative colitis frequently cause aphthous-like ulcers in the mouth, particularly during active flares. Behcet’s disease produces oral ulcers that look nearly identical to canker sores but tend to be more numerous, longer-lasting, and more painful, and they appear alongside ulcers in other parts of the body. Iron deficiency anemia and pernicious anemia can also produce small, shallow mouth ulcers that mimic canker sores.
If a sore hasn’t healed within two weeks, if you’re getting large or numerous ulcers that keep returning, or if you notice other symptoms like joint pain, genital sores, eye inflammation, or unusual fatigue alongside your mouth ulcers, those patterns point to something beyond ordinary canker sores and are worth bringing to a doctor or dentist.