A canker sore is not an infection. It is not caused by a virus, bacterium, or fungus, and it cannot be spread to another person through saliva, kissing, or sharing utensils. Canker sores (formally called aphthous ulcers) are small, shallow ulcers that form inside the mouth, and they result from your body’s own immune response rather than any invading pathogen.
Why People Confuse Canker Sores With Infections
The most common source of confusion is the cold sore, which looks similar but is a completely different condition. Cold sores (fever blisters) are caused by the herpes simplex virus, usually type 1. They are highly contagious, typically appear on or around the lips, and spread through direct contact. Canker sores, by contrast, only appear inside the mouth on soft tissues like the inner cheeks, tongue, or the base of the gums. They are never contagious.
A quick way to tell them apart: if the sore is on the outside of your lip or the skin around your mouth, it’s likely a cold sore and viral in origin. If it’s inside your mouth on non-keratinized tissue, it’s almost certainly a canker sore and not infectious.
What Actually Causes Canker Sores
There is no single confirmed cause. The National Institute of Dental and Craniofacial Research states that canker sores “do not have a known cause,” but research points strongly to an overactive immune response. Your immune system mistakenly attacks healthy cells lining the inside of your mouth, creating a localized area of inflammation and tissue breakdown. The fact that canker sores respond well to anti-inflammatory medications supports this explanation.
Several triggers can set off this immune overreaction:
- Minor mouth injuries from biting your cheek, aggressive brushing, dental work, or braces
- Stress and fatigue, which can shift immune function
- Certain foods, particularly acidic or spicy items like citrus, tomatoes, and chocolate
- Nutritional deficiencies in iron, zinc, folate, or B vitamins
- Hormonal changes, especially around menstruation
- Sodium lauryl sulfate, a foaming agent in many toothpastes
Some people get a canker sore once or twice a year. Others deal with recurring outbreaks where new sores develop before old ones heal. That pattern of recurrence can feel like an infection cycling through, but it’s still your immune system misfiring rather than a pathogen replicating.
How Canker Sores Heal
Most canker sores are small, less than 1 centimeter across, and shallow. These minor sores typically heal on their own within one to two weeks without scarring. The pain usually peaks in the first few days, then gradually fades.
Major aphthous ulcers are larger, deeper, and can take significantly longer to resolve. In people with weakened immune systems, particularly those with HIV/AIDS, healing times can stretch to months. Major ulcers sometimes leave scars.
There’s also a less common type called herpetiform ulcers, which appear as clusters of tiny sores. Despite the name (which refers to their herpes-like appearance), these are not caused by the herpes virus and are not infectious.
Treatment Options
Because canker sores aren’t infections, antibiotics and antiviral medications won’t help. The primary treatment approach focuses on reducing inflammation and managing pain. Topical corticosteroids and anti-inflammatory agents are the standard first-line options. These come as gels, pastes, or mouth rinses that you apply directly to the sore.
Over-the-counter options include protective pastes that coat the ulcer and numbing gels containing benzocaine. For mild sores, rinsing with warm salt water or a baking soda solution several times a day can ease discomfort and promote healing. Avoiding acidic, spicy, or rough-textured foods while you have an active sore makes a noticeable difference in pain levels.
Severe or frequently recurring canker sores that don’t respond to topical treatments may require systemic therapy, meaning oral medications that work throughout the body rather than just at the sore site.
Can a Canker Sore Become Infected?
While canker sores themselves aren’t infections, the open wound they create inside your mouth can potentially develop a secondary bacterial infection. Your mouth contains hundreds of bacterial species, and an ulcer provides an entry point. Signs that a canker sore may have become secondarily infected include increasing pain after the first few days instead of improving, spreading redness around the sore, swelling in the surrounding tissue, pus or unusual discharge, and fever.
This scenario is uncommon in people with healthy immune systems, but it’s worth being aware of, especially if a sore isn’t following the expected healing timeline.
When Canker Sores Signal Something Else
Frequent or unusually severe canker sores can sometimes be a sign of an underlying health condition. Several systemic diseases are associated with recurrent mouth ulcers:
- Celiac disease, an autoimmune reaction to gluten that damages the intestinal lining
- Inflammatory bowel diseases like Crohn’s disease and ulcerative colitis
- Behçet’s disease, a rare condition causing widespread inflammation throughout the body
- Immune deficiencies, including HIV/AIDS
Warning signs that your canker sores may warrant further investigation include sores that are unusually large, sores lasting longer than two weeks, frequent outbreaks where new ulcers appear before previous ones heal, sores extending onto the outer lip border, high fever accompanying the sores, or pain severe enough to prevent eating or drinking. In these cases, the sores themselves still aren’t infections, but they may be pointing to a condition that needs diagnosis and treatment.