Most canker sores heal on their own within one to two weeks without any complications. A canker sore that becomes secondarily infected, though, will show clear warning signs: increasing pain instead of improving pain, spreading redness or swelling around the sore, and pus draining from the area. If your canker sore is following a normal trajectory of gradually feeling better over several days, it’s almost certainly fine. If it’s getting worse, that’s when to pay attention.
What Normal Healing Looks Like
Understanding the typical timeline helps you spot when something has gone off track. A canker sore usually announces itself with a tingling or burning sensation a few hours before it appears. The area turns red, swells slightly, and starts to hurt. Within one to three days, the swelling develops into the characteristic whitish or yellowish spot with a well-defined red border. Pain peaks during these first few days.
After that, the trajectory is steadily downward. Pain fades over the next few days, and the sore disappears completely in about a week. Minor canker sores, which make up the vast majority of cases, rarely need any treatment at all. The key pattern to remember: a normal canker sore hurts most in its first few days and then gradually improves from there.
Signs a Canker Sore Is Infected
A secondary bacterial infection reverses that normal healing pattern. Instead of slowly getting better, things get worse. The specific signs to watch for are:
- Increasing pain: Pain that intensifies after the first few days, rather than fading, suggests something beyond normal inflammation.
- Spreading redness or swelling: A normal canker sore has a contained red border. If redness or swelling is expanding outward into the surrounding tissue, bacteria may be involved.
- Warmth around the sore: The tissue near the sore feels noticeably hot to the touch.
- Pus or unusual drainage: Any yellowish or greenish discharge coming from the sore is a strong indicator of infection. This is different from the normal whitish center of a canker sore, which is a fibrin coating (part of the healing process), not pus.
- Fever: A localized mouth sore shouldn’t cause a systemic fever. If you develop one alongside a worsening sore, the infection may be spreading beyond the original site.
The single most reliable signal is the direction of change. Normal canker sores improve day by day. Infected ones get worse.
Why Canker Sores Sometimes Get Infected
Your mouth is full of bacteria, and a canker sore is essentially an open wound on your inner cheek, tongue, or gum tissue. Most of the time your immune system keeps those bacteria in check while the sore heals. But if you bite the sore again, irritate it with rough or acidic food, or have a weakened immune system, bacteria can take hold and cause a secondary infection.
Poor oral hygiene increases the risk. Brushing gently (but consistently) around the sore, and rinsing with warm salt water a few times a day, helps keep bacterial levels down without further irritating the tissue. Avoiding very spicy, acidic, or crunchy foods while the sore is open also reduces the chance of reinjury.
Canker Sore vs. Cold Sore vs. Something Else
Before deciding your canker sore is infected, make sure you’re actually looking at a canker sore. The most common mix-up is with cold sores (fever blisters), and the distinction is straightforward: canker sores occur inside the mouth, while cold sores appear on the outside, usually around the border of the lips. Cold sores also look different. They form clusters of small fluid-filled blisters, whereas canker sores are typically a single round white or yellow sore with a red border. Cold sores are caused by the herpes simplex virus and are contagious; canker sores are not contagious, and their exact cause remains unknown.
A more serious concern is oral cancer, which can sometimes mimic a stubborn canker sore. The differences are important. Canker sores have a characteristic flat or slightly sunken center with clearly defined edges. Oral cancer is more likely to appear as a raised patch, a hardened area, or a thickened spot inside the mouth. The most telling difference is persistence: oral cancer sores simply don’t heal. Other warning signs include unexplained mouth bleeding, loose teeth, lumps on the lips or tongue, or changes in taste or the texture of your mouth lining.
When a Sore Needs Medical Attention
Two clear thresholds should prompt a visit to your doctor or dentist. First, any canker sore lasting longer than two weeks. Normal canker sores resolve well before that mark, so a two-week sore suggests either infection, an underlying condition, or something other than a canker sore entirely. Second, any sore larger than about one centimeter (roughly the size of a pea). Sores that large are classified as major canker sores and are more likely to need treatment.
Beyond those benchmarks, the infection signs listed above all warrant a call to your provider, especially pus drainage, spreading redness, or fever. A localized mouth infection that goes untreated can, in rare cases, progress to cellulitis, a deeper soft-tissue infection. Cellulitis carries its own set of risks, including abscess formation and, very rarely, the infection entering the bloodstream. These serious complications are uncommon from a simple canker sore, but they illustrate why a worsening sore shouldn’t be ignored.
What Treatment Looks Like
Minor canker sores that are healing normally need no treatment. Over-the-counter topical gels or rinses can help manage pain while you wait for the sore to close. Warm salt water rinses, done several times a day, are a simple and effective way to keep the area clean.
If your sore does show signs of infection, a healthcare provider will typically examine it and may prescribe a medicated mouth rinse or a topical treatment to address the bacterial component. For sores that are large, unusually painful, or persistent, prescription-strength options exist that go beyond what you can buy at the pharmacy. The goal is to clear the infection and allow the tissue to resume normal healing, which it usually does quickly once the bacterial load is under control.