A normal canker sore is a small, round ulcer with a white or yellow center and a clean red border, found inside the mouth. An infected canker sore looks noticeably different: the redness spreads beyond the original border, the surrounding tissue swells, and you may see pus draining from or around the sore. The sore itself often appears larger or more irregular than it did when it first formed, and the pain intensifies rather than gradually fading over time.
What a Normal Canker Sore Looks Like
Understanding the normal version helps you spot when something has gone wrong. A typical canker sore is a single, round or oval ulcer inside the mouth, on the inner cheeks, tongue, soft palate, or floor of the mouth. It has a white or yellow center with a well-defined red border. It hurts, sometimes intensely, but the pain peaks in the first few days and then slowly improves. Most canker sores heal within two weeks without any treatment.
The key feature of a healthy canker sore is that it follows a predictable arc: it appears, it hurts, it gradually shrinks, and it disappears. Infection disrupts that arc.
Visual Signs of Infection
When a canker sore becomes secondarily infected with bacteria, the appearance changes in several distinct ways:
- Spreading redness. The red border that normally surrounds the sore expands outward into the surrounding tissue. Instead of a neat ring, you see a widening zone of inflamed, angry-looking skin or mucosa.
- Increased swelling. The tissue around the sore puffs up noticeably. You might feel a firm or boggy area when you press your tongue against it.
- Pus or discharge. A normal canker sore doesn’t produce pus. If you see a yellowish-green or whitish discharge draining from the sore, or the center looks thicker and more opaque than the typical white-yellow film, that points to bacterial involvement.
- Growing size. Instead of shrinking after the first week, an infected sore stays the same size or gets larger. The edges may become ragged or irregular rather than maintaining the smooth, round shape of a standard canker sore.
- Warmth. The area around the sore may feel warmer than the surrounding tissue when you touch it with your tongue or finger.
Pain is also a useful signal. Normal canker sore pain improves day by day. Infected canker sore pain escalates, often becoming throbbing rather than the steady sting you started with.
Symptoms Beyond the Sore Itself
An infection doesn’t always stay local. When bacteria from a mouth ulcer trigger a broader immune response, you may develop a fever, sometimes reaching 104°F (40°C) in severe oral infections. Swollen lymph nodes under your jaw or along the side of your neck are another sign that your body is fighting something beyond a simple sore. General fatigue and feeling unwell can accompany these symptoms.
If you notice difficulty swallowing, difficulty opening your mouth fully, or swelling that extends into the jaw or neck, that suggests the infection is spreading into deeper tissue. Oral infections can progress through stages, starting as localized inflammation, then developing into cellulitis (a spreading soft-tissue infection), and potentially forming an abscess. While this progression is more commonly associated with tooth infections than canker sores, any open wound in the mouth provides a pathway for bacteria into surrounding structures.
Canker Sore vs. Cold Sore
Before assuming your sore is an infected canker sore, make sure it’s actually a canker sore. Cold sores (fever blisters) look different and behave differently, and confusing the two can lead you in the wrong direction.
Canker sores appear inside the mouth and present as a single round ulcer with a white or yellow center. Cold sores appear outside the mouth, typically around the border of the lips, and look like a cluster of small, fluid-filled blisters grouped together. Cold sores are caused by the herpes simplex virus and are contagious. Canker sores are not viral and not contagious. If what you’re seeing is a cluster of tiny blisters on or near your lip, that’s likely a cold sore rather than an infected canker sore.
When Healing Takes Too Long
The two-week mark is the line that matters. A standard canker sore heals within two weeks. If yours persists past that point, something is interfering with normal healing, whether that’s a secondary infection, an underlying immune issue, or a condition that mimics canker sores but isn’t one. A sore lasting longer than two weeks needs professional evaluation, particularly if you have a weakened immune system, have recently started a new medication, or notice other symptoms like skin rashes or drooling.
Recurrent canker sores that keep coming back in waves are a separate issue from infection, but frequent recurrence can increase the odds of secondary infection simply because there’s an open wound in your mouth more often.
How Infected Canker Sores Are Treated
Most canker sores never become infected and don’t need treatment beyond basic pain management. When infection does set in, the approach depends on severity.
For mild cases, your dentist or doctor may recommend an antimicrobial mouth rinse to reduce the bacterial load. Prescription-strength rinses containing certain antibiotics have been shown to increase pain-free days by about 40% compared to rinsing with a placebo. These rinses help control bacteria at the surface while your immune system handles the rest.
For more significant infections with spreading redness, pus, or systemic symptoms like fever, oral antibiotics may be necessary. The goal is to stop the infection from progressing into deeper tissue. In rare cases where an abscess forms, drainage may be needed.
At home, keeping the area clean matters. Rinsing gently with warm salt water several times a day helps remove debris and bacteria. Avoid spicy, acidic, or rough-textured foods that irritate the sore and slow healing. Don’t poke at or try to drain the sore yourself.
What Raises Infection Risk
Not every canker sore gets infected. Certain factors make it more likely. Poor oral hygiene gives bacteria easier access to the open wound. A weakened immune system, whether from illness, medication, or chronic stress, reduces your body’s ability to keep bacteria in check. Repeatedly irritating the sore by biting the area, eating rough foods, or brushing aggressively over it can break down the healing tissue and create an entry point for infection. Tobacco use also slows oral wound healing and changes the bacterial balance in your mouth.
Large canker sores (sometimes called major aphthous ulcers) carry higher infection risk simply because the wound is bigger and takes longer to close. These sores can exceed a centimeter in diameter and sometimes take six weeks or more to heal, giving bacteria a much wider window of opportunity.