An infected root canal typically shows visible signs both on the tooth itself and in the surrounding gum tissue. The tooth may darken to a gray or brown shade, the gums nearby can develop a small pus-filled bump, and in more advanced cases, noticeable swelling spreads into the cheek, jaw, or neck. Some of these signs are obvious in the mirror, while others only show up on an X-ray.
Changes to the Tooth’s Color
One of the earliest visual clues is the tooth changing color. When the nerve tissue inside a tooth dies from infection or trauma, pigment deposits within the tooth structure can turn it gray or brown. In some cases, a reactive process called calcification turns the tooth yellow instead. The discoloration usually looks different from a surface stain because it comes from inside the tooth, giving it a dull, washed-out appearance compared to the teeth around it.
This color change can happen whether the tooth has already had a root canal procedure or is infected for the first time. Either way, a single tooth that looks noticeably darker than its neighbors is a strong visual signal that something has gone wrong internally.
Gum Boils and Pus Drainage
The most distinctive visual sign of a root canal infection is a gum boil, clinically called a parulis. It looks like a small, pimple-like bump on the gum tissue, usually sitting right next to the infected tooth. The bump is raised, often whitish or yellowish at its center, and contains pus. It forms because the infection at the tooth’s root tip creates pressure that needs somewhere to go, and the pus eventually tunnels through the bone and gum tissue to reach the surface.
When a gum boil drains on its own, it releases a thick, yellow fluid with a foul taste. You might notice this taste suddenly appearing in your mouth even when you haven’t eaten anything. The bump may shrink after draining, then refill and swell again over days or weeks. This cycle of swelling and draining is a hallmark of a chronic infection that won’t resolve without treatment.
Swelling in the Face and Neck
When the infection is more acute, swelling can spread well beyond the gum line. Where it shows up depends on which tooth is infected. Upper teeth tend to cause swelling in the cheek or along the upper jaw. Lower molars are more concerning because the infection can extend into the spaces under the tongue, beneath the jaw, and down toward the neck.
Minor gum inflammation from plaque buildup looks quite different from an abscess. Gingivitis causes red, puffy gums that bleed easily but stay confined to the gum line and don’t produce lumps or facial swelling. An infected root canal, by contrast, makes the tooth extremely sensitive to touch or tapping, and the swelling tends to feel firm or tight rather than soft. You may also notice tender, swollen lymph nodes under your jaw or along your neck, which is the body’s immune response kicking in.
Facial swelling that makes it hard to breathe or swallow, or swelling accompanied by fever, signals that the infection has spread into deeper tissue. This is a medical emergency.
What It Looks Like on an X-Ray
Some infections are invisible to the naked eye and only appear on dental X-rays. The classic sign is a dark shadow at the tip of the tooth’s root, where the infection has dissolved the surrounding bone. Dentists look for this dark area as evidence that the bone around the root is breaking down.
The catch is that standard dental X-rays aren’t always sensitive enough to catch early infections. A periapical lesion only becomes visible on a traditional X-ray once 30 to 50 percent of the bone structure in that area has already been lost. That means a tooth can be actively infected for some time before the damage shows up on film. Three-dimensional imaging (CBCT scans) can detect bone loss earlier, which is why some dentists use them when they suspect an infection but the standard X-ray looks normal.
Signs You Can Feel but Not See
Not every infected root canal announces itself visually. Pain is often the first and most noticeable symptom, typically a severe, throbbing ache that can radiate into the jawbone, ear, or neck. The tooth may become sensitive to hot and cold temperatures, with lingering pain that continues even after you’ve stopped eating or drinking. Some people experience a constant dull ache, while others feel sharp pain only when biting down.
Fatigue and a general feeling of being unwell can accompany the infection, especially if fever develops. These systemic symptoms indicate the body is fighting a significant bacterial load, not just a localized irritation.
Previously Treated Teeth Can Get Reinfected
Root canal treatment has roughly an 89 percent success rate, which means a small but real percentage of treated teeth develop new infections later. A previously treated tooth that starts darkening, develops a gum boil, or becomes painful again is showing signs that bacteria have re-entered the canal system. This can happen if the original seal breaks down, if a crack develops in the tooth, or if bacteria were trapped inside during the initial procedure.
The visual signs of reinfection look the same as a first-time infection: discoloration, gum swelling, pus drainage, and dark shadows on X-rays around the root tips. The difference is that you’ll also see the white filling material from the original root canal procedure visible on the X-ray, which helps the dentist assess whether retreatment or a surgical approach is needed.