Swollen gum behind a molar is most often caused by trapped food or bacteria in an area that’s hard to clean. If the swelling is behind your very last molar, there’s a good chance a wisdom tooth is involved, either pushing through the gum or stuck partway. But even without a wisdom tooth in the picture, several other common problems can cause that same puffy, tender spot.
Pericoronitis: The Most Common Culprit
If the swelling is right behind your last molar (or on top of a partially visible wisdom tooth), the likely cause is pericoronitis. This happens when a flap of gum tissue, called an operculum, partially covers a tooth that hasn’t fully come in. A small pocket forms between the gum flap and the tooth surface, and it’s nearly impossible to keep clean with a toothbrush. Food gets wedged in, bacteria thrive in the warm, low-oxygen space, and the surrounding tissue becomes inflamed and infected.
Pericoronitis is especially common between ages 17 and 25, when wisdom teeth are trying to erupt. But it can also happen around a second molar if that tooth came in at an odd angle or never fully broke through. The telltale signs include a swollen, reddish flap of gum that’s painful to touch, a bad taste in your mouth, and sometimes pain that radiates toward your ear or jaw. In mild cases, the swelling comes and goes. In more stubborn cases, it keeps returning every few weeks.
Food Impaction Between or Behind Molars
Food getting packed tightly between teeth or behind the last molar is one of the most overlooked causes of localized gum swelling. Unlike a small piece of spinach stuck in your teeth, true food impaction means debris is being forcefully wedged into the gum line during chewing. This is more likely if you have a gap between your back teeth, a tilted or rotated molar, a worn-down chewing surface, or a filling or crown that doesn’t quite match the shape of the original tooth.
When food sits against the gum tissue repeatedly, it irritates and inflames the area. Over time, this can progress from simple redness and puffiness to deeper gum problems, including the early stages of gum disease. If you notice the swelling tends to flare up after meals, especially after eating fibrous or crunchy foods, food impaction is a strong possibility.
Gum Disease Localized to One Spot
Gum disease doesn’t always affect your whole mouth evenly. It can show up in one isolated pocket, particularly behind molars where brushing and flossing are most difficult. Plaque builds up along the gum line, bacteria colonize the space between the tooth and gum, and the body’s immune response causes swelling, redness, and sometimes bleeding.
In its earliest stage (gingivitis), the inflammation is reversible with better cleaning. Gingivitis affects up to 90% of people at some point. But when plaque extends deeper and the gum starts to pull away from the tooth root, a periodontal pocket forms. Pockets deeper than 3 millimeters indicate gum disease, and pockets deeper than 5 millimeters are difficult to manage with brushing alone. Behind the last molar, these pockets can develop without any obvious symptoms other than occasional swelling or tenderness, because the area is so far back that you rarely see or feel it directly.
A Dental Abscess
If the swelling is intense, throbbing, and getting worse rather than better, an abscess may be forming. There are two types relevant here. A periapical abscess starts at the tip of the tooth root, usually from an untreated cavity or a cracked tooth that allowed bacteria inside. A periodontal abscess forms in the gum tissue alongside the root, often when a deep gum pocket gets sealed off and bacteria multiply inside the trapped space.
Both types can cause significant swelling behind a molar, along with sharp or pulsing pain, sensitivity to heat, and sometimes a small pimple-like bump on the gum that oozes a salty or foul-tasting fluid. An abscess won’t resolve on its own. The infection needs to be drained, and the underlying cause (decay, a cracked tooth, or a deep gum pocket) needs to be addressed.
What You Can Do at Home
Warm salt water rinses are the most effective immediate step. Dissolve about half a teaspoon of table salt in eight ounces of warm water and swish gently for 30 seconds, focusing on the swollen area. Repeat this three to four times a day. The salt water helps draw fluid out of the inflamed tissue and creates an environment that’s less friendly to bacteria.
Keep the area as clean as possible, even though it hurts. Use a soft-bristled toothbrush angled toward the gum line behind the molar. If you can, gently irrigate the space with a water flosser on a low setting to flush out trapped food. Over-the-counter pain relievers like ibuprofen can help reduce both pain and inflammation. Avoid chewing on that side, and stay away from very hot, spicy, or crunchy foods that could irritate the tissue further.
These measures can calm a mild flare-up, but they won’t fix the underlying problem if there’s an impacted wisdom tooth, a deep pocket, or an abscess involved.
What Happens at the Dentist
Your dentist will examine the area and likely take an X-ray. For the back of the mouth, a small periapical X-ray (the kind where you bite down on a film) is generally better at identifying bone loss and detailed defects than a panoramic image, though a panoramic view is useful for seeing the position of wisdom teeth and the overall layout of the jaw.
If pericoronitis is the diagnosis and your wisdom tooth is partially impacted or poorly positioned, extraction is usually recommended to prevent repeated infections. In some cases, the dentist may remove just the gum flap covering the tooth (a procedure called an operculectomy) if the tooth itself is healthy and has room to fully erupt. For gum disease, treatment focuses on deep cleaning to remove plaque and bacteria from below the gum line. An abscess will need drainage and possibly a short course of antibiotics, typically lasting 3 to 7 days depending on severity.
Signs the Swelling Needs Urgent Attention
Most gum swelling behind a molar is uncomfortable but not dangerous. However, infections in this area can occasionally spread into the deeper tissues of the jaw, throat, or neck. Watch for these warning signs: you can’t open your mouth as wide as normal (a condition called trismus, sometimes referred to as lockjaw), you have difficulty swallowing or breathing, swelling is spreading visibly into your cheek, under your jaw, or down your neck, or you develop a fever. Any of these signals that the infection has moved beyond the local gum tissue and needs prompt treatment.