Pericoronitis: the swollen gum behind your last molar

If the gum behind your last lower molar is swollen, tender to touch and leaving a bad taste, and there is a wisdom tooth partly through underneath it, the usual answer is pericoronitis. The word means inflammation around a crown. What is inflamed is the hood of gum lying over the part of the tooth that has not finished erupting.

The pain sits further back than ordinary toothache — behind the last tooth rather than in it — and it often refers up towards the ear or along the jaw on the same side. Biting is uncomfortable because the upper wisdom tooth comes down onto swollen gum instead of onto a tooth. Swallowing can be sore on that side. The taste is the giveaway: stale, metallic or frankly unpleasant, and back within minutes of rinsing.

Most episodes are not that. They are localised to the gum around one tooth, they announce themselves over a day or two, and they are dealt with in the chair. But the distance between a localised flap and a spreading infection is short in the lower jaw, which is why those signs are set out first. Everything below assumes none of them applies to you. If one appears at any point — including overnight, and including after things had seemed to be settling — that list overrides everything else on this page.

A tooth that erupts normally pushes through the gum, and the gum then tightens around its neck in a cuff a toothbrush can clean. A third molar frequently stops halfway. There is often not enough room between the back of the second molar and the front edge of the ramus — the part of the jaw that rises towards the joint — so one or two cusps break through and the rest of the crown stays covered. The tissue left lying across the back of that tooth is the operculum.

Underneath it is a pocket. The space between the underside of the flap and the enamel is warm, dark, permanently moist, and open to the mouth only through a slit you cannot get a brush into. Saliva, food debris and bacteria go in. Very little comes out on its own. Bristles stop at the surface of the flap, floss passes in front of it, and mouthwash swilled around the mouth barely enters it. This is the whole mechanism, and it explains almost everything else about the condition.

The bacteria in that pocket are not the same population as the plaque on the front of your teeth. Low oxygen under a flap of tissue favours anaerobic organisms, and those are the ones that produce the odour and the taste patients describe. It is also why the taste returns so quickly after rinsing: the rinse clears the mouth, not the pocket.

One further piece of anatomy completes the cycle. The upper third molar often erupts further than the lower one, because there is more room in the upper jaw and the tooth has fewer obstacles. Once the lower operculum swells, that upper tooth bites into it with every chew. That contact swells it further, the swollen flap moves further into the bite, and it is caught again. This is why so many episodes involve the sensation of repeatedly biting the same spot, and why the flap is often marked with an indentation exactly where the upper cusp lands.

Taken together, this is why pericoronitis is overwhelmingly a lower jaw problem, and why it clusters in the late teens and twenties when third molars are erupting. Upper third molars can develop it, but less often: gravity drains the pocket, and the upper flap is less often in the line of the bite.

A typical first episode builds over 24 to 48 hours. It starts as an awareness of the area rather than pain — something feels rough or raised back there, and the tongue keeps finding it. Then it becomes tender, then sore to bite on, then sore at rest. Lying flat makes it throb, which is why the night is often when it stops being ignorable.

Alongside the pain there are usually two or three of the following: a bad taste that a rinse only briefly shifts, a sore throat confined to that side, mild stiffness on opening wide, a tender lymph node under the jaw on the same side, and mild discomfort on swallowing. In a mirror, with a phone torch, the flap looks red and puffy rather than pale pink, sometimes with a white line or dent across it from the opposing cusp. Pus sometimes appears at the edge of the flap on its own, and it tastes exactly as bad as it sounds. Do not press or squeeze the flap to find out whether it will.

Left alone, many localised episodes quieten over several days to a week. That is the single most misleading thing about the condition. It settles, the flap goes quiet, and the reasonable conclusion is that whatever it was has gone. The pocket, the flap and the tooth's position are all exactly as they were, which is why the pattern over months matters more than the severity of any one episode.

Not everything that hurts behind the last molar is pericoronitis. The table below is for orientation rather than for self-diagnosis: the distinction is made in the chair, and it does not need to be made by you.

Flare-ups rarely come out of nowhere, and the trigger is usually identifiable in hindsight. The tooth erupts a fraction further and opens the pocket wider. A hard or fragmented food — popcorn husks, seeds, crisps, granola — packs under the flap and stays there. The opposing upper tooth catches the flap once, and the swelling that follows makes the next catch more likely.

The other common pattern is a run of poor sleep, a heavy work or exam period, or a cold or flu in the week beforehand. The pocket is colonised all the time; what changes is how well the tissue is holding the line against it. This is not a reason to blame yourself for an episode, and it is not a condition caused by inadequate brushing. No amount of brushing reaches under an operculum. It is a reason to expect flare-ups at the times you can least accommodate them.

Smoking is worth naming separately. It impairs the small vessels supplying gum tissue and slows its recovery, and people who smoke tend to have more trouble with a partly erupted third molar than people who do not. Vaping applies heat and irritants to the same tissue and is not a workaround.

Self-care has a defined job here: reduce the bacterial load in the pocket, control the inflammation, and stop the upper tooth traumatising the flap while all that happens. It cannot change the position of the tooth, it cannot drain a collection of pus, and it cannot stop an infection that has already left the gum. It runs alongside being seen rather than instead of it.

The rinse is the part most often done badly. Use warm water, not hot — half a teaspoon of salt in a cup. Take a mouthful, tilt your head so it sits over the affected side, and hold it there for twenty or thirty seconds rather than swilling it around. Then let it fall out of your mouth. Repeat after every meal and before bed. Swilling for two seconds and spitting achieves almost nothing; the point is contact time against the flap.

Antibiotics are the thing patients most expect and the thing least likely to be the answer. Localised pericoronitis — normal jaw opening, no fever, swelling confined to the gum — is usually managed by cleaning under the flap rather than by a prescription. Antibiotics reach tissue through the bloodstream; they do not flush debris out of a pocket, so they leave the cause untouched and the episode tends to return once the course finishes. They are indicated when the infection has moved beyond the gum, which is a specific judgement rather than a default.

Give self-care about 48 hours to show a direction, with the emergency list above still standing over the whole of that time. If the pain, swelling and taste are all trending down, you are managing an episode that will probably settle, and the useful conversation is about what happens next rather than about tonight. If nothing has improved in two days — or if anything is worse at any point, particularly jaw opening — that is the point to be seen rather than to persist.

The lower third molar sits at a junction of tissue planes. Behind and below it are spaces that run under the jaw, up beside the muscles that close the mouth, and inwards towards the floor of the mouth and the throat. An infection that escapes the gum does not spread randomly; it follows those planes. Which one it takes determines what you notice, and one of them is why this condition is taken more seriously than its usual behaviour would suggest.

Jaw stiffness is the most useful thing you can track yourself. The muscles that close the jaw sit directly over and beside the area, and when inflammation reaches them they stop lengthening properly. Most adults can fit roughly three stacked fingers between the front teeth at full opening. Two fingers means the muscles are involved and you should be assessed the same day. One finger, or less, is an emergency department finding rather than a dental appointment finding — not because of the stiffness itself, but because of where the infection has to have reached to cause it. Treat this as a rough self-check rather than a measurement, and let a clinician confirm it.

Swallowing is the other one that matters. Soreness on swallowing food is common with an ordinary episode. Pain or difficulty swallowing your own saliva is a different statement about anatomy. So is drooling because swallowing hurts, a voice that sounds muffled or thick, swelling under both sides of the jaw, or a tongue that feels pushed up in the mouth. Those describe the floor of the mouth and the spaces beside the airway, and the assessment for them is in a hospital.

Pain is a poor guide to which row you are in. A localised episode can be genuinely severe, and an infection that has escaped into a tissue space sometimes hurts less as it spreads, because the pressure that caused the pain has found somewhere to go. Swelling, jaw opening, temperature and swallowing are the reliable signals. If you feel worse in yourself while the tooth itself feels easier, that is a reason to be seen, not a reason to relax.

Some people should move a step earlier than the rows above suggest. Anyone who is immunosuppressed, having chemotherapy, taking a bone-modifying medication, or living with poorly controlled diabetes has less margin, and the same infection in those circumstances moves faster and is managed sooner. Say so when you call, because it changes how quickly you are seen.

At the spreading end, treatment is drainage and antibiotics. Where the airway is a consideration that happens in hospital, with antibiotics given through a drip and the airway watched while the swelling is brought under control. The tooth responsible is usually removed, either during that admission or once the acute phase has settled.

Cleaning under the flap

The first and most useful thing done is irrigation. A blunt, curved syringe is passed under the operculum and the pocket is flushed with saline or an antiseptic solution until what comes back is clear. Debris that has been there for days comes out. Local anaesthetic is used where the area is too tender to tolerate it otherwise, and the site is examined afterwards for a collection of pus that needs releasing. The change in taste is often immediate; the change in swelling takes a day or two to follow.

Taking the opposing tooth out of the bite

If the upper third molar is traumatising the flap with every chew, treating the lower gum without addressing that leaves half the problem in place. Two options exist: adjusting the cusp that is doing the damage, or removing the upper third molar. Upper third molars are generally a more straightforward removal than lower ones, and taking one out breaks the cycle immediately. It is a genuine option in its own right rather than a fallback, and it is a reasonable thing to ask about.

Antibiotics, when they are indicated

Antibiotics are indicated where the infection has moved beyond the gum: facial swelling, fever, limited opening, a tender node, or a patient whose immune system is compromised by disease or medication. They are not the treatment for a localised flap, and prescribing them for one treats the wrong problem while leaving the pocket exactly as it was. If you have had two or three courses for the same tooth in a year, the courses are not the treatment plan — that pattern is itself the finding.

Drainage

Where pus has collected, it is released, either from under the flap or through a small incision if it has formed a pocket in the surrounding tissue. Relief tends to follow drainage rather than follow the antibiotic, and that is the correct order of expectations to hold.

A single episode in a tooth that is genuinely still erupting, in someone in their late teens, with space behind the second molar for the tooth to come into, is a reasonable thing to treat and then watch. Some of those teeth complete their eruption, the flap recedes, the pocket disappears, and the problem does not return. Whether removal is the right recommendation after one settled episode depends on whether the tooth has anywhere left to go, and that question is asked before the recommendation is made.

So the question after a first episode is not how bad it was. It is whether the anatomy that produced it is going to change. That is answered from the angle of the tooth, the space available in front of the ramus, how far the roots have formed, and your age — third molar roots are generally complete in the early twenties, and a tooth that has not erupted by then is unlikely to travel much further. A panoramic radiograph settles most of it in one image.

Recurrence is the usual reason because nothing about a resolved episode changes the conditions that produced it. The flap is still there, the pocket is still uncleanable, the opposing tooth still lands where it landed. Recurrent pericoronitis is one of the established reasons to remove a third molar; a single episode that has settled, on its own, usually is not. That distinction is worth holding onto, because the two situations are answers to different questions: whether this tooth can still erupt, and whether it has already shown that it will not.

Cutting the flap away — operculectomy — comes up in this conversation. The tissue is removed, the area heals, and the gum re-forms over a tooth that is still in the same position, so the pocket often returns within months. It can be reasonable where the tooth is well positioned, fully or nearly erupted, and the only remaining obstacle is a rim of gum. Where the tooth is impacted, it treats the symptom and leaves the cause.

The second molar is the other thing at stake and the one patients hear least about. A wisdom tooth tipped forward against it holds plaque against a surface no brush reaches, and decay that starts on the back of a second molar is difficult to access, difficult to restore well, and occasionally the second molar is lost. It is a healthy tooth being damaged by its neighbour, and it is one of the reasons a quiet impacted third molar is sometimes still worth removing.

The history matters more than it sounds like it should: how many episodes, how far apart, whether jaw opening was affected on any of them, whether antibiotics were taken and whether they helped, and whether any episode involved facial swelling or a hospital visit. Two mild episodes eighteen months apart, and two episodes six weeks apart that both restricted jaw opening, are different situations, and what should be done differs with them.

The examination measures jaw opening, looks at the flap and how much of the crown is exposed, checks the node under the jaw, examines the back surface of the second molar and the gum behind it, and identifies whether the opposing upper tooth is landing on the flap. A panoramic radiograph shows the angle, the depth, the state of the roots and the relationship to the nerve canal below. Where the roots appear to cross that canal, a cone-beam scan is taken specifically to establish whether they genuinely contact it, because that changes the consent conversation and sometimes the plan.

Removal during an acute episode is usually deferred until the acute phase is controlled, and it is worth knowing why rather than assuming you are being put off. Local anaesthetic works less predictably in acutely inflamed tissue, and a jaw that opens two fingers is not a jaw in which a lower third molar is straightforward to reach. Settle the episode, then operate on a quiet field. Where the infection is spreading, that logic reverses and the tooth comes out sooner.

What is used to keep you comfortable during a removal is agreed at the consultation, and it depends on the procedure, how many teeth are involved and your medical history. Local anaesthetic is part of every plan; the rest of that conversation happens with your medication list in front of both of you, and it can be revisited before the day.