Swollen Gum with Pus: Telling a Periodontal Abscess from a Periapical One

A swelling on the gum that discharges pus looks much the same whatever caused it. Underneath, however, there are two substantially different conditions, arising in different tissues, and requiring different treatment. Getting the distinction right is the whole of the diagnostic work in this situation, and getting it wrong leads to treatment that does not resolve the problem.

This article explains what each one is, how they are told apart, and what follows from the answer.

A periapical abscess begins inside the tooth. Bacteria reach the pulp — usually through decay, sometimes through a crack or a failing restoration — and the pulp becomes infected and dies. Infection then exits through the tiny opening at the tip of the root and establishes in the bone around it. From there it may track through bone and break out through the gum, producing a swelling.

A periodontal abscess begins outside the tooth, in the tissues supporting it. An existing gum pocket becomes blocked — by debris, by a piece of food, by calculus obstructing the opening, or after vigorous instrumentation — and the bacteria already present multiply in a space that can no longer drain. Pus accumulates within the pocket wall.

Both produce a swollen, tender gum with pus. The tooth involved may be sore to bite on in either case. Yet one originates in a dead nerve and the other in a living tooth with a diseased attachment.

Several pieces of evidence are combined.

Pulp vitality testing. This is the single most decisive test. A tooth with a periapical abscess has a non-vital pulp and will not respond to cold or electric pulp testing. A tooth with a periodontal abscess usually retains a vital pulp and responds normally. Testing neighbouring teeth for comparison is part of the procedure.

Periodontal probing. A periodontal abscess is associated with a deep pocket on the affected side, often with pus expressible from the pocket opening. A periapical abscess typically shows normal probing depths, unless a narrow tracking defect is present.

Position of the swelling. Periodontal abscesses tend to appear alongside the root, closer to the gum margin. Periapical swellings usually appear further up, level with the root tip.

Radiographs. A periapical abscess often shows a dark area at the root tip, though early lesions may not yet be visible. A periodontal abscess typically shows bone loss along the side of the root.

History. Periapical abscesses often follow a period of severe toothache that then stopped — the pain ceasing when the pulp died. Periodontal abscesses often arise in a mouth with known gum disease, or shortly after something became lodged in a pocket.

The tracking test. Where there is a small opening discharging pus — often called a gum boil, properly a sinus tract — a fine gutta-percha point can be inserted and a radiograph taken. The point follows the tract back to its source, which may be several teeth away from where the swelling appears.

That small pimple on the gum, often causing little or no discomfort, that appears, discharges, disappears and comes back deserves separate explanation, because patients routinely dismiss it.

It is the surface opening of a channel running from an area of infection to the mouth. Its existence means the infection has found a route to drain. Drainage relieves pressure, and pressure is what causes pain — which is why a tooth with a discharging sinus is often entirely comfortable.

This is the trap. The absence of pain is a consequence of the drainage, not of resolution. Meanwhile the infection at the source continues, and bone destruction continues with it, often silently over months or years. Our article on a bad taste from a ruptured abscess covers this, and our article on apical periodontitis and infection spreading into the jawbone covers what is happening at the root tip.

A gum boil that causes no discomfort is not a minor thing. It is a chronic infection with an outlet.

A periapical abscess is treated by addressing the infected pulp space. That means root canal treatment — cleaning and disinfecting the canal system and sealing it — or extraction of the tooth. Draining the swelling relieves the pressure and is often done at the same visit, but drainage alone does not treat the source.

A periodontal abscess is treated by draining and debriding the pocket. That involves removing the deposits and debris from the root surface within the pocket, allowing it to drain, and then managing the underlying periodontal disease. Root canal treatment would achieve nothing, because the pulp is healthy.

There is a third possibility that complicates matters: a combined lesion, where infection from the root tip and infection from the pocket have joined. These require treatment of both components, usually starting with the endodontic element, and the prognosis is more guarded.

This is why the answer to "can I just have antibiotics?" is usually no. Antibiotics do not reach into a necrotic pulp chamber, which has no blood supply, and they do not remove the calculus and biofilm on a root surface within a pocket. They may reduce swelling and make a patient feel better while the cause continues undisturbed. Our article on whether antibiotics can replace root canal treatment addresses this directly.

Pus is the residue of the immune response — neutrophils that have migrated to the site, together with tissue debris and bacteria. Its formation indicates a substantial local defensive effort.

It also creates its own problem. Accumulating within a confined space, it generates pressure, and pressure both causes pain and pushes the process outwards into whatever tissue plane offers least resistance. This is why an abscess expands rather than staying put, and why relieving pressure is often the fastest route to comfort.

Our article on dental abscess symptoms covers the general picture, and our article on telling a toothache from an abscess covers the earlier stages.

Most gum abscesses are urgent in the sense of needing prompt dental attention rather than emergency care. The features that change that are:

• Facial swelling that is spreading, particularly beneath the jaw or towards the neck

• Difficulty swallowing, difficulty breathing, or a change in voice

• Swelling in the floor of the mouth

• Inability to open the mouth fully

• Fever, rigors, or feeling severely unwell

• Swelling involving the area around the eye with any visual change

Any of these means an emergency department rather than a dental appointment. Our article on fever with facial swelling and our article on whether a tooth abscess can be life-threatening cover why these thresholds exist.

Rinse gently with warm salty water, several times daily. Take over-the-counter analgesia according to the packet instructions. Keep cleaning the area, gently — plaque accumulation makes a periodontal abscess worse. Avoid chewing on that side. Avoid applying heat to the face. Do not attempt to squeeze or open the swelling, which risks pushing infection into surrounding tissue.

If the swelling bursts on its own and you feel much better, still attend the appointment. That is drainage, not resolution.

The two origins call for different prevention.

Periapical abscesses follow untreated decay, failed restorations and cracks. Regular examination with radiographs identifies these before the pulp is involved, and toothache that resolves on its own still warrants assessment because the resolution may mean the nerve has died.

Periodontal abscesses arise from existing pockets. Reducing pocket depth through periodontal treatment and maintaining plaque control at home is what reduces the risk. Our article on how long it takes to treat gum disease covers the process, and our article on how early detection of gum disease reduces long-term costs makes the wider case.

• A periapical abscess originates in a dead pulp; a periodontal abscess originates in a gum pocket around a vital tooth.

• Pulp vitality testing is the most decisive way of telling them apart, alongside probing depths and radiographs.

• A gum boil is a drainage channel from a chronic infection; the absence of pain reflects drainage, not healing.

• Periapical abscesses need root canal treatment or extraction; periodontal abscesses need pocket debridement.

• Antibiotics alone reach neither a necrotic pulp chamber nor biofilm on a root surface.

• Spreading swelling, fever, restricted opening or difficulty swallowing require emergency care.

If you have a swollen gum discharging pus, an examination with vitality testing, probing and radiographs will establish which type of abscess it is. You can contact our team or speak to our emergency dentist service. Treatment may involve gum disease treatment or root canal treatment depending on the diagnosis.

This article is provided for general information only and does not constitute dental or medical advice. A dental abscess requires professional assessment, and the diagnosis can only be established through clinical examination, vitality testing and radiographs. If you develop spreading swelling, fever, restricted mouth opening, or difficulty swallowing or breathing, seek emergency care immediately.

Next review due: 9 September 2027