What Does an Infected Root Canal Feel Like?
An infected root canal causes symptoms that often feel similar to the original toothache that led to treatment. The infection means bacteria have returned to the inside of the tooth or the area around the root tip.
A root canal procedure removes the pulp (the soft tissue containing nerves and blood vessels inside a tooth) and seals the internal canals. When this treatment succeeds, the tooth remains comfortable and functional for years. However, in some cases, bacteria survive inside the tooth or find a new path back in. When that happens, infection can develop again. Research indicates that initial root canal treatment has a high success rate, but approximately 2% to 14% of treated teeth may require further intervention due to persistent or recurrent infection. [4] [2]
The most recognizable symptoms of a reinfected root canal include persistent or returning pain in the treated tooth, swelling in the gum tissue near the root, and a small pimple-like bump on the gum called a fistula (also known as a sinus tract). A fistula is the body's way of draining the infection. You may also notice a bad or salty taste in your mouth, which comes from the draining fluid. Sensitivity to heat, pressure when biting, or a feeling that the tooth sits "higher" than neighboring teeth are also common signs.
These symptoms can appear weeks, months, or even years after the original root canal. Some reinfections develop slowly and cause mild, intermittent discomfort. Others produce sharp, constant pain. The timeline and severity depend on the cause of failure and how far the infection has spread.
- Returning or new pain in a tooth that previously had root canal treatment
- Swelling in the gum, cheek, or jaw near the treated tooth
- A pimple-like bump (fistula) on the gum that may drain fluid
- Bad taste or persistent unpleasant odor in the mouth
- Sensitivity to hot foods or drinks
- Pain or discomfort when biting or pressing on the tooth
- Darkening or discoloration of the treated tooth
Why Do Root Canals Fail?
Root canals fail when bacteria re-enter the tooth's internal canal system or persist in areas the original treatment did not reach. Several specific factors can lead to this outcome.
Incomplete Cleaning or Missed Canals
Teeth can have complex internal anatomy. Some teeth, especially molars, have extra canals that are narrow, curved, or hidden. If a canal is missed during the first procedure, bacteria remain inside the tooth. These bacteria can multiply and cause a new infection over time. Studies have identified missed canals as one of the leading causes of endodontic treatment failure. [2] [5]
Even when all canals are found, some branches are too small to clean mechanically. Tiny side canals, called lateral or accessory canals, may harbor bacteria that survive the procedure. Persistent bacteria such as Enterococcus faecalis have been commonly identified in teeth with failed root canals, as they can survive in the harsh environment of a cleaned canal. [5] This is one reason reinfection can occur despite careful treatment.
Broken Seal or Delayed Restoration
After the canals are cleaned and shaped, they are filled with a rubber-like material called gutta-percha and sealed. A crown or permanent filling is then placed on top. If the seal between the filling material and the tooth wall breaks down, bacteria from saliva can leak back into the canal system. This is called coronal leakage.
Delays in placing a permanent crown or filling after root canal treatment increase the risk of coronal leakage. A temporary filling is not designed to protect the tooth long-term. The longer a tooth goes without a permanent restoration, the greater the chance of recontamination. Research has shown that teeth restored with a full crown after root canal treatment have significantly higher long-term survival rates compared to teeth left with only a filling. [3] [6]
Cracks, Fractures, and Root Resorption
A crack in the root of a treated tooth creates a direct pathway for bacteria to reach the canal system. Cracks can develop from heavy biting forces, grinding (bruxism), or trauma. Root-canal-treated teeth lose their internal blood supply, which can make them more prone to fracture over time, though the extent of increased brittleness is debated in the literature. Placing a crown helps protect against fracture. [6]
Root resorption is a less common cause. This occurs when the body's own cells break down the root structure, creating openings that bacteria can exploit. Resorption can happen internally (inside the canal) or externally (on the outer root surface).
When to See a Dentist or Endodontist
You should contact a dental provider if you notice any returning symptoms in a tooth that has had root canal treatment. Early evaluation gives the tooth the best chance of being saved.
Most reinfected root canals develop gradually and are not life-threatening emergencies. A mild ache or a small gum bump that drains on its own can typically wait a few days for an appointment. However, certain signs indicate a more serious situation that warrants same-day or emergency care.
- Seek immediate care if you have facial swelling that is spreading to the eye, neck, or floor of the mouth
- Seek immediate care if you develop a fever above 101°F (38.3°C) along with dental pain or swelling
- Seek immediate care if you have difficulty swallowing, breathing, or opening your mouth
- Schedule a prompt appointment for returning pain that lasts more than a few days
- Schedule a prompt appointment for a gum pimple (fistula) that appears near a treated tooth
- Schedule a prompt appointment for new sensitivity to heat or pressure on a tooth that previously had a root canal
Why See an Endodontist Instead of a General Dentist?
An endodontist is a dentist who has completed an additional two to three years of advanced training focused entirely on the inside of the tooth. They treat root canal infections and failures daily. According to the American Association of Endodontists, endodontists perform an average of 25 root canal procedures per week, compared to fewer than two per week for a general dentist. [2]
Endodontists also use specialized tools such as dental operating microscopes and cone-beam computed tomography (CBCT, a type of 3D X-ray). These tools help them find missed canals, identify cracks, and plan retreatment with greater precision. If your treated tooth is showing signs of failure, a referral to the endodontics page can connect you with a specialist trained for exactly this situation.
How Is a Failed Root Canal Diagnosed?
Diagnosis involves a clinical exam, imaging, and specific tests that help locate the source of infection and determine whether the tooth can be saved.
Your endodontist will start by reviewing your symptoms and dental history. They will examine the tooth visually and check for swelling, a fistula, or discoloration. They will also tap on the tooth (percussion testing) and press on the surrounding gum tissue (palpation) to identify tenderness.
X-Rays and 3D Imaging
A periapical X-ray (a small film that captures the entire root of the tooth) is usually the first imaging step. It can reveal a dark area around the root tip, which indicates bone loss caused by infection. The X-ray also shows whether the original root canal filling extends to the full length of the root or whether there are gaps. [3]
In many cases, the endodontist will also take a CBCT scan. This 3D image provides a more detailed view of the tooth and surrounding bone from every angle. It is especially useful for finding missed canals, vertical root fractures, and resorption that may not be visible on a standard 2D X-ray. The American Association of Endodontists and the American Academy of Oral and Maxillofacial Radiology have published joint guidelines supporting the use of CBCT in complex endodontic cases. [2]
Vitality and Probing Tests
Pulp vitality testing uses cold or electric stimulation to determine whether a tooth has a living nerve. A previously root-canal-treated tooth should not respond to cold, since the nerve has been removed. If the tooth does respond, it may mean the nerve tissue was not fully removed during the first procedure.
Periodontal probing measures the depth of the gum pocket around the tooth. A deep, narrow pocket along one side of a root can indicate a crack. The endodontist combines all of these findings to determine the cause of failure and whether the tooth is a candidate for retreatment or surgery.
Treatment Options for an Infected Root Canal
The two main treatment options are retreatment (redoing the root canal from the top of the tooth) and apicoectomy (a surgical approach through the gum to remove the infected root tip). The best choice depends on the cause of failure.
Root Canal Retreatment
Retreatment is the most common approach. The endodontist removes the existing crown or filling, takes out the old root canal filling material, and re-cleans and reshapes the canals. They look for missed canals, remove any remaining infected tissue, and place new filling material before resealing the tooth. [2]
Retreatment is typically recommended when the original failure is related to missed canals, incomplete cleaning, or a broken seal. It is a nonsurgical procedure performed through the biting surface of the tooth, similar to the original root canal. The tooth will need a new crown or permanent restoration afterward.
Success rates for nonsurgical retreatment vary depending on the complexity of the case. A systematic review of the literature on secondary root canal treatment outcomes found a weighted pooled success rate of approximately 77%, with individual studies reporting rates ranging up to about 89% depending on case selection and follow-up length. [8] Teeth with significant bone loss, severe resorption, or extensive cracks may have a lower chance of long-term success.
Apicoectomy (Root-End Surgery)
An apicoectomy is a surgical procedure in which the endodontist makes a small incision in the gum, removes the last few millimeters of the root tip, and places a small filling to seal the end of the root. The surrounding infected tissue is also removed. [2]
This approach is typically recommended when retreatment is not practical. For example, if the tooth has a post and core (a metal or fiber post cemented into the canal to support a crown), removing it for retreatment could risk fracturing the root. Apicoectomy is also an option when the infection is localized at the very tip of the root and the rest of the root canal filling appears intact.
Modern apicoectomies are performed with dental operating microscopes and ultrasonic instruments, which allow for smaller incisions and more precise root-end preparations. Published data on modern microsurgical apicoectomies report success rates of approximately 90% to 95% when using contemporary techniques and biocompatible root-end filling materials such as mineral trioxide aggregate (MTA). [7] Healing typically involves mild swelling and discomfort for a few days.
When Extraction Is the Only Option
In some cases, neither retreatment nor apicoectomy can save the tooth. A vertical root fracture that extends below the bone level, severe root resorption, or extensive bone loss may make the tooth unsalvageable. When extraction is necessary, the tooth can typically be replaced with a dental implant, a fixed bridge, or a removable partial denture. [3]
Your endodontist will discuss all options with you and explain the reasoning behind their recommendation. Getting a clear diagnosis before committing to extraction is important, because a tooth that seems hopeless on a 2D X-ray may turn out to be treatable when viewed with a CBCT scan or under a microscope.
Cost of Treating a Failed Root Canal
Treatment costs depend on the tooth's location, the complexity of the case, and the type of procedure performed. Costs vary by location, provider, and case complexity.
Root canal retreatment for a front tooth (single canal) is generally less expensive than retreatment for a molar (three or four canals) because molars require more time and more complex instrumentation. As a general range, retreatment may cost between $700 and $1,500 or more for a molar. An apicoectomy typically falls in the range of $900 to $1,300 per root, though multi-rooted teeth cost more. These ranges are approximate and can vary significantly by geographic region and individual practice.
Diagnostic imaging adds to the total cost. A periapical X-ray is usually a minor expense, while a CBCT scan may cost between $150 and $400 depending on the area imaged. Many dental insurance plans cover a portion of endodontic retreatment and apicoectomy, though coverage levels vary widely.
Ask the endodontist's office for a treatment estimate before your appointment. Many offices can also submit a pre-authorization to your insurance company so you know your expected out-of-pocket cost in advance.
Find an Endodontist Near You
If a previously treated tooth is causing pain, swelling, or other symptoms, an endodontist can determine whether the root canal has failed and recommend the most appropriate next step. Endodontists have the advanced training, microscopes, and 3D imaging needed to identify the exact cause of failure and perform retreatment or surgery when needed. Visit the endodontics page to find an endodontist in your area and schedule an evaluation.
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