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Root Canal Infections: Symptoms, Causes and Prevention

A root canal infection starts inside the tooth, where you cannot see it. This guide explains how it happens, how to recognise it and how root canal treatment deals with it.

Key points

  • A root canal infection begins in the pulp inside the tooth, where you cannot see it, and it can follow deep decay, a crack, a failed filling or injury.
  • Typical signs are throbbing tooth pain that is worse at night or on biting, lingering sensitivity to hot or cold, darkening, swelling or a bump on the gum, though sometimes there are no symptoms.1
  • Root canal treatment removes the infected pulp, cleans and shapes the inside and fills and seals it.1
  • If an infected tooth is not treated, the NHS says it may need to be extracted.1
  • Treating cavities early, replacing leaking fillings and visiting the dentist regularly help to prevent infections.

Beneath the enamel and dentine of a tooth lies the pulp, a soft tissue with nerves and blood vessels, which runs through narrow canals in the roots. When bacteria reach the pulp, it becomes inflamed and then infected. That is a root canal infection, and it is one of the commonest reasons for toothache.

How the pulp becomes infected

  • Deep decay gradually reaches the pulp.
  • A crack or fracture lets bacteria in.
  • A deep or leaking filling that has failed.
  • Injury, even if the tooth does not look damaged.
  • Repeated dental work on the same tooth.

Root canal treatment is advised when the pulp of a tooth is infected or damaged, and the dentist decides after an examination and X-rays whether the tooth can be saved.1

Symptoms

  • Tooth pain caused by an infected pulp,1 often throbbing, worse at night or on biting.
  • Lingering sensitivity to hot or cold, long after the trigger has gone.
  • Deep decay that has reached the pulp.1
  • A chipped, broken or cracked tooth that has let infection in.1
  • A dental abscess, which is a collection of pus at the tooth root.1
  • A tooth that has turned darker.
  • A bump on the gum, a bad taste or swelling.

Sometimes there are no symptoms at all, and the infection is found on an X-ray.

Why it should be treated

If an infected tooth is not treated with root canal treatment, the NHS says it may need to be extracted.1 A dental abscess needs urgent dental treatment and will not clear up on its own.6 Swelling that spreads to the eye, face or neck, or affects breathing or swallowing, needs a hospital emergency department. See our guide to a tooth abscess.

What root canal treatment does

Root canal treatment removes the infected pulp, cleans and shapes the inside of the tooth and fills and seals it so that it does not become infected again.1 The NHS says treatment usually takes two or more appointments of one to two hours each, with a temporary filling between visits.1 In a review of 10 trials, the healing rate of single-visit and multiple-visit treatment was similar, and short-term pain after filling was less common after a single visit.5

A local anaesthetic is given, so you are awake but should not feel pain.1 Extra anaesthetic is often needed for an inflamed tooth, so tell the dentist if you feel anything.3 Our page on root canal treatment and our guide to the procedure step by step explain more.

How well it works

Root canal treatment is more successful than many people expect, although the figures depend on how success is measured.4 In a review of 14 studies, between 86 and 93 percent of treated teeth were still in place after 2 to 10 years, and survival was higher for teeth with a crown, with contact on both sides, that were not bridge or denture supports and that were not molars.2 The authors of another review rated the quality of the evidence as sub-optimal.4

Pain

Patients usually have pain before treatment, and in a systematic review the share with pain fell from 81 percent before treatment to 40 percent after a day and 11 percent after a week.3 The area around the tooth may feel swollen and sore afterwards, and it should get better in a couple of weeks.1 Take paracetamol or ibuprofen as directed if you have pain.1

After treatment

  • Your jaw and face may stay numb for several hours, so be careful not to bite your cheek or tongue.1
  • Avoid knocking or biting hard on the tooth, and keep your follow-up appointments, including the one to restore it.1
  • After root canal treatment the tooth is usually restored with a filling or a crown, and teeth with a crown after the treatment survived longest in a review.12 See dental crowns.
  • Look after your teeth and gums and keep your regular check-ups.1

Alternatives

If a tooth is too badly broken or too weakened to be restored, it may need to be taken out.1 Our guide to root canal, extraction or implant explains how that choice is made.

Preventing root canal infections

  1. Treat cavities early, before they reach the pulp.
  2. Brush twice a day with fluoride toothpaste, and clean between your teeth.
  3. Replace broken or leaking fillings.
  4. Protect your teeth during sport with a mouthguard.
  5. Repair cracked teeth promptly.
  6. Visit your dentist regularly.

Recognising an infection that is spreading

An infected tooth can develop into a dental abscess, a collection of pus at the root. The NHS lists signs such as severe, throbbing toothache, swelling in the face or cheek, a high temperature, a bitter taste and difficulty opening the mouth.6 See a dentist the same day if you have these.

Go to a hospital emergency department, or call 112, India's emergency number, if swelling spreads to the eye, jaw or neck, or if you have trouble breathing or swallowing.6 These are signs that the infection is spreading beyond the tooth and need urgent medical care, so do not wait for a routine appointment.

Pain between visits

Root canal treatment may take more than one visit, and some soreness between visits is common. The tooth can feel tender when you bite, and the gum may be sore from the clamp and instruments. A painkiller taken as the label directs usually helps. Avoid chewing on that tooth, because a temporary filling is not as strong as the final one.

Contact the clinic if pain gets worse, if swelling appears, if the temporary filling falls out or if you have a fever. Do not wait until the next appointment, because a tooth left open between visits can become reinfected.

Questions to ask before treatment

  • Is my tooth suitable for root canal treatment, and what are the chances of saving it?
  • How many visits will I need, and how long will each take?
  • Will I need a crown afterwards, and when?
  • What happens if the infection does not settle?
  • What is the alternative, and what would it involve?
  • What should I do if I have pain or swelling between visits?

A clear explanation of the plan, the alternatives and the follow-up is a good sign, and a second opinion is reasonable if you are unsure.

Myths and facts

Myth: root canal treatment is painful. A local anaesthetic is given, and pain before treatment is usually worse than after.13

Myth: it is better to pull the tooth. If a tooth can be saved and restored, keeping it is usually a good aim.

Myth: root canal treatment causes illness elsewhere. There is no good evidence for that, and an untreated infection is the greater concern.

What your dentist looks for

At the examination, the dentist asks about your pain, taps the tooth, checks how it responds to cold and looks at the gum for swelling or a bump. An X-ray shows the root and the bone around it, and may show a dark area at the root tip where infection has damaged the bone. These findings, together with your symptoms, show whether the pulp is inflamed, infected or dead, and whether the tooth can be saved.1

A tooth that has darkened

A tooth that turns grey or dark may have a dead pulp, often after a knock. It may cause no pain, but it can still hold infection. Tell your dentist if a tooth changes colour, because it can be treated, and the colour can often be improved afterwards.

Frequently asked questions

Can a root canal infection go away on its own?

No. An infected pulp does not heal on its own, and an abscess needs urgent treatment.6

How long does a root-treated tooth last?

In a review of 14 studies, between 86 and 93 percent of treated teeth were still in place after 2 to 10 years.2

Will the tooth need a crown?

Often, particularly a back tooth, because teeth with a crown afterwards survived longest.2

What if the pain continues after treatment?

Contact your dentist. Some tenderness is normal, but pain that gets worse needs checking.

Is a darkened tooth always infected?

Not always, but it is worth a check, because a dead pulp can hold infection without pain.

Can a root canal infection spread?

Yes, if untreated, and swelling that spreads to the face or neck needs urgent help.6

Can a root canal infection go away on its own?

No. The infection inside the tooth cannot clear by itself, and pain can disappear when the nerve dies while the infection remains. A dentist needs to treat the tooth.

Do I need antibiotics for a root canal infection?

Not always. Antibiotics do not treat the cause inside the tooth, so the dentist decides when they are needed, usually when there is spreading infection or a fever.

About the author

Portrait of Dr Anupam Sinha

Dr Anupam Sinha, BDS, MDS – Orthodontics. BDS (2002), KLE Vishwanath Katti Institute of Dental Sciences, Belgaum. MDS in Orthodontics (2006), SDM College of Dental Sciences, Dharwad. His thesis work won a national research award in 2005, and he holds a certification in aesthetic dentistry. He sees patients at Orion Dental Clinic in Greater Kailash 1, South Delhi. Read the full profile of Dr Anupam Sinha.

Sources

  1. NHS website. Root canal treatment (patient information). Page review date 3 October 2025. https://www.nhs.uk/conditions/root-canal-treatment/
  2. Ng YL, Mann V, Gulabivala K. Tooth survival following non-surgical root canal treatment: a systematic review of the literature. International Endodontic Journal, 2010;43(3):171-189. https://pubmed.ncbi.nlm.nih.gov/20158529/
  3. Pak JG, White SN. Pain prevalence and severity before, during, and after root canal treatment: a systematic review. Journal of Endodontics, 2011;37(4):429-438. https://pubmed.ncbi.nlm.nih.gov/21419285/
  4. Ng YL, Mann V, Rahbaran S, Lewsey J, Gulabivala K. Outcome of primary root canal treatment: systematic review of the literature, part 1. Effects of study characteristics on probability of success. International Endodontic Journal, 2007;40(12):921-939. https://pubmed.ncbi.nlm.nih.gov/17931389/
  5. Su Y, Wang C, Ye L. Healing rate and post-obturation pain of single- versus multiple-visit endodontic treatment for infected root canals: a systematic review. Journal of Endodontics, 2011;37(2):125-132. https://pubmed.ncbi.nlm.nih.gov/21238790/
  6. NHS website. Dental abscess (patient information). Page review date 18 March 2026. https://www.nhs.uk/conditions/dental-abscess/

Medically reviewed by Dr Anupam Sinha, BDS, MDS – Orthodontics, on . General information for patients from Orion Dental Clinic, last updated 23 October 2026. It is not advice for one person, so ask a dentist what suits you. To report an error, contact us.