From Tooth Decay to Root Canal Treatment: What Happens at Each Stage

How decay starts

Bacteria on the teeth turn sugar into acid, which draws minerals out of the enamel; saliva and fluoride help put them back. UK prevention guidance explains that in the early stages, decay can be reversed¹. When mineral loss wins for long enough, decay spreads into the dentine, the softer layer under the enamel, until the surface breaks down and a cavity forms.

How often and how much sugar you have matters. A 2022 systematic review found moderate-quality evidence that decay is lower when sugars make up less than 10% of daily energy², and low-quality evidence for a stricter limit of 5% for free sugars.

Early decay may not cause any symptoms at first³. Once a hole forms, it can cause toothache, a sharp pain with hot, cold or sweet things, and white, brown or black spots on the tooth.

The stages, and what each one usually needs

1. Early decay, before there is a hole. Fluoride toothpaste helps, and a dentist can prescribe or apply stronger fluoride treatments³, such as a varnish or a mouthwash, to help reverse early decay. A review of 44 trials ranked the options that work without drilling by type of lesion. For early decay in the grooves of back teeth, for example, sealants combined with fluoride varnish⁴ were among the most effective, with low to moderate certainty.

2. A cavity. Once there is a hole, you will probably need a filling³.

3. A deep cavity close to the nerve. For a deep cavity in a tooth with no symptoms, or only brief pain when something triggers it, European guidance suggests removing the decay selectively⁵, leaving some softened dentine over the nerve, or in two stages, rather than drilling until only hard dentine is left. The aim is to avoid exposing the nerve; the recommendation is conditional and the evidence very low in certainty. The same guidance says that in extremely deep decay, exposing the nerve should be anticipated. So the need for nerve treatment sometimes becomes clear only during the filling.

4. Decay has reached the pulp. If decay reaches the pulp, you may need root canal treatment³, and sometimes the tooth has to be removed. In root canal treatment the dentist removes the inflamed or infected pulp, cleans the canals inside the roots and seals them, then restores the tooth with a filling or a crown.

There is a third option for some teeth. When the nerve is exposed while decay is being removed, and the tooth had no pain or only pain when triggered, removing only part of the pulp (a pulpotomy) seems to be as effective as root canal treatment⁵ in the research so far. That evidence is of low certainty; for pain after treatment it rests on one trial of 54 people, and the guidance itself assumes that not every dentist does this in adult teeth.

Saving the tooth or taking it out

Root canal treatment keeps your own tooth. In pooled studies, about 86 to 93 in 100 root-treated teeth were still in place⁶ 2 to 10 years later; most of those studies looked back at patient records. Still in place does not mean trouble-free. Complete healing seen on an X-ray is a different, stricter measure: a review of 63 studies published between 1922 and 2002 found pooled success rates of 68% to 85%⁷ on the strictest criterion. The two sets of figures come from different studies and cannot be compared directly. After treatment, the NHS says the area may feel swollen and sore⁸, but it should get better in a couple of weeks.

Is an implant a better choice? A review of 143 studies found that the long-term survival of root-treated teeth and of single implant crowns was similar⁹, although the two were rarely compared directly. The same review found limited data suggesting that removing a tooth without replacing it had worse effects on people's well-being.

After root canal treatment, teeth that received a crown survived longer in observational studies. The one trial in a Cochrane review that compared a crown with a filling could not show a difference¹⁰ in failures after 3 years. It was small (117 people), included only premolars restored with a post and had a high risk of bias, so it cannot show that the two do equally well. Your dentist decides with you, based on how much of the tooth is left.

What you can do at home

• Brush twice a day with fluoride toothpaste. UK guidance advises adults to brush last thing at night and at one other time, with a toothpaste of 1,350 to 1,500 ppm fluoride¹¹, and to spit out rather than rinse, so the fluoride is not washed away. Young children use less: only a smear of toothpaste under 3 years and a pea-sized amount from 3 to 6.
• Know what toothpaste does. In adults, fluoride toothpaste reduced new decay compared with toothpaste without fluoride¹², with moderate certainty. That is prevention; the review did not test whether toothpaste reverses decay that has started.
• Have sugar less often. Keep sugary food and drinks to fewer, smaller occasions, and avoid them at bedtime. The same UK guidance recommends this.
• Ask about stronger fluoride if you keep getting decay. For adults with active decay, a dentist may consider prescribing a higher-fluoride toothpaste¹¹ until the decay is under control.

Home remedies: an honest verdict. Xylitol is not established as a treatment for decay: the main evidence¹³ is a fluoride toothpaste with added xylitol in children, from two studies by the same authors, with low certainty, and the evidence in adults is inconclusive. Trials of propolis reported less plaque and fewer bacteria, but that evidence is very uncertain, and decay itself was rarely measured¹⁴. Home "silver" products are not the same as silver diamine fluoride, a professional treatment that a dentist applies to decay.

UK guidance found no evidence that any particular check-up interval¹ changes how decay develops. Ask your dentist how often you should be seen.

When to see a dentist

Book an appointment if you notice a hole or a dark or chalky spot on a tooth, food catching in one place, or pain with hot, cold or sweet things.

See a dentist if toothache lasts more than 2 days or does not go away with painkillers¹⁵. Ask for an urgent appointment if your face, cheek or jaw is swollen or you have a high temperature.

While you wait, painkillers such as ibuprofen or paracetamol can help, if they are safe for you. Follow the pack and do not go over the maximum daily dose. Swallow the tablets¹⁶: holding one against the tooth or gum can damage the tissue. This self-care advice covers the first 24 hours. If you still need pain relief after that, ask a pharmacist, and keep trying to get a dental appointment.

Emergencies. Call 112 or go to the nearest emergency department if you have swelling that makes it hard to breathe, speak or swallow, a swollen or painful eye or sudden problems with your eyesight, swelling in your neck, a lot of swelling inside your mouth¹⁷, or if you find it hard to open your mouth.

Frequently asked questions