The next ten minutes
Pick it up by the crown
The crown is the white part you normally see. Never touch the root, the pointed part that was in the gum. The root is covered in living cells, and those cells are the entire reason a replanted tooth can knit back in. Handling them destroys them.
Do not clean it
No scrubbing, no soap, no disinfectant, no toothbrush, no cloth. If it is visibly dirty, rinse it for a couple of seconds in milk or in saline. Never in tap water: water kills the root cells quickly.
Put it in milk, or in your cheek
Milk is the best thing most people have to hand, because it is close to the right saltiness and it keeps the root cells alive. Saliva is second best: hold the tooth inside your cheek, if you can do so without any risk of swallowing it. Saline works. A dry pocket or a tissue does not.
If you can, put it back in the socket
For an adult tooth, and only if you feel able: rinse briefly in milk, press it gently back into the socket the right way round, and bite softly on a clean cloth to hold it. The socket is the best storage medium there is. If you cannot, the milk is fine, and going straight to the clinic is what matters.
Call and set off
Say on the phone that it is a knocked-out tooth. That call moves everything else out of the way.
Why the hour matters
The root of a tooth is wrapped in a thin layer of living cells called the periodontal ligament. When a tooth is replanted, it is those surviving cells that re-attach it to the bone. They begin to die as soon as the tooth leaves the socket, and they die much faster in air and in tap water than in milk or saliva.
Replanted within the hour, a tooth often survives. After that the odds fall away sharply, and after several dry hours replanting is rarely worth attempting. This is why the storage instruction above is not fussiness: the difference between a tooth in milk and a tooth in a tissue is frequently the difference between keeping it and not.
Baby teeth are the exception
A knocked-out baby tooth is not put back. Replanting it risks damaging the permanent tooth developing in the bone above it. Still call, because the child should be checked for damage to the socket and to the teeth beside it, and because a lost baby tooth sometimes needs a space maintainer.
If you are not sure whether it is a baby tooth or an adult one, put it in milk and call. That covers both.
What happens at the clinic
The socket is cleaned and the tooth repositioned, then splinted to the teeth on either side so it is held still while the ligament re-attaches. An X-ray checks the position and looks for fractures in the socket that are not visible from outside, and your tetanus status is asked about if the injury happened outdoors.
A replanted tooth usually needs a root canal in the following week or two, because the nerve rarely survives the injury even when the tooth does. That is planned properly at a normal appointment, not done at three in the morning.
You will also be told honestly if the tooth cannot be saved. That is a real outcome, and knowing it on the night is better than a splint that fails in a month.
If the tooth is loose but still in
Do not wiggle it, and do not test it to see how loose it is. Leave it exactly where it is, eat nothing, and call. A loosened tooth that is stabilised quickly very often settles completely. One that has been pushed back and forth for two days usually does not.