Dental fillings eventually need attention. Like any part of a tooth's surface, a filling will experience wear and tear and requires maintenance. But the best (and easiest) results are found with replacement—meaning a dentist will remove and replace a filling when the time comes. This is less complicated than attempting to preserve an older filling. Based on their observations, a dentist can decide when a filling needs to come out. But what happens when the filling makes its own decision?
A filling doesn't fall out of a cavity for no reason, so the reason must be identified, and some reasons are more critical than others. One of the most simple reasons is optimism. With larger areas of decay or damage, your dentist may be optimistic that a filling will suffice. A dental filling is less expensive and invasive than other types of dental restorations, so it's often the first line of treatment. But the surface area in question may be too large for a filling to maintain its structural integrity. It may fall off because your specific bite pattern exerted pressure on the filling in a way that wasn't anticipated. The filling will keep failing, and a more comprehensive measure is needed. Your filling may be replaced with a dental crown, which is a porcelain cap fitted over the entire tooth.
Most forms of tooth decay have a bacterial element. Unless it's properly removed, cariogenic bacteria (capable of causing tooth decay) may continue to weaken the tooth. This is why the decayed portions of a tooth are removed, slightly enlarging the existing cavity, so that the prepared cavity can be filled with tooth-colored dental resin, or whichever material your dentist has selected. If primary decay isn't comprehensively removed, it may become secondary decay—continuing beneath the surface of the filling. This destabilizes the filling, causing it to detach. The cavity can be prepared to a greater depth before it's filled again, and this should prevent the recurrence of secondary decay.
There are times when a lost filling isn't surprising, and that's when the filling was intended to be temporary. These are routinely applied during dental work that requires a period of monitoring. For example, a dentist may apply a temporary filling after root canal work. This lets them make sure that all the infected tissues inside the tooth were removed. After sufficient monitoring time, the temporary filling is replaced with a permanent one. Temporary fillings are softer than their permanent versions, and can easily fall out. Contact your dentist if this happens. They may advise caution (the tooth will be more sensitive), but may not need to replace the temporary filling if placement of a permanent filling has already been scheduled.
A falling filling isn't a dental emergency, but it should be reported to your dentist so it can be treated without delay.