Very bad toothache is often caused by a tooth abscess. When a hole (or cavity) in a tooth gets large enough, the living part of the tooth (or pulp) becomes damaged. The pulp can then die leaving an empty space inside the tooth where the pulp used to be. Bacteria from the mouth can get inside the tooth and cause an infection (known as an abscess) at the very end of the root (or root tip).
This infection can then spread to form an abscess around the root tip. The infection inside the tooth's root (or root canal) builds up and eventually spreads into the bone at the end of the root canal (root tip) to form a rounded area which can usually be seen on an x-ray. Toothache caused by an abscess often lasts for long periods, and the tooth may well be tender to bite on.
Abscesses often become increasingly more painful, and sometimes the infection spreads even more to form a bump on the gum overlying the root ("gumboil"), or sometimes it causes a swollen face.
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