When do you prescribe an antibiotic after a root canal treatment?

The use of an antibiotic in adjunct with a root canal treatment is based on the judgement of the dentist treating a patient. In my practice, most patients whose pulps were nonvital(necrotic or no longer living) are given antibiotics to take after root canal treatment. My thinking is that the contents of the root canal system are most likely contaminated with bacteria and the patient will benefit by taking an antibiotic after being treated. A small percentage of non vital cases 'flare up' after root canal instrumentation. These flare ups can involve periapical swelling and pain and a worsening of the infection. Antibiotic may help prevent the flare up,lessen any that occurs and will most likely help with the resolution of any infection already present.

For vital cases(teeth with living, non infected pulps)most times antibiotic is not prescribed. After a root canal treatment of a vital tooth there also can be some periapical tenderness and even some slight swelling, but antibiotics are not prescribed unless I suspect an infection may have developed.