Dr. George Meinig, author of Root Canal Cover Up, said it best when he quoted Dr. Weston A. Price as follows:
This quote may be long and sound complex; however, it speaks directly to why root canals should be avoided. Whenever a tooth is root canaled it sets up a situation that is identical to the one described above."... when infected teeth produce disturbance in other parts of the body, it is not necessary that the quantity of infection be large, nor is it demonstrated that it is necessary that organisms pass throughout the body or to the special tissues involved, but the evidence at hand strongly suggests that soluble poisons may pass from the infected teeth to the lymph or blood circulation, or both, and produce system disturbance entirely out of proportion to the quantity involved.
The evidence indicates that toxic substances may under certain circumstances sensitize the body or special tissues so that very small quantities of the organisms, which produce the toxin, may produce very marked reactions and disturbances." (Source: Root Canal Teeth and Cavitations by Lowell Weiner DDS, FAGD)
Harmful substances are produced by the infected root canaled tooth which can have serious effects on constitutional health. When these infections and their circulating toxins occur unchecked for prolonged periods of time, systemic damage often results.