Here, Stangle. A chronic cannabis-user's study.

http://www.medicalcannabis.com/PDF/Chronic_Cannabis.pdf

Also,
http://www.marijuanalibrary.org/brain2.html
One of the safest qualities of THC, delta-9 tetrahydrocannabinol, the primary psychoactive substance in marijuana, is the natural limit the body places on the drug's effects.

It has long mystified scientists how most individuals can consume enormous quantities of marijuana with few or no obvious ill effects. But the explanation will not surprise regular marijuana users.