French scientists have identified a natural painkiller in human saliva and demonstrated in pain studies in rats that it is up to 6 times more potent than morphine (Wisner A et al. Proc Natl Acad Sci U S A. doi:10.1073/pnas.0605865 103 [published online ahead of print November 13, 2006]).
The compound, opiorphin, appears to work by enhancing the body's own defenses against pain by preventing the breakdown of enkephalins, chemicals that activate opiate receptors to block pain signals from reaching the brain. Studies with rats subjected to a painful injection (of a formalin solution into a paw) or pain caused by walking across a pin-covered surface revealed that a 1-mg/kg dose of opiorphin had as much pain-suppressing effect as a 3- to 6-mg /kg dose of morphine.
Stephenson J. Natural Painkiller Found. JAMA. 2006;296(23):2791. doi:10.1001/jama.296.23.2791-c