Chemically speaking, we are all potheads. Raphael Mechoulam of Hebrew University in Jerusalem discovered that astounding fact in 1992, and now the reasons why are finally emerging. Numerous experiments with genetically altered mice and rats have shown that when natural brain compounds, called endocannabinoids, are missing or their receptors are blocked, the animals are more susceptible to pain, cannot control their appetites, have trouble handling anxiety and are less able to cope with stress.