"I thought marijuana was all-natural; how can it hurt your body if it's natural?"
Answer from Scientist:
"Many drugs of abuse come from plants, and therefore can be considered "natural." For example, heroin comes from poppy plants and cocaine comes from the coca plant. That doesn't mean they are healthy—think tobacco. Similarly, marijuana comes from a plant that contains THC, which is "psychoactive" (it affects mood and behavior) and can be addictive."