web-md
Heavy Marijuana Use Doesn't Damage Brain
http://my.webmd.com/content/article/70/80972.htm
http://www.askmen.com/sports/health/20_mens_health.html
Brain Damage
Marijuana is psychoactive because it stimulates certain brain receptors, but it does not produce toxins that kill them (like alcohol), and it does not wear them out as other drugs may. There is no evidence that marijuana use causes brain damage. Studies performed on actual human populations will confirm these results, even for chronic marijuana users (up to 18 joints per day) after many years of use.
Though studies are unconclusive about the degree of cancer that it causes.
