A study done in 2007 with synthetic cannabinoids has shown that neurogenisis can occur... that is cannabis can stimulate the creation of new brain cells. Even heavy use of cannabis has been proven not to damage the brain.
Whats more, a cannabinoid has been shown to provide neuroprotecting properties. That is, it can limit secondary damage from head injuries and speed recovery time.
Why were we told smoking cannabis kills brain cells?
The reason people started saying cannabis killed brain cells was because of the Heath/Tulane Study of 1974. Heath reported that rhesus monkeys when given the equivalent of 30 columbian strength joints a day began to atrophy and die after 90 days. (that is 2700 joints, or average 1.25 joints and hour every 24 hours for 90 days straight. When Heath killed the test monkeys and the control monkeys and found dead brain cells in the test monkeys, he concluded that cannabis was responsible for the brain damage.
However, it has since been revealed how the tests were conducted.
Not only were the monkeys given 63 columbian strength joints in 5 minutes, the smoke was administered through a facial mask with no additional oxygen. As you will find in any CPR or first aid manual 3 to 5 minutes of oxygen depravation will cause brain damage. The monkeys were suffocating.