This one is only a partial myth – technically true but very misleading. Marijuana is proven to impair your short-term memory while you are under it’s effects. Use of marijuana causes a temporary change in thoughts, information processing as well as perceptions. However, once the effects wear off, so does the memory impairment.
In fact, studies have shown that information that was learned previously is easily reachable by participants under the influence of marijuana. However, it must also be stated that these same participants showed a diminished capacity to learn and recall new information (while stoned), but this capacity only lasted for the duration of the intoxication.
Many studies have also addressed the question of whether there are severe deficiencies in cognitive function resulting from heavy marijuana use. These studies were performed on both humans and monkeys. “Most reports have show that, while there are deficits in the performance of complex cognitive
tasks in long-term cannabis users, although there is little evidence that these are qualitatively or quantitatively more severe than those seen after acute drug use”.
So, lets sum this up.. Basically, marijuana has carcinogens in the smoke that can harm you. When you are stoned, your memory can be impaired. When you are just stopping using marijuana, you may still experience a reduction in brain function. However, it seems that once you have gone through the “just stopping” phase, there are no long-term negative effects.