This does not mean that it is proven that marijuana has no long term effects on the brain. Current research techniques may not be sensitive enough to detect brain changes. Studies only test a small portion of users. There may be certain people who are more easily damaged by marijuana chemicals who may not have been represented in the research. With many conditions and drugs besides marijuana, changes to the brain occur but the individual adapts and overcomes them through a process of relearning.
Many medicines affect senior citizens differently than they affect young people.
The same could be true of marijuana. Cerebral atrophy in young cannabis smokers. Absence of cerebral atrophy in chronic cannabis users. Evaluation by computerized transaxial tomography. Computed tomographic examination of heavy marijuana smokers.