Do people who smoke mary jane really lose brain cells?

Brain function. The research is equivocal about actual brain cell loss with use of marijuana. (mri scans don't show this.) however, brain function definitely changes for the worse while under the influence -- and possibly longer, especially with chronic use. http://www.drugabuse.gov/publications/research-reports/marijuana-abuse/how-does-marijuana-use-affect-your-brain-body.