I apologize for my cheesy Bone Thugs N Harmony reference in the title, it just needed to be done. Of all of the bad things our parents told us not to do I think cigarettes and marijuana might have topped the list. As Reefer Madness such as the image on the leftcontinuously dies out and marijuana becomes more mainstream by the month we can learn more and more about both positive and negative effects of marijuana in a more scientific and objective way. But I found a study that might turn the head of just about anyone. This Study was conducted to see the effects of combined nicotine and marijuana together. Marijuana users and tobacco users both show smaller hippocampus size. Now ask any scientist and they will tell you that smaller hippocampus size should lead to worse memory. Incredibly, when marijuana smokers also smoke cigarettes their memory is better, although their hippocampus. Now explaining further, the smaller hippocampus size is being related to better memory.  Now this only occurs when then is an interaction between the two substances. Lets just say that I have a friend who is a frequent user of both and has the best memory of all of the people that I know. What is interesting about this study is that a previous confounding variable may become a leading variable in this study. The study mentions that nicotine use was rarely even considered in previous studies on the effect of marijuana on the brain. Although I do believe that further studies need to be conducted on this subject I believe that this study, as I interpret it, may suffer from the Texas Sharpshooter Problem. The study was conducted by just bringing an outside variable into a previous experiment(effects of marijuana on the brain). The study was conducted to see the interaction between the 2 variables(Marijuana and Nicotine), whatever the result may be. That being said, roughly 70% of marijuana users also use tobacco or nicotine products, and further tests need to be conducted.

http://www.brainhealth.utdallas.edu/blog_page/study-finds-nicotine-changes-marijuanas-effect-on-the-brain