Studies have shown that marijuana can cause schizophrenia. You are at risk if you have certain gene combinations, which we don't yet have tests for. (Roughly 1% of the world's population will develop schizophrenia in their lifetime, and you are at higher risk if you smoke marijuana.) http://www.cbc.ca/documentaries/natureofthings/2010/downsideofhigh/facts.html On the other hand, there are studies that show marijuana can reduce the size of brain tumors, while at the same time enhancing positive brain cell growth (which seems like a strange contradiction to me o_O). A cursory search will reveal endless links on these subjects. There are many things about our biology, and brains, that we just don't know yet. While, it's not yet clear what the long term effects are, short term studies have shown both positive and negative results. If you're willing to subject yourself to something that, in all likelihood, will alter your brain function, that's entirely up to you.