Apparently, a substance called Cannabidiol found in cannabis has been scientifically  proven to cure cancers. Basically, it destroys a gene in cancers that makes it spread to other parts of the body.  It even kills off the most aggressive types of cancer. As bad luck would have it Cannabidiol is the part of cannabis that doesn't get you high and you can't smoke enough weed to get enough of the substance to kill the cancer. 

I'm not holding my breath on big pharma developing and producing this natural cure for cancer, unless they can secure the rights to all the types of pot, they won't even bother. 

Source: http://www.digitaljournal.com/article/333260