As always, grannys advice rocks! Weed can only have a negative effect on your life if you allow yourself to get lazy. If you find enjoyable ways to excersize your body and mind WHILE stoned in addition to while sober, you will find your world opening up in ways you were unable to even comprehend previously.

Also oddly enough, a Rastafarian lifestyle includes smoking to expand your mind and consciousness as opposed to simply self-medicating. (That statement was in no way an assumption of your reasonings behind smoking weed, just had to add that in there )