Whoa...lets back up here. You do not go to rehab for pot. I can promise you that. Your sister's best friend went because she was doing something other than that.
Marijuana has no addictive qualities to it, nor does it have any more affect on the body than say caffeine, which is probably worse for you in the long run.
Now, lots of other drugs can have adverse effects.
Raistlin, a lot of the drugs were not really ever invented, they've been here as long as humans have been.