Hi, there! (:
Well for starters - you shouldn't take your friend's medication because it can internally harm you. Chances are if it's prescribed by a doctor for someone other than you it's not made for your body which can lead to a very dangerous reaction on your end.
Another reason is that it's not actually legal.
But lastly, it can lead to a road of addiction, or even you going and trying to experiment further, and it could lead to long term damage.
I hope all is well, and you pass! Good luck, rockstar! (: If you have any follow up questions, or even a comment on what i've wrote - let me know! (:
