i hate something and i dont know why?

this is dumb but im having a panic attack over it. i found out my friend lied to me and is smoking weed again and yes, i know, weed is not bad, and its natural. but i have a bad past concerning alcohol and drugs. it makes me so uncomfortable to the ppint that i shake and cry.  i dont know what to do. i would never ask my friend to stop doing something they enjoy unless i knew for sure it was harming them. so i want to know how i can cope easier with something like this. it's so incredibly stupid and i know that but i can't stop how it initially makes me feel, i can only try and grow and cope. i just need help. i think it isn't weed that scares me, it's just the fact that drugs change people. and eventually, weed stops having an effect on some people, and they turn to other, harder drugs. and that scares me. what do i do?