iv def had small amounts of shin and elbow pain over the years but im pritty used to it not. i cant say iv ever had any knee problems that i think are from muay thai. but knee problems can be horrible, it can affect every aspect of your game and keep you immobilized at times.
smoking weed or taking some oxies from time to time is pritty good. but as iv learned, smoking weed and cigarettes does really fuck up your cardio. it makes a world of difference, so try to keep that to a minimal.