There is one way to prevent a bad high, and it's purely mental. If your friend thinks weed is sketchy in any way, there is a great chance he will have a bad high. This is why experienced smokers usually never get bad highs, because they understand that weed is just a harmless herb. New smokers don't realize this right away, as years of anti-marijuana propaganda is in the back of their minds. Marijuana is a slight psychedelic, and therefore it can bring out things like that from the back of your mind during your high, resulting in a bad experience.