I'm going to cut off my best friend because I am unable to support his new relationship
Why do you not like your friend's relationship?
I overheard that she was involved in drugs. I don't want to hang out with people who make my friend turn to the dark side.
I understand your plight here. Maybe you can inform your friend of how you feel about the relationship. If he wants to continue the relationship despite what you have said, then you can end the friendship.
I'm not going to be friends with someone who is doing drugs.
It is very understandable to feel this way. It sucks having to lose a friend in this manner, but you must abide by the values you have.