Drastic change in a person mood?

I know someone who a couple months was someone completely different than who they are now. Before they were really reserved, layed back, understandable. Now they talk out loud to themselves all day, Think out loud, anger isuues, and cant have a conversation without talking over you every single time. Im concerned maybe theyre on drugs or maybe hangig around the wrong group of friends. What could cause this person to change so much in a short amount of time. Thank you for your opinions.