Mac and Dennis have known each other since high school. They became friends despite coming from entirely different social circles - according to Dennis' recollections of high school, Mac was a drug dealer with no friends while Dennis was immensely popular ("Underage Drinking: A National Concern"). However, we later learn that Dennis was not as popular as he claims to have been, and spent all of high school getting high under the bleachers with Mac and Charlie ("The High School Reunion, Part 2: The Gang's Revenge"). Even though they were friends, Mac slept with Dennis' prom date and covered it up by telling Dennis that Tim Murphy did instead. Dennis doesn't find out the truth until years later ("The High School Reunion, Part 2: The Gang's Revenge"). By season four, Dennis and Mac have been