I think heroin has a half life of 4-8 hrs, and methadown 20-36hrs, i understand that it takes 36hrs for half the drug dose to leave your body, but theres only 2 day deference between the two drugs but methadown takes months to kick and heroin weeks. thats the part i dont understand but theres only 2 day deference between the two. anyone with knowledge please try in a laymen terms to explain. thanks for any help in advance.