Methadone often causes slow movements and loss of sense of balance, because it's used to make people relief and relaxed, that's why this drug is given to people that suffer extreme pain, or even chronicle illness.
Actually, methadone was created during World War II by Germans to avoid pain from injuries during the war, that's why nowadays it's used to relief pain. Other side effects are: restlessness, nausea or vomiting, slow breathing, heavy sweating, and sexual problems.
