1) British dance girl group 2) English musical duo 3) Habit-forming 4) Vocal duet 
(Humans as organisms) a substance on which a person can become physically dependent
[adj] - causing or characterized by addiction
addictive A reference to a drug, etc.: to which one may become addicted; causing dependence, habit-forming.
habit-forming adjective causing or characterized by addiction; `addictive drugs`; `addictive behavior`
producing or tending to cause addiction: an addictive drug. · more than normally susceptible to addiction: an addictive personality.