One who is addicted, pshycicly/physicly to a chemical based drug. Obsessive use and out of control behaviour are accompanied.
2- Heroin,crack, meth, Cocaine, Speed, Ecstasy, and PCP, are the most common drugs a "Drug Addict" May be Addicted to.
Amy Winehouse is a crazy drug addict.
Amy winehouse is a crack whore. (Crack whores are Drug addicts)