Single word for object that is addictive
One may get addicted to any of various addictives:
noun
addictive (plural addictives)
. A drug that causes an addiction.
. Anything that is very habit-forming.
[wiktionary]
Vice
As in "John's vice is cigarettes"
It does have a much wider usage though - can be used to refer to an action, habit or attribute that is considered a weakness.