There's controversy with dependence vs addiction (specifically with marijuana which is classified as not being physically addictive, but users can develop a psychological "dependency" upon it). With true addiction the brain's neurotransmitters lock onto the substance and the feeling becomes one where the body feels like it needs the substance to function, to feel "normal" or to even survive.
I caught a headline today - but not sure where - that over 2,000,000 Americans are addicted to painkillers.