It's an overused distinction - but certain drugs when used over long periods of time result in such large biochemical changes to the human body that if they are suddenly withdrawn, the person may experience various medical emergency situations that require hospitalization (delirium tremens with alcohol, seizures with benzodiazepines, dehydration due to vomiting and diarrhea with opiates).

These extreme physical effects are seen at the upper end of abuse limits, and the majority of people who legitimately suffer from addiction don't need round-the-clock medical care if they quit - and even those who do can generally avoid such problems if they go through a tapering-off process over the period of about a month.