Couple of weeks go by it ain't even like I'm gettin' high
Now I need it just not to feel sick,

from Eminem – Deja Vu Lyrics on Rap Genius

Meaning

He first started taking drugs to get high — recreational use. Now he can’t live without it. Opiates actually cause you to get “dope sick” when you are not on them. Once your body becomes accustomed to it you need it just to normal and not even get high. Your body gets used to it, and eventually, it becomes one with your body, so if you end up skipping your dose, you end up feeling sick.

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