system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 if nicotine flushes out of your body within a week, why do ex smokers have physical cravings even long time after quitting?
gpt
The nicotine doesn't directly cause the cravings. Instead, when you use nicotine for an extended period of time your body starts to assume it's always going to be there, so it adjusts itself appropriately.

When you stop ingesting nicotine suddenly your body doesn't have it anymore and so you feel like your lacking something. Just like a wound takes a while to heal after the knife has left, it takes a while for your body to adjust itself back to the way it was after the nicotine is gone.

This applies to any substance we use to alter our body from caffeine, to alcohol, to heroin.