Body & HealthFact
Nicotine fits a lock that was built for something else.
Your brain uses the messenger acetylcholine for attention and muscle commands. Nicotine resembles it so closely that it activates the same receptors, which is why they're called 'nicotinic'. After inhalation it reaches the brain in roughly 10 to 20 seconds and triggers, among other things, a release of dopamine. Under constant stimulation the brain adds extra receptors, which raises demand.
Useful: Speed is part of the problem: the faster a substance reaches the brain, the more tightly it binds to habit.
Source: en.wikipedia.org