What is it?
Nicotine in the body
Only about ten seconds after a smoker inhales this chemical, it proceeds to be absorbed through the skin, and travel to the bloodstream of the smoker's body, eventually leading to the brain. It stimulates your adrenal glands, which increases heart rate and constricts blood vessels. It also speeds up the process in your brain which processes dopamine, a transmitter in your brain that controls the brain's pleasure center. This causes your brain to crave nicotine and cigarettes, because every time the chemical is inhaled, it stimulates the pleasure center of your brain.
Nicotine in the brain
Your brain has thousands of neurotransmitters within it. A neurotransmitter is what sends your brain information in which it's several receptors receive. Nicotine imitates the neurotransmitter acetylcholine, and binds to those receptors. While normal neurotransmitters send signals to your brain at a pace, nicotine sends constant signals, increasing the smoker's addiction.
How addictive is it?
Nicotine is very addictive. People may say that "cigarettes aren't that bad," or that "It's easy to quit," but that's not true at all. Nicotine is actually just as addictive as heroin or cocaine. That's right, it's as hard to quit using highly illegal and deadly drugs like cocaine and heroin as it is to quit smoking.