Like all other addictive drugs, nicotine activates the brain's reward system. Nicotine is quickly absorbed during smoking and travels fast to the pleasure-reward areas of the brain, producing a satisfying, positive feeling. To recreate this sensation, more nicotine needs to be smoked and before you know it you're addicted.
Besides stimulating the brain's reward system, nicotine affects other parts of the brain and body to create a physical and psychological dependence. It stimulates regions of the brain responsible for emotions, memory, and alertness, resulting in increased alertness and a sense of well-being. It also decreases appetite and affects the parts of the brain that regulate nerves, hormones, and the organ systems of the body.
As with all addictions, continued use of nicotine can cause physical and chemical changes as the body adapts to the constant presence of nicotine. This is called "neuroadaptation." These changes result in increased tolerance to the drug as more drug is needed to create the same effect.
Why do withdrawal symptoms happen?
When you stop smoking, the supply of nicotine to the brain is interrupted. Because your body has become used to the effects of nicotine and needs it to maintain the feeling of "normal," sudden removal of nicotine throws your body out of balance and causes symptoms of withdrawal. Some of these symptoms include anxiety, irritability, fatigue, dizziness, headaches, poor concentration, short-term memory loss, and hunger. It will take some time for your body to re-adapt to functioning normally without nicotine.