Quitting is so difficult because of nicotine, which is as addictive as heroin and cocaine. Nicotine exists naturally in tobacco and is carried deep into the lungs when smokers inhale. Once in the lungs, it enters the bloodstream and spreads throughout the body, causing a variety of changes.

Like caffeine, nicotine is a stimulant. It increases activity in the brain and produces a "kick" or "buzz" caused in part by stimulation of the adrenal glands, which discharge adrenaline. This rush of adrenaline stimulates the body and causes a sudden release of glucose, as well as an increase in blood pressure, respiration, and heart rate. Nicotine also suppresses insulin output from the pancreas, which means that smokers always have elevated blood sugar levels. Nicotine also creates a pleasant feeling by releasing the neurotransmitter dopamine.

However, as the nervous system adapts to nicotine, it develops tolerance, meaning it requires more and more nicotine to attain the buzz. That's why long-term smokers tend to smoke much more than people just starting out. When smokers who have developed a tolerance to nicotine try to cut back or quit, the absence of nicotine leads to withdrawal symptoms such as dizziness, headaches, irritability, and insomnia.

It is important to realize that withdrawal has both physical and psychological components. Physically, the body reacts to the lack of nicotine, but the psychological effects can be daunting as well. For some people, the feel, smell, and sight of a cigarette and the ritual of obtaining, handling, lighting, and smoking the cigarette are all associated with pleasure and can make withdrawal and cravings even worse. Withdrawal can be very uncomfortable, but hang tight. Symptoms usually peak a few days after quitting and then subside over time. You can read more about withdrawal in "What is withdrawal, and how do I quench cravings?"