Withdrawal symptoms and nicotineA smokers nervous system becomes accustomed to functioning with nicotine.
When you stop smoking, the reduced nicotine intake will disturb the balance of the central nervous system, causing withdrawal symptoms.
The most common withdrawal symptoms are:
Cravings for tobacco
Irritation
Anger
Weight gain
Concentration problems
Depression
Headaches
Fatigue
Constipation
Restlessness
Insomnia
Dizziness
Anxiety
Fortunately, the majority of these symptoms tend to disappear after a few weeks. Some people may experience cravings, concentration problems and an increased appetite over a longer time period.
http://www.netdoctor.co.uk/smoking/withdrawalsymptoms_000507.htm