Nicotine is the addictive component of tobacco smoke. It is readily absorbed through the oral mucosa and lungs and peak blood concentrations are achieved within 30 seconds of a puff of a cigarette. The drug acts on the central nervous system, causing transient euphoria, a feeling of relaxation, improved concentration and memory, and reduced appetite.

Nicotine produces withdrawal symptoms of anxiety, difficulty in concentrating and irritability, which are relieved by the next cigarette. Eventually, smokers establish a steady blood concentration of nicotine through a regular smoking pattern, preventing withdrawal cravings.

Psychological and behavioural components contribute to dependence on smoking in roughly equal measure to the physiological addiction, and are of two types:

• Associations that reinforce the habit, which can be positive (eg, social drinking and following meals), or negative (eg, stressful situations)
• Ritual behaviour associated with lighting, holding and inhaling a cigarette, which the smoker associates with the reward of a measure of nicotine.

Other physiological effects of nicotine include stimulation of the autonomic nervous system, which increases heart rate, raises blood pressure and causes vasoconstriction. It also increases the stickiness of blood platelets, leading to increased risk of clotting.

Nicotine raises the levels of serotonin, catecholamines, pituitary hormones and vasopressin in the blood and brain. It also increases gastric acid secretion, which may lead to peptic ulceration.