“Nicotine is known to switch on receptors on the surface of cells in certain parts of the brain, causing these neurones to release the neuro-transmitter dopamine, a chemical that is associated with feelings of pleasure. This effect leads to a person’s addiction.

More than 50 per cent of people suffering from clinical depression smoke, while the figure rises to 95 per cent for schizophrenics. But smoking among the general public has dropped to about 25 per cent. ‘The assumption is that people with psychiatric conditions are self-medicating,’ said McGehee. ‘They are smoking because the nicotine in particularly helpful in alleviating their condition.'” (Guardian.UK)