The mentally ill also smoke more cigarettes per day -- often up to two packs. They have a disproportionately high rate of tobacco-related disease and mortality, such as cardiovascular disease or cancer, with a correspondingly heavy financial burden to the health-care system.

Doctors erroneously believe mental disorders will worsen if they take away a person's tobacco.

'Not a single study shows that symptoms get worse,' Hitsman said, according to a Feinberg release.

He examined 13 randomised clinical trials that measured psychiatric symptoms during smoking cessation treatment. Seven studies showed that psychiatric symptoms actually improved during smoking cessation treatment, and six showed no changes.

His paper appeared recently in the Canadian Journal of Psychiatry.