Smoking just one cigarette a day is associated with about half the risk of developing coronary heart disease (CHD) or a stroke as smoking 20 per day, according to the BMJ.

The meta-analysis of 141 studies estimated that men and women who smoked, on average:
• One cigarette daily were 48 and 57 per cent respectively more likely to develop CHD and 25 and 31 per cent respectively more likely to have a stroke than non-smokers
• Five cigarettes daily and they were 58 and 76 per cent more likely to develop CHD and 30 and 44 per cent more likely to develop a stroke
• 20 cigarettes daily and they were two- (relative risk [RR] 2.04) and three-times [RR 2.84] more likely respectively to develop CHD and about twice as likely to experience a stroke (RR 1.64 and 2.16 respectively).

Light smoking is associated with about half the cardio-vascular risk of smoking 20 daily. Men smoking one and five cigarettes daily had 46 and 57 per cent of the excess CHD risk caused by smoking 20 cigarettes per day; women had 31 and 43 per cent excess risk.

Men smoking one and five cigarettes daily had 41 and 52 per cent of the excess stroke risk arising from smoking 20 a day, while women had 34 and 44 per cent of the excess risk.

The risk was even higher in studies that adjusted for multiple risk factors. “We have shown that a large proportion of the risk of coronary heart disease and stroke comes from smoking only a few cigarettes,” the authors comment. “This has important consequences for smokers who believe that light smoking carries little or no harm.”

^{BMJ 2018; 360:j5855}