Smoking just one cigarette a day has a much higher risk of developing coronary heart disease and stroke than expected - about half the risk of smoking 20 per day - concludes a review of the evidence published by The BMJ today.
The researchers say their findings have important consequences for many smokers and health professionals who believe that smoking only a few cigarettes carries little or no harm. They argue that smokers should stop completely instead of cutting down to significantly reduce their risk of heart disease and stroke.
This information might be particularly helpful at the start of a new year, when many smokers are likely to be thinking about stopping or smoking less.
Individual studies have reported that smoking only one to five cigarettes per day is associated with a higher than expected risk of heart disease. To investigate this further, a team of researchers led by Professor Allan Hackshaw at the UCL Cancer Institute at University College London analysed the results of 141 studies and estimated the relative risks for smoking one, five, or 20 cigarettes per day.
They found that men who smoked one cigarette per day had 46% of the excess risk of heart disease and 41% of the excess risk of stroke associated with smoking 20 cigarettes per day (much higher than the expected 5%).
For women, those who smoked one cigarette per day had 31% of the excess risk of heart disease and 34% of the excess risk of stroke associated with smoking 20 cigarettes per day. Women's heart disease risk was more than doubled with one cigarette per day, when only studies that controlled for several factors were included in the analysis.
"We have shown that a large proportion of the risk of coronary heart disease and stroke comes from smoking only a couple of cigarettes each day," say the authors. "This probably comes as a surprise to many people. But there are also biological mechanisms that help explain the unexpectedly high risk associated with a low level of smoking."
They acknowledge some study limitations, but say their paper is the first to combine results across many studies covering both coronary heart disease and stroke, "making it a valuable reference that can be used to strengthen public health campaigns, and provide a strong health incentive for smokers to stop completely (particularly women)."
Cardiovascular disease, not cancer, is the greatest mortality risk for smoking, causing about 48% of smoking-related premature deaths.
The authors conclude: "No safe level of smoking exists for cardiovascular disease. Smokers should quit instead of cutting down, using appropriate cessation aids if needed, to significantly reduce their risk of these two common major disorders."
In a linked editorial, Kenneth Johnson, Adjunct Professor at the University of Ottawa outlines the major public health implications of these results, and says "only complete cessation is protective and should be emphasised by all prevention measures and policies."
The take home message for smokers is that "any exposure to cigarette smoke is too much," he adds. "The message for regulators dealing with newly marketed 'reduced risk' products is that any suggestion of seriously reduced coronary heart disease and stroke from using these products is premature."