Giving up smoking could cut a person's risk of coronary heart disease (CHD) by more than a third in as little as 2 years, researchers have found.

Julia Critchley and S Capewell note that, while the importance of smoking in heart disease is beyond doubt, the speed and magnitude of risk reduction when a smoker with CHD quits is still subject to debate.

In an attempt to resolve this issue, they conducted a systematic review of 20 large, prospective, cohort studies in CHD patients, who were followed up for at least 2 years. Smoking status had to have been assessed on at least two occasions to determine whether or not smokers had quit.

Together, the study incorporated results from over 12,000 smokers with CHD, 5659 of whom had quit smoking. Participants were followed for between 2 and 26 years, and mortality rates assessed.

Critchley and Capewell report that the CHD benefits associated with quitting smoking did not seem to increase with longer follow-up, indicating that the majority of risk reduction may occur within a couple of years of quitting.

Overall, the pooled crude risk reduction from the 20 studies was 0.64. "This 36 per cent risk reduction appears very substantial in comparison with other secondary preventive therapies such as cholesterol lowering, which have received greater attention in recent years," the researchers say.

Reporting their findings in the Cochrane Library journal, they conclude: "Given the overwhelming evidence of the benefits of stopping smoking, and growing evidence of the best methods of helping patients to achieve this goal, it is unlikely that further work exploring the magnitude and speed of effect of smoking cessation is needed."