Using the negative exponential distribution to quantitatively review the evidence on how rapidly the excess risk of ischaemic heart disease declines following quitting smoking

► No previous review has formally modelled the decline in IHD risk following quitting smoking. ► We estimated when the excess risk becomes half that of continuing smoking (the half-life H). ► Based on 41 independent data blocks we estimated H as 4.40 (95% CI 3.26 to 5.95) years. ► Individual estimates are extremely variable: <2 years for 10 blocks and >10 years for 12 blocks. ► Claims that excess risk approximately halves after quitting for 1 year do not reflect the overall data.