Women who give up smoking before the age of 40 reduce their excess mortality risk by 90% and those who stop before 30 by > 97%, results from the Million Women Study have shown.

This UK prospective cohort study aimed to quantify accurately the excess mortality associated with smoking in women, together with the effects of cessation, by reviewing a large cohort born in the 1940s.

From 1996 to 2001 participants were recruited into the Million Women Study through the NHS Breast Screening Programme. As part of the study participants completed a questionnaire which included lifestyle and smoking information. Participants were followed up by postal questionnaire at three years, when information on stopping smoking was first sought, and eight years. Date and cause of death were automatically notified to the study authors up to 2011, a mean of 12 years after entry.

Patients with a baseline history of cancer (other than some skin cancers), heart disease, stroke and current treatment for respiratory disease were excluded leaving a cohort of 1.2 million women, age 50-69 years, who were included. Mean age at recruitment was 55, and 6% died during the average 12 years of follow-up, at a mean age of 65.

At baseline 20% were current smokers, 28% former smokers and 52% never smokers. Smokers were more likely to live in deprived areas, drink more than 14 units of alcohol weekly, and not to undertake strenuous exercise. Regular exposure to second-hand smoke in never smokers was 13%. Of the current smokers at baseline, 23% had stopped at the three-year survey and 44% at eight years.

At the 12-year mortality follow-up those who were current smokers at baseline had almost three times the mortality rate of never smokers.

The risks increased steeply with the amount smoked, but even for those smoking fewer than 10 cigarettes a day at baseline mortality was doubled.

The age at which women had started smoking affected overall mortality decades later, with those starting at age 15 at greatest risk. Common causes of death included cancer, vascular and respiratory diseases. All results were adjusted for alcohol intake.

Overall, the benefits of stopping were greater for those stopping earlier. Those stopping in the 25-34 year group avoided about 97% of the excess overall mortality compared with those who continued smoking, while those who stopped by about 40 avoided 90%.

Stopping smoking, at whatever age, reduced excess mortality, though even those who had stopped at ages 25-34 years had, decades later, measurably higher lung cancer mortality than never-smokers.

Former smokers who quit at age 25-34 or 35-44 had respective relative risks of 1.05 (95% CI:1.10-1.11) and 1.20 (95% CI:1.14-1.26) for all cause mortality and 1.84 (95% CI:1.45-2.34) and 3.34 (95% CI:2.76-4.03) for lung cancer mortality.

This paper confirms the very significant excess in mortality in women who smoke, and the authors concluded that female smokers lose at least 10 years of their lifespan.

GPs now have very specific and accurate information on the harms associated with smoking in women, and the substantial benefits of stopping, particularly at a relatively young age.

Dr Jez Thompson

• Pirie K, Peto R, Reeves GK et al. The 21st century hazards of smoking and benefits of stopping: a prospective study of one million women in the UK. Lancet published online Oct 27 2012.http://dx.doi.org/10.1016/S0140-6736(12)61720-6