Smoking cigarettes throughout adulthood reduces life expectancy by about 11 years in women but quitting avoids much of the extra risk, a new large study shows.

The Million Women Study in the UK recruited 1.3 million British women who were born in the early 1940s to look at the hazards of smoking and the benefits of stopping at various ages.

Women in North America took up smoking decades later than men. (Jonathan Alcorn/Reuters)

In most of Europe, Canada and the U.S., the popularity of smoking among young women reached its peak in the 1960s, decades later than for men.

Among women in the study who smoked cigarettes through their adult lives, the mortality rate was three times that of women who never smoked or who stopped well before middle age, Sir Richard Peto of the University of Oxford and his co-authors said in Saturday's issue of the journal Lancet.

"Stopping before 40 years of age, and preferably well before, avoids more than 90 per cent of this excess mortality; stopping before 30 years of age avoids more than 97 per cent of it," the study's authors concluded.

"This does not, however, mean that it is safe to smoke until 40 years and then stop, for women who do so have throughout the next few decades a mortality rate 1.2 times that of never-smokers."

Study participants were recruited from 1996 to 2001. They filled in questionnaires about the lifestyle, medical and social factors and were resurveyed by mail three and eight years later.

At the start of the study, 20 per cent of them were smokers, 28 per cent were ex-smokers and 52 per cent had never smoked. By 2011 six per cent had died.

The excess mortality among smokers was mainly from diseases that are known to be affected by smoking, such as lung cancer, chronic lung disease, heart disease and stroke, the researchers said. A little of the excess wouldn’t be caused by smoking, they added.

Quitting sooner the better

Smokers were more likely to live in economically poor areas, drink more than 14 units of alcohol a week and avoid strenuous exercises. Those factors were taken into account in the analysis.

A journal commentary accompanying the study called the main findings "simple and unequivocal."

Even women considered social smokers having a handful of cigarettes a day had twice the mortality rate of never smokers, Professor Rachel Huxley at the University of Minnesota in Minneapolis said in her commentary.

"For adults who already smoke, stopping works extraordinarily well — and the sooner the better," Huxley wrote.

Women who smoke have more than four times the risk of dying of heart disease in the near future than nonsmokers the same age, she said, compared with nearly double the odds in men. Huxley speculated physiological or behavioural differences could explain the gender gap.

The research was funded by Cancer Research UK and Medical Research Council.