Scientists say women could end up living for 10 years longer, if they gave up smoking early enough.
A new study of 1.3 million women found that those who kick the habit at 30 can avoid 97% of their excess risk of premature death.
The study found that smokers had three times the chances of dying over nine years compared with non-smokers, mostly due to smoking-related diseases such as lung cancer, chronic lung disease, heart disease or stroke.
The risk rose steeply with the quantity of tobacco smoked, but even light smokers doubled their likelihood of dying.
Former lung cancer patient June Atherton smoked until she was 50 and said she wishes she had given up sooner.
"I have actually stopped people in the street and said to them, 'You're such a beautiful girl, please don't smoke'," she said.
"I'm sure they thing, 'Wow, crazy lady', but it is such an important message."
The authors of the Million Women Study wrote in The Lancet medical journal: "Smokers lose at least 10 years of lifespan.
"Although the hazards of smoking until age 40 years and then stopping are substantial, the hazards of continuing are 10 times greater."
Stopping before age 40 years avoids more than 90% of the excess mortality caused by continuing smoking; stopping before age 30 years avoids more than 97% of it.
The study, which was conducted in the UK, also concluded that two thirds of all deaths of smokers in their 50s, 60s, and 70s are caused by smoking.
You can see the full study on The Lancet website here (requires registration, but is free to read.)
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