Just one cigarette could be enough to trigger a fatal heart attack, doctors have found.
Every time a smoker lights up, the risk of a seizure over the following hours rises, according to the research.
For a single cigarette can tip the balance within the body, leading to a blood clot large enough to stop the heart.
Doctors have known for years that long-term smokers are more likely to suffer heart attacks, but the study is the first to show there is an immediate effect.
Researchers found that heart attack victims who had smoked a cigarette six hours before their seizure had bigger blood clots in their arteries.
While small clots may have no effect or result in only a minor attack, large clots can kill by choking off blood from the heart.
'We know that the long-term effects of smoking raise the risk of cardiovascular disease,' Dr Murray Mittelman, of the Beth Israel Deaconess Medical Centre in Boston told an American medical conference yesterday.
'This study shows a short-term association with the formation of bigger clots that may increase the risk or severity of the heart attack.
'Put simply, smoking a cigarette can enlarge the blood clot and tip the balance.'
Dr Mittelman's team studied 902 patients who had suffered heart attacks and were given angiograms to show up any blockages in the arteries, which were then cleared.
After the operations, the patients were asked if they smoked and if so, how long before the heart attack symptoms they last had a cigarette.
Doctors found a correlation between blood clot size and the amount of time since the last cigarette smoked, although they do not know why smoking had this immediate result.
The average clot size of those who smoked within six hours of the attack was 23sq mm, while the clots in those who had smoked between six and 24 hours before the symptoms were on average half that size.
Smokers who had their last cigarette more than a day before the heart attack had the smallest clot, at 6.7sq mm.
Clive Bates, director of the anti-smoking charity ASH, said: 'The message has got to be that one cigarette can kill you, and that giving up will give you immediate health benefits and may even save your life.'
With more than 270,000 Britons suffering a heart attack every year, and 20 per cent of heart attack deaths smoking related, a spokesman for the British Heart Foundation said: 'Smoking is the biggest cause of preventable disease and premature death in the UK.
'This new study could well open up a new avenue of research', reports, Mail Online.