Think twice before lighting up during your coffee break because it could do more damage to your heart than either vice alone, according to researchers.
The authors advise smokers quit
A Greek team Athens Medical School found smoking and caffeine acted together to have a harmful effect on arteries and blood flow.
Scientists have suspected the two might work synergistically rather than having just an additive effect on the heart.
The findings appear in the Journal of the American College of Cardiology.
There is already a large body of evidence about the heart risks associated with smoking.
Some studies have found caffeine can have a detrimental effect on the heart.
Increasingly, researchers have been looking at whether a combination of the two might be even more damaging to the heart.
The current research had two parts - an acute study of the immediate effects of smoking and caffeine in 24 people, and a study that looked at the long-term consequences in 160 people.
In each study, the researchers evaluated the stiffness of the aorta, the body's main artery leading from the heart, and the way blood flowed in the arteries.
These factors give an indication of cardiovascular disease risk.
Smoking and caffeine separately had a negative effect on aortic stiffness and blood flow.
In the acute study, the final impact of the two stimuli was larger than the sum of the separate impacts of the two stimuli alone.
The researchers said other studies had shown smoking and caffeine combined together had an unfavourable effect on blood pressure and increased the risk of heart attacks.
Kick the habit
But they added: "The underlying mechanism at the basic level responsible for the interactive effect of the two is unclear at this stage.
"Given the frequent combination of smoking and caffeine intake, these effects on arterial function may have important implications," they said.
Lead author Dr Charalambos Vlachopoulos said: "For individuals who have not yet quit smoking, it is advisable not to smoke while consuming coffee or other caffeine-containing beverages, as very frequently is the case.
"A healthy heart may be able to compensate, at least in the short run, for such unfavourable conditions, but for patients with impaired cardiac function these effects may be deleterious."
June Davidson of the British Heart Foundation said:
"Large volumes of research have confirmed the deeply detrimental effect that smoking has on cardiovascular health.
"Previous studies have also suggested that there may be a link between drinking lots of coffee and atherosclerosis - 'furring' of the coronary arteries.
"Therefore, it is possible that the combined effect combining smoking and caffeine may lead to exacerbated heart health problems."
She said stopping smoking was the single most important thing someone can do to avoid developing heart disease.
Amanda Salford from Action on Smoking and Health agreed.
"If people consume large amounts of coffee and smoke then they might want to consider what sort of impact that might have on the heart.
"Quitting smoking has got to be the number one priority."
A spokeswoman from the British Coffee Association said: "The level of caffeine used was equivalent to two to three cups of coffee which the general UK population would be unlikely to consumer in one sitting.
"The researchers themselves are also unclear as to the mechanisms involved and indeed state 'our study refers to young, apparently healthy adults and results might not be directly extendable to other population groups'.
"Taken in context with the overwhelming scientific evidence it does not change the overall position that coffee drinking in moderation, four-five cups per day, is perfectly safe for the general population and may confer health benefits.
"This position is supported by the British Heart Foundation."
Simon Clark, director of the smokers' lobby group FOREST, said: "Cigarettes and coffee are a classic combination, like gin and tonic or cheese and pickle.
"A smoker without caffeine is like Posh without Becks. A common sense approach should be everything in moderation, nothing in excess."