A small amount of dark chocolate a day can thin the blood and cut the risk of clots in much the same way as taking aspirin, US researchers have said.
An alternative to aspirin?
Researchers carried out tests on 139 "chocoholics" who were disqualified from another study because they could not give up their habit.
Previous research has suggested that chocolate is good for the heart.
The study, by Johns Hopkins University, featured at an American Heart Association meeting in Chicago.
The Johns Hopkins team started out examining the effects of aspirin on platelets, the tiny solid particles in blood that clump together to form a clot.
However, 139 people who signed up for the study were disqualified because they were unable to give up eating chocolate as required.
Researchers have known for almost two decades that dark chocolate can lower blood pressure and has other beneficial effects on blood flow.
But the "chocoholic" group gave the Johns Hopkins team an ideal chance to probe why this might be a little further by carrying out a biochemical analysis.
They carried out tests comparing how long it took platelets taken from the "chocoholics" and others who had not eaten chocolate to clump together when they were run through a mechanical blood vessel system.
Platelets from those who stayed away from chocolate clotted faster, at 123 seconds, compared with 130 seconds for the chocolate group.
Researcher Professor Diane Becker said: "What these chocolate 'offenders' taught us is that the chemical in cocoa beans has a biochemical effect similar to aspirin in reducing platelet clumping, which can be fatal if a clot forms and blocks a blood vessel, causing a heart attack.
"Eating a little bit of chocolate or having a drink of hot cocoa as part of a regular diet is probably good for personal health, so long as people don't eat too much of it, and too much of the kind with lots of butter and sugar."
Professor Becker said two tablespoons of dark chocolate a day was enough to have a beneficial effect.
It is thought compounds called flavonoids, in which chocolate is rich, may be the key ingredient.
Vicky Evans, a cardiac nurse at the British Heart Foundation, said: "The chemicals in cocoa beans may have some benefits for our circulation but chocolate is more often part of the problem for heart health rather than a solution.
"This is particularly the case for milk chocolate which tends to be higher in fat and sugar.
"We are certainly not suggesting people totally avoid chocolate - everyone can enjoy a treat from time to time - but there are more effective ways of looking after your heart than using chocolate as a medicine.
"Eating five portions of fruit and vegetables a day is a far better way to get heart-protective flavonoids without having to worry about the fat and sugar packed into each chocolate bar."