The new finding disproves the suggestion that folic acid supplements could reduce in the blood the level of an amino acid linked to a higher risk of heart attack, known as homocysteine.
During the study it was observed that though homocysteine levels did fall by 30 per cent after a year of treatment with folic acid and B12, there was no corresponding fall in heart attacks or strokes.
The researcher behind the study said that in the group given folic acid, there was a decline in strokes, but an increase in cancer, though neither was significant.
Published in the Journal of the American Medical Association, the study involved more than 3,000 patients in two Norwegian hospitals between 1999 and 2006.
The subjects were administered folic acid plus vitamin B12 and vitamin B6, or folic acid plus B12, B6 alone, or a placebo.
"Our findings do not support the use of B vitamins as secondary prevention in patients with coronary artery disease," Times Online quoted the research team as saying.
Since folic acid fortification of flour began in the US and Canada ten years ago, deaths from stroke have fallen faster than in England and Wales, where fortification has been discussed endlessly but never implemented.