Taking B vitamins to ward off heart attacks and stroke does no good and may even be harmful, say experts.
The supplements are safe to take when recommended
Scientists had thought that these drugs might be useful by lowering levels of a blood substance called homocysteine which has been linked heart risk.
However, a large study looking at this has found no benefit even though homocysteine went down with these supplement pills.
The work was revealed in Stockholm at a European Society of Cardiology meeting.
The Norwegian Vitamin Trial (NORVIT) researchers from the University of Tromsø looked at 4,749 heart attack survivors who had been divided into four groups.
In addition to their standard heart medicines, the groups received either daily folic acid (itself a B vitamin), daily vitamin B6, both folic acid and vitamin B6 or a dummy drug for three years.
After three and a half years, those who had been taking either folic acid or vitamin B6 alone had only a small increase in the risk of cardiovascular disease (heart attack or stroke), compared with those who had received the placebo.
However, those who had taken both folic acid and vitamin B6 each day had a 20% increased risk of heart attack and stroke, despite their homocysteine levels going down by up to 30%.
The results also showed there was a 40% increase in the risk of new cancers in the group taking folic acid, which the researchers said warranted further investigation.
Author Professor Kaare Harald Bønaa said: "The results of the NORVIT trial are important because they tell doctors that prescribing high doses of B vitamins will not prevent heart disease or stroke.
"B vitamins should be prescribed only to patients who have B vitamin deficiency."
Professor Peter Weissberg, medical director of the British Heart Foundation, said: "People should not be taking folic acid and vitamin B6 to stop them having a heart attack because it won't.
"The study shows a significant increase in heart attacks and strokes."
However, he said there was no reason for pregnant women and those hoping to conceive to stop taking folic acid by itself. Folic acid is recommended for such women to reduce the risk of birth defects.
Patrick Holford, founder of the Institute for Optimum Nutrition, questioned the validity of the study findings.
"Given the extensive cocktail of drugs these patients were on, and the late stage they were in the disease process, it's unlikely there was much room for improvement.
"We are still awaiting the definitive trial that takes people with high blood homocysteine levels, which is the indicator of B vitamin need, gives them B16, B12 and folic acid, and measures the reduction in heart attacks or strokes."