The study discovered that about a third of the patients (34.5 per cent) taking the vitamin B supplements experienced significant reclosing of arteries, compared to only 26.5 percent taking a placebo.
The findings contradict research published in 2002 , in which researchers from the UK, China and Australia concluded that folic acid supplements could help keep the arteries healthy and protect against heart disease.
The researchers purported, two years ago, to have discovered that folic acid supplementation could help reduce homocysteine - an amino acid known to influence the narrowing of the arteries levels - and keep arteries clear.
In this study, the volunteers were given a daily dose of 10 milligrams of folic acid for a year. An ultrasound scan was used to test the function of their arteries. The study found that taking folic acid supplements reduced homocysteine levels by 12 percent over the course of a year and improved endothelial function.
The most recent study, by German and Dutch researchers, suggested quite the opposite. The scientists studied 636 patients who underwent angioplasty - opening the clogged arteries with a small balloon - before being treated with stents, small metal scaffolds designed to prop open the arteries.
High rates of "restenosis," in which arteries close up again, meant patients had to go back for repeat surgery. Of those taking vitamins, 15.8 per cent had to have repeat angioplasties, compared with only 10.6 percent of those taking the placebo.
The latest results "raise the disturbing possibility that a therapy has previously been considered safe may actually be harmful," University of Pennsylvania cardiologist Howard Herrmann concluded in an editorial accompanying the new study.