NEW ORLEANS — Men who take antidepressants are more likely to have thickening of the arteries and higher heart and stroke risks than those who do not, said a study of middle-aged male twins.

The difference translates to about a four-year age gap, making the twin taking the pills physically older than the twin who does not, said the study presented at a major cardiology conference in New Orleans on Saturday.

The study is the first to examine the link between vascular disease and antidepressant use, and looked at 513 twins from the Vietnam Era Twin Registry, the authors said. The average age of the subjects was 55.

Researchers found that the inner lining of the carotid artery, which supplies oxygen to the brain, was five percent thicker in men who took antidepressants compared to their twin brothers who did not.

"There is a clear association between increased intima-media thickness (IMT) and taking an antidepressant, and this trend is even stronger when we look at people who are on these medications and are more depressed," said Amit Shah, a cardiology fellow at Emory University in Atlanta, Georgia.

The study said antidepressant use was associated with a 37 micron increase in carotid IMT, or about five percent.

Previous research has suggested each additional year of life is linked to a 10 micron increase in IMT, and each 10 micron jump is linked to a 1.8 increased risk of heart attack or stroke.

Antidepressants raise the level of chemical messengers like norepinephrine and serotonin, which may have the negative effect of restricting blood vessels, though more research is needed to determine exactly why the difference was observed, the study's main author said.

"Because we didn't see an association between the depression itself and a thickening of the carotid artery, it strengthens the argument that it is more likely the antidepressants than the actual depression that could be behind the association," said Shah.