Niacin is vitamin B3, a water soluble vitamin that helps regulate many key metabolic processes. Niacin has been studied for its role in reducing the risk of heart disease and lowering LDL ("bad") cholesterol. It is also a very potent agent for raising HDL (good") cholesterol. When used for this purpose, niacin has to be taken at doses much higher than found in daily multivitamins and functions as a drug. Because of this, it should be used at these dosages only under a doctor's supervision.
Hemorrhagic strokes stem from a ruptured blood vessel in the brain. The break in circulation interferes with normal blood flow and causes bleeding into an area of the brain. These strokes can result from hypertension, which increases pressure on arterial walls that may already be damaged by arteriosclerosis. A ruptured aneurysm (a weakened bulge in the wall of an artery) can also cause a hemorrhagic stroke, and risk is increased in individuals whose blood has been thinned by anticoagulant medications such as Coumadin and aspirin.
I discussed your question about whether niacin (vitmain B3) might play a role in hemorrhagic stroke with Stephen R. Devries, M.D., preventive cardiologist and associate professor of medicine at Northwestern Memorial Hospital in Chicago. He said that to his knowledge, no scientific data links niacin to hemorrhagic strokes. He noted that a recent clinical trial tested whether niacin added to statin therapy would work better than a statin alone in reducing long-term cardiovascular events in certain individuals. Participants in this trial had a history of cardiovascular disease, low HDL, high triglycerides; their LDL was under control. The trial was stopped early because no apparent benefit was seen and because of an increased risk of ischemic stroke (not hemorrhagic stroke) in those taking niacin plus a statin. But even here there are serious questions about whether niacin really was a contributing factor since some participants had stopped taking it as long as four years before their strokes.
Here's more information on the causes and treatment of stroke.
Andrew Weil, M.D.