Johnson & Johnson said yesterday that it will soon send letters to thousands of U.S. physicians advising them of a possible increased risk of stroke among elderly patients taking its popular anti-psychotic drug Risperdal.

A spokeswoman said the company also plans to change the package-insert label of the pill, which generates annual global sales of $2.1 billion, to note the possible stroke risk.

The diversified health care company sent a similar warning letter to Canadian doctors and pharmacists in October. It cited 37 reports of stroke or related events such as blood clots and hemorrhages, including 16 deaths, among patients who have taken its drug.

The company also cited two clinical trials of elderly dementia patients in which "a higher proportion of patients taking Risperdal experienced strokes or related events than those who received placebo [sugar pills]."

The firm, however, noted in the Canadian letter that the elderly are generally at increased risk of stroke.

Shares of Johnson & Johnson closed down 5 cents, at $57.21, on the New York Stock Exchange.

Risperdal is the company's second-biggest-selling medicine. Although only approved for schizophrenia, it is widely used to control behavioral disorders in elderly patients with dementia and Alzheimer's disease -- such as delusions, aggression and anxiety.

Risperdal and rival schizophrenia medications already include information in their labels about strokes seen in patients taking them either in clinical trials or after the drugs reached the market. Risperdal's label will be changed, however, to include more specific information.