Apr 14 (Reuters) - Johnson & Johnson on Thursday said it will soon send letters to thousands of US physicians advising them of possible increased risk of stroke among elderly patients taking its blockbuster antipsychotic drug Risperdal.

A J&J spokeswoman told Reuters the company also plans to change the package insert label of the pill, which has annual global sales of $2.1 billion, to note the possible stroke risk.

"An update to the Risperdal label is indeed being made, and we will be sending out letters to health care professionals soon."

The diversified healthcare giant last October sent a similar warning letter to Canadian doctors and pharmacists, citing 37 reports of stroke or stroke-like events, including 16 deaths, among patients who have taken its drug.

Moreover, the company 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."

J&J noted in the Canadian warning letter, however, that the elderly are generally at increased risk of stroke.

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

Risperdal is J&J'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 drugs already include information in their labels about strokes seen in patients taking them in either clinical trials or after the drugs reached the market.

Risperdal's label will be changed, however, to include more specific information about strokes in the elderly.

Larry Sasich, a pharmacist and research analyst for consumer watchdog group Public Citizen, said worrisome safety trends have cropped up in various clinical trials that tested Risperdal in Alzheimer's patients.

He said 29 cases of stroke and stroke- related events were seen among 764 patients tested in four specific trials, or in about 4 percent of patients, compared with only 2 percent of those who received placebos.

"And there were four deaths among patients taking Risperdal, compared with only one death in those taking placebos," Sasich said.

"The Risperdal label clearly states that there is no evidence this drug is safe or effective in treating dementia, and it looks like doctors are hurting people by prescribing it for this condition," Sasich said.

Sasich said the incidence of stroke among elderly Alzheimer's patients should spur U.S. regulators to further examine whether younger schizophrenia patients are also unacceptably prone to them.

Mario Corso, a drug analyst for Leerink Swann & Co., said he does not expect the label change to badly hurt the drug's sales.

"We do not expect a major change in prescribing behavior, as these patients are very sick, with confounding factors of cardiovascular disease and vascular dementia in many cases, making the drug effects difficult to decipher," Corso said in a research report.