Updated from 9:52 a.m. EST

ATLANTA -- A clinical study has found that adding the blockbuster anticoagulant Plavix to aspirin isn't significantly better than aspirin on its own in lowering the risk of heart attacks, strokes or cardiovascular deaths.

Plavix, the second-biggest-selling drug in the world, is marketed by Sanofi-Aventis ( SNY) and Bristol-Myers Squibb ( BMY). Last year, sales of Plavix were nearly $6.3 billion, trailing only Lipitor from Pfizer ( PFE).

The results from the study, known as Charisma, don't affect Plavix's currently approved use as an anticlotting agent, but Bristol-Myers and Sanofi-Aventis had been hoping to gain expanded indications for the drug.

The trial found that combining aspirin and Plavix, also known as clopidogrel, didn't demonstrate a statistically significant reduction in the chance of a patient having a heart attack, stroke or death by a cardiovascular cause when compared with a placebo and aspirin. More than 15,000 patients took part.

" A broad population of patients with either established atherothrombotic disease or multiple risk factors for atherothrombotic events," were enrolled in the Charisma trial, according to Plavix's makers.

The results of the trial were presented at the American College of Cardiology meeting in Atlanta. The Charisma trial was coordinated by the Cleveland Clinic and was supported by grants from Sanofi-Aventis and Bristol-Myers.

For patients who had heart disease, adding clopidogrel to aspirin and other standard therapy reduced the relative risk of a recurrent heart attack, stroke or cardiovascular death by 12.5% vs. a placebo and aspirin, a result Bristol-Myers and Sanofi-Aventis said was statistically significant. Patients in this group accounted for nearly 80% of the trial's participants.

However, patients with multiple risk factors but no clearly established vascular disease didn't benefit from combining Plavix with aspirin. For these patients, the risk of death was greater with the combination treatment.

Severe bleeding occurred in 1.7% of the Plavix group, compared with 1.3% of the patients getting the placebo. Moderate bleeding was seen in 2.1% of the Plavix patients, compared with 1.3% in the placebo group.

"The rate of severe bleeding was not significantly greater with clopidogrel than with placebo, but a trend prompting concern was noted, and clopidogrel was associated with a significant increase in the rate of moderate bleeding," said Dr. Deepak Bhatt of the Cleveland Clinic, a lead researcher in the trial.

The data were also published in the New England Journal of Medicine. Shares of Bristol-Myers slipped 5 cents to $22.86. Sanofi-Aventis was up $1, or 2.3%, to $44.87.