So the question is, why?

"We know that there's differences in the way racial and ethnic groups handle drugs, for example. The way they metabolize drugs. There are genetic differences. There are important cultural and social differences."

Cultural and social differences, he says, may be reflected in a willingness to participate in a clinical trial or follow instructions if you do.

Combining the results of studies done in several countries can mask geographic differences. A drug that seems generally beneficial across the board can be very effective in one country, but nearly useless in another. And O'Connor says that could hurt patients.

"Once these trials suggest a benefit and get approved, they're going to be used by everybody across the globe," O'Connor says. "And what you don't want to have is a situation that you approve a drug or a device based on a global trial, and then realize five years later that, well, it didn't work in this part of the world or that part of the world."

O'Connor's study of how beta blockers vary in effectiveness in different geographic regions is published online by the Journal of the American College of Cardiology.