by Sari Harrar for O, The Oprah Magazine

About 20 years ago, scientists realized that testing drugs and medical procedures primarily on men told them little about how a woman might react. But animal researchers have just recently begun to conclude that using only males means they’re getting only half an answer.

“Seventy-nine percent of pain studies are on male lab animals only, even though far more women have problems with pain,” says Jeffrey Mogil, PhD, professor of pain studies at McGill University and a premier sex-difference researcher in the field of pain genetics. “We’re missing valuable information because so many researchers don’t look at both sexes when studying lab animals.”

“Sex matters in ways we haven’t really thought about before,” says Virginia M. Miller, PhD, a professor of physiology at the Mayo Clinic in Rochester, Minnesota. One of Miller’s lab studies (with female pigs) unearthed a possible reason why premenopausal women get little or no heart protection from low-dose aspirin: The linings of female arteries produce a mix of chemicals that don’t respond to aspirin the way males’ do. This helps explain why the conventional wisdom about aspirin—that it protects everyone’s heart from dangerous blood clots—was off the mark.