The TIME 100

In some ways, Steve Nissen's undergraduate years at the University of Michigan—which included crusading against the Vietnam War—were the toughest period of his life. But his extracurricular activism was just the training required for his work today: a health crusader who devotes his time equally to developing medical protocols that can keep people well and exposing ones that don't.

Nissen, 58, a heart specialist at the Cleveland Clinic, started his climb to medical prominence in 1987 when he recognized that newer ultrasound imaging devices could be miniaturized and threaded into the beating heart to reveal the exact composition of plaques representing the early stages of artery damage. Once he could characterize plaques and measure their size, he could evaluate how well anticholesterol medications were doing their job. Drugmakers soon came his way, figuring they could save millions of dollars in product development.

Nissen, however, used this and other techniques to take a hard look at drugs that were already on the market or being considered for it. He led the effort that linked COX-2 inhibitors such as Vioxx and Celebrex with heart attacks. He probed FDA data on a blockbuster diabetes drug that was likely being approved and published his findings that the product carried high cardiovascular risks. The drug has not been okayed.

It's not always easy to be this brash with industry, but Nissen's intravascular ultrasound lab has become so powerful that it lures industry leaders back—even if the service they receive sometimes includes more brutal honesty than they want. The activist continues to crusade, but these days lots of folks are listening.

Oz is a professor at New York Presbyterian/Columbia University