America’s Most Popular Drug Has a Puzzling Side Effect. We Finally Know Why.

Statins, one of the most extensively studied drugs on the planet, taken by tens of millions of Americans alone, have long had a perplexing side effect. Many patients—some 5 percent in clinical trials, and up to 30 percent in observational studies—experience sore and achy muscles, especially in the upper arms and legs. A much smaller proportion, less than 1 percent, develop muscle weakness or myopathy severe enough that they find it hard to “climb stairs, get up from a sofa, get up from the toilet,” says Robert Rosenson, a cardiologist at Mount Sinai. He’s had patients fall on the street because they couldn’t lift their leg over a curb.

But why should an anticholesterol drug weaken muscles in the arms and legs? Recently, two groups of scientists stumbled upon an answer. They didn’t set out to and , their suspicions separately landed on mutations in a gene encoding a particularly intriguing enzyme.