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 study statins. They weren’t studying cholesterol at all. They were hunting for genes behind a rare disease called limb girdle muscle dystrophy, in which muscles of the upper arms and legs—sound familiar?—become weak and waste away. After both teams tracked the disease through a handful of families in the U.S. and a Bedouin family in Israel, their suspicions separately landed on mutations in a gene encoding a particularly intriguing enzyme.

The enzyme is known as HMG-CoA reductase, and to doctors, it is not obscure. It is, in fact, the very enzyme that statins block in the process of halting cholesterol production. And so, the answers to two mysteries suddenly became clear at once: Dysfunction in this enzyme causes muscle weakness from both limb girdle muscular dystrophy and statins.

This connection between a rare disease and a common drug stunned the researchers. “It seemed too good to be true,” says Joel Morales-Rosado, a pathologist who worked on one of the studies as a postdoctoral researcher at the Mayo Clinic. “One of the first things you learn in medical school is association between statins and myopathy.” Now the answer as to why— along with…

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