The reason statins can make your muscles sore or weak was unclear—until scientists accidentally stumbled upon an answer.

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.

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