Testosterone levels in men can go up and down throughout the day. Katherine Streeter for NPR hide caption

itoggle caption Katherine Streeter for NPR

Katherine Streeter for NPR Katherine Streeter for NPR hide caption

itoggle caption Katherine Streeter for NPR

Some men take testosterone hoping to boost energy and libido, or to build strength. But at what risk? iStockphoto hide caption

itoggle caption iStockphoto

Prescriptions for testosterone medications, such as Testim and AndroGel, to men 40 and older rose threefold over the decade ending in 2011 MICHAEL BRYANT/MCT/Landov hide caption

itoggle caption MICHAEL BRYANT/MCT/Landov