Testosterone replacement therapy is popular. In men with a deficiency, injections should provide pep, a stronger libido and lower anxiety. Now many with normal levels hope to achieve the same benefits, as well as slower ageing, even though the jabs are unproven and risk side-effects including infertility. Adherents include Joe Rogan, a podcaster; Robert F. Kennedy junior, America’s health secretary; and Robbie Williams, a singer portrayed as a monkey in a recent biopic.