Gynecomastia is estimated to affect anywhere from 30 to 70 per cent of males at some stage of their lives. And "gyne" or "gyno", as it's commonly known, is not only an unwelcome (and ironic) side-effect of anabolic-androgenic - that is, muscle-building and masculinising - steroids: it can also be caused by other hormonal disruptors such as alcohol, cigarettes, cannabis, obesity, puberty (usually temporarily) and old age. But [Tom] Powell says his gyno was "99 percent" likely due to steroids.
Source: wiktionary