The Take That star confessed to receiving the sex drive-boosting hormone after originally asking Los Angeles doctors for human growth hormone.
"To cut a long story short I went to get some HGH. It's what all the old fellas are on out there in LA that's making them look 40 instead of 60," Williams told Esquire magazine. "It's improving their health, their memory, their hair, skin.
"Could give you cancer. I weighed that up. Thought I'd have it anyway. Went to see a Hollywood doctor. Had my blood tests. Went back. He said, 'You don't need HGH. You've got the testosterone of a 100-year-old man'."
Williams, who has been injecting himself for two years now, said: "And then everything made sense. It was kind of an epiphany that day. It has changed my life. I feel I'm getting a second wind."
The 37-year-old is currently on tour with Take That, after rejoining the 'Love Love' group last year.
Discussing bandmates Gary Barlow, Mark Owen, Jason Orange and Howard Donald, Williams quipped that he is only the "third weirdest member" of the man-band.
> Take That 'to holiday together after Progress tour'
Watch Take That's video for 'Love Love' below: