The 37-year-old Take That singer thought getting a boost of Human Growth Hormone (HGH) would help fight his feelings of lethargy and make him healthier but was told by medical experts his problems stemmed from a lack of the hormone - which boosts a man`s sex drive - and says the supplementary injections have changed his life.
He told Britain`s Esquire magazine: "I went to the doctor to get some Human Growth Hormone (HGH).
"It`s what all the old fellas are on out there in Los Angeles that`s making them look 40 instead of 60.
"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.`
"And then everything made sense. It was kind of an epiphany that day."
Robbie married US actress Ayda Field last August and insists he will always remain faithful to her - though he admits he could be "tempted" to cheat.
When asked if he could ever see himself cheating on Ayda, he replied: "Actually, no, because I don`t want to break Ayda`s heart. That`s the last thing I want to do. I met Ayda, I fell in love, that`s what happened."
The `Angels` hitmaker also spoke of his belief that people naturally want to cheat and branded marriage "made-up bull".
He said: "The rules aren`t set up right. Because people are f***ing outside of their marriages, outside of their relationships. I think that means we`re built to f***.
"And marriage, that whole institution, is made-up bull."