Actor Ben Foster has revealed he took performance-enhancing drugs while preparing to play disgraced seven-time Tour de France winner Lance Armstrong.

The biopic about the high-profile drug cheat premiered at the Toronto Film Festival on Sunday (local time).

Foster, whose colleague described him as an “immersive actor”, said the drug taking was done under strict supervision.

“Doping is not the reason why Lance Armstrong won but it did shape a culture at the time, and it certainly shaped him, and I wanted to understand — on a personal level, on a cellular level — what that experience is like,” Foster told journalists in Toronto.

“I took a calculated risk, with a doctor supervising, and what I came away with is that drugs work — they work,” he said.

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However, he said there were obvious downsides to doping – especially for a part in a movie.

“Coming off those drugs is the difficult part. That’s where your health concerns will come in.

“And it took a while for me to right myself. But that was a calculated risk and part of the joy of the job, I suppose.”

While Foster didn’t reveal what type of performance-enhancing drugs he took, fellow actor Chris O’Dowd who portrays Irish journalist David Walsh said he wasn’t surprised by the revelation.

“In a way I’m not surprised, he’s a very immersive actor, very focused and committed,” O’Dowd said.

“It hadn’t struck me that he would, but of course that makes perfect sense.”