Reese Witherspoon says film 'Wild' hardest of career

Oscar-winning actress Reese Witherspoon said her new film "Wild" was the hardest role of her career, at the biopic's premiere at the London Film Festival on Monday.

Witherspoon plays the role of Cheryl Strayed, a woman who trecked across a vast tract of wilderness in the United States for three months to recover from the death of her mother, a marriage breakup and heroin abuse.

"It was more challenging in a way that I've never had before," Witherspoon said.

"It's definitely the hardest movie I've ever made, physically, emotionally, mentally, but when I got through it I was so happy."

Initially, Witherspoon had carried a backpack filled with newspaper for filming, but this was replaced by a bag packed with weights by Jean-Marc Vallee to make her walk more realistically.

"I've never been as strong as I was after that movie. I had some back problems."

The film also depicts drug taking and explicit sex scenes, which Witherspoon admitted she struggled with.

The actress, who also produced the film, was joined in London by the real life Strayed, whose memoir was adapted by novelist Nick Hornby for the film.

The film was highly acclaimed at the Toronto Film Festival and has been tipped for Oscar success.

The 38-year-old won the Oscar for best actress in 2006 for her role in another biopic, "Walk the Line", the tale of US singer-songwriter Johnny Cash.