Today on the Oprah Winfrey show: balderdash, superstition and a dose of iffy medical advice from some New Age healthcare "gurus" who certainly haven't been recommended by your doctor.

The world's most influential chat show host has been thrust to the centre of a heated debate, following allegations that she has abused her legendary influence to help peddle alternative treatments that are ineffective, expensive and dangerous.

Ms Winfrey, below, whose advice on fashion, literature and pretty much everything else is treated like gospel by tens of millions of Americans, was last week forced to speak out against criticism of the doctors and health "experts" who preach from her billion-dollar sofa.

In a lengthy cover story headlined "Crazy talk: Oprah, wacky cures, and you", Newsweek dissected the credentials of Winfrey's favourite talking heads, accusing her of failing to differentiate between bona fide medical professionals and opinionated imposters who simply "gush nonsense." The 6,000 word article claimed guests have offered questionable endorsements of ineffective new plastic surgery techniques, unproven hormone therapies and dangerous cancer "cures".

Among those named was the actress Suzanne Somers, 62, who in January was allowed to sing the praises of the 60 different dietary supplements, together with various "bioidentical hormones", she takes each day to fight ageing. Most have no proven benefits. Also mentioned was Jenny McCarthy, the actress and partner of Jim Carrey, who used a recent appearance on the programme to claim that the MMR vaccine had caused her son's autism. Despite overwhelming scientific evidence that there is no such link, McCarthy's comments passed "virtually unchallenged", Newsweek's article said.

Most bizarrely of all, Winfrey allowed a physician called Christiane Northrup to claim – in contradiction to almost all scientific evidence – that "in many women, thyroid dysfunction develops because of an energy blockage in the throat region [after] a lifetime of 'swallowing' words one is aching to say".

On Thursday, amid mounting controversy, Winfrey issued a statement to the TV show Entertainment Tonight. It failed to address any of Newsweek's individual concerns, but instead claimed her audience was educated enough to make their own decisions about healthcare. "For 23 years, my show has presented thousands of topics that reflect the human experience, including doctors' medical advice and personal health stories that have prompted conversations between our audience members and healthcare providers," it read. "I trust viewers, and know that they are smart and discerning enough to seek out medical opinions to determine what may be best for them."

That line of argument is unlikely to cut much ice with scientific experts, though. Winfrey's personal "brand" relies firmly on her ability to inspire trust and tap into the hearts, minds and wallets of Middle America. "It's about time one of the big media players pointed out that she has been promoting fake therapies," said P Z Myers, a prominent scientific commentator and associate professor at the University of Minnesota. Despite Winfrey's "message of positive self-esteem for women", he described the theories some of her guests have advanced as "credulous glop".

The controversy comes at an unwelcome time for Winfrey, whose business empire has suffered during the recent financial downturn. The circulation of her magazine, O, has fallen by around 10 per cent, while her afternoon TV show is currently drawing just over six million viewers, down from more than nine million in 2004. Although her Chicago-based business, Harpo, remains highly profitable (she made an estimated $275m last year) some commentators wonder whether, at 55, Winfrey can carry on for ever.

In a further sign of the times, Forbes magazine last week announced that, for the first time in recent years, it had dethroned Winfrey from the number-one spot on its "Power List" of the world's most influential celebrities, in favour of Angelina Jolie. "Oprah is still our biggest earner, but when it comes to fame, Angelina Jolie is hands down the most famous woman on the planet," said Macey Rose, a senior editor at the title.