Popularised by celebrities like Kim Kardashian, Jennifer Hudson and Kelly Clarkson, Dr Siegal's Cookie Diet recommends one "real" meal a day, plus six of the specially-formulated cookies.
The ingredients include milk, eggs, sugar, wheat and a "secret amino acid protein mix" believed to curb hunger, reports The Daily Telegraph.
But, according to dietician Susie Burrell, the program could be most dangerous.
"It is not helping people to get over that need for sweet by saying you can eat cookies and eat healthy," Burrell said.
According to her, meal replacement programs were nutritionally limited and unsustainable.
"The key thing is it is not re-programming eating habits. As soon as you go off the cookies, you haven't learnt how to eat for fat loss," she said.
She said similar diets had been proven to work, but only while the person continued with the regime.
"If it is being followed, initially you generally see quite good results. Calories are being controlled and there are no extras slipping in," she said.
"The issue is people can't stick to them for long periods of time. As soon as they go back to eating normal food it is actually harder to lose weight than before. because their calories have generally been controlled for a period, they have suffered a drop in metabolic rate," she added.