A leading British dentist has said there is only "weak evidence" that flossing prevents gum disease and cavities - despite it being recommended by most in the profession.
Professor Damien Walmsley, 58, of Birmingham University, said the time and expense required for reliable studies meant the health claims often attributed to floss were unproven.
Prof Walmsley, who is also a scientific adviser to the British Dental Association, said: "The difficulty is trying to get good evidence. People are different and large studies are costly to do ... until then you can't really say yes or no."
He added "more sophisticated trials" were needed.
Earlier this year, the US government dropped the flossing recommendation from its guidelines because they must be legally based on scientific evidence.
A Public Health England (PHE) spokeswoman said it keeps "abreast of the evidence base and will consider these findings".
She said: "Some people may not have large enough spaces in between their teeth to use an inter-dental brush, so flossing can be a useful alternative.
"Patients should speak to their dentist if they have any concerns."
An Associated Press investigation looked at the most rigorous research of the past decade. Twenty-five studies in leading journals found evidence for flossing is "weak, very unreliable", of "very low" quality, and carries "a moderate to large potential for bias".
One review conducted last year said: "The majority of available studies fail to demonstrate that flossing is generally effective in plaque removal."
Another 2015 review cites "inconsistent/weak evidence" for flossing and a "lack of efficacy".
One study did credit floss with a slight reduction in gum inflammation. However, the reviewers ranked the evidence as "very unreliable".
Floss can occasionally cause harm, with poor technique leading to damaged gums and teeth and also dislodging bad bacteria, which can lead to infections.
The British Dental Association said: "Small inter-dental brushes are best for cleaning the area in between the teeth, where there is space to do so.
"Floss is of little value unless the spaces between your teeth are too tight for the interdental brushes to fit without hurting or causing harm."
Dentist Levi Spear Parmly is credited with inventing floss in the early 19th century. By the time the first floss patent was issued, in 1874, the applicant noted that dentists were widely recommending its use.