For decades health authorities and dentists have insisted that daily flossing is necessary to prevent cavities and gums so diseased that your teeth fall out.
But, as it turns out, all that flossing may be overrated.
New dietary guidelines for Americans, issued by the departments of Agriculture and Health and Human Services, quietly dropped any mention of flossing without notice.
A leading British dentist Prof Damien Walmsley, of Birmingham University, also said there was only "weak evidence" that flossing prevents gum disease and cavities — despite it being recommended by most in the profession.
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."
The Associated Press reported that officials had never researched the effectiveness of regular flossing, as required, before cajoling people to do it.
The American Academy of Periodontology on Tuesday acknowledged that most of the current evidence fell short because researchers had not been able to include enough participants or “examine gum health over a significant amount of time.”
The revelation has caused a stir among guilt-ridden citizens who strive to floss daily but fall short of that lofty goal. Among experts, however, it has been something of an open secret that flossing has not been shown to prevent cavities or severe periodontal disease.
"It is very surprising that you have two habits, flossing and toothbrushing without fluoride, which are widely believed to prevent cavities and tooth loss, and yet we don't have the randomised clinical trials to show they are effective," said Dr Philippe Hujoel, a professor of oral health sciences at the University of Washington in Seattle.
The American Dental Association’s website says flossing “is an essential part of taking care of your teeth and gums.”
Last year, Dr Edmond R Hewlett, a spokesman for the group and a professor of restorative dentistry at the University of California, Los Angeles, said: "We're confident that disturbing the bacteria in plaque with brushing and flossing is, indeed, beneficial."
Actually, that’s only half proven. Brushing with fluoride does prevent dental decay. That flossing has the same benefit is a hunch that has never been proved. If it’s any consolation, there is some mediocre evidence that flossing does reduce bloody gums and inflammation known as gingivitis.
Early gingivitis is a long way from severe periodontal disease. Still, some dentists argue that despite a lack of rigorous study, flossing matters if it can reverse initial gum problems.
“Gum inflammation progresses to periodontitis, which is bone loss, so the logic is if we can reduce gingivitis, we’ll reduce the progression to bone loss,” said Dr. Sebastian G. Ciancio, the chairman of the department of periodontology at the University at Buffalo.
Severe periodontal disease may take five to 20 years to develop. "It's a very insidious, slow, bone-melting disease," said Dr Wayne Aldredge, the president of the American Academy of Periodontology, who practices in New Jersey.
Those who quit flossing are “rolling the dice,” he said. “You don’t know if you’ll develop periodontal disease, and you can find out too late,” he said.
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.