A fifth of athletes surveyed said their oral health actually damaged their training and performance.
Researchers said athletes, as a group, had worse dental health than other people of a similar age.
The beaming smiles of gold-medal winners Usain Bolt, Jessica Ennis-Hill and Mo Farah are some of the defining memories of London 2012.
But a team at University College London says many competitors had dental problems.
"Our data and other studies suggest that, for a similar age profile, the oral health of athletes is poor. It's quite striking," said lead researcher Prof Ian Needleman.
He said eating large amounts of carbohydrates regularly, including sugary energy drinks, was damaging teeth.
He added that the stress on the immune system from intense training may leave athletes at risk of oral disease and that a fixation on training, preparation and other aspects of health may leave little time or awareness of oral health.
The study looked at those visiting the dental clinic at the Games, which offered free check-ups and mouth guards.
Competitors using the clinic may have been more likely to have dental problems than other athletes, but the research group say their findings are consistent with previous studies.
Of the 302 athletes examined, from 25 sports, 55% had evidence of cavities, 45% had tooth erosion and 76% had gum disease.
One in three said their oral health affected their quality of life and one in five said it affected training or athletic performance.
Teeth to training
Oral health is already a suspect in other seemingly unrelated conditions such as heart disease. People who do not brush twice a day are at higher risk of a heart attack and inflammation is common to both.
The researchers suggest inflammation elsewhere in the body may also affect recovery time and susceptibility to injury.
They added that tooth pain and the resulting impact on diet and sleep may also damage performance.
Prof Needleman, who is also director of the International Centre for Evidence-Based Oral Health, told the BBC: "We know the differences at the high end of elite sport are small, it would not be surprising if oral health was having an impact on those differences.
"Many sports medics have anecdotes about athletes missing medals at major competitions as a result of oral health problems."
