Dentists agreed red skin, known medically as erythema, was likely to have been caused by performing oral sex.
In a bizarre case, a man was left with a circular-shaped mark in his mouth that doctors claim was caused by performing fellatio.
Dentists in Mexico stumbled across the 47-year-old's red lesion in the roof of his mouth during a routine check-up. Baffled as to what it could be, they quizzed the patient, who hasn't been identified and wasn't aware he had the mark.
They eventually agreed the red skin, known medically as erythema, was likely to have been caused by performing oral sex.
Medics from near Monterrey published their tale in BMJ.
Writing in the BMJ Case Reports, they claimed more dentists should be aware that fellatio can lead to oral lesions.
Explaining how it happened, they said: 'The contact of the palate with the penile glands may cause a haematoma due to blunt trauma.'
According to lead author of study, Dr Luis Alberto Mendez, the dilation of the blood vessels in the mouth caused by sucking also helped it to form.
The man presented to the School of Dentistry at the Universidad de Montemorelos after he noticed a filling had broken.
The dentists then examined the rest of his mouth and discovered a 'circular-shaped erythematous lesion'.
Upon questioning, the man admitted to being sexually active with men and having recently used marijuana and cocaine.
He also revealed that he regularly performs oral sex, with his last instance being just three days before his dental visit.
Writing in the journal, Dr Mendez and colleagues added that other causes of such lesions can include playing a wind instrument.