Oral sex may create an environment for a common vaginal condition called bacterial vaginosis or BV, according to research in the journal PLoS Biology.
BBC reported that BV is not a sexually transmitted infection. It is an imbalance of the usual bacteria found in the vagina.
Women who have it may have no symptoms, but some get a strong-smelling discharge.
BV is not usually serious, but should be treated because having BV makes women more vulnerable to catching sexually transmitted diseases and getting urinary infections.
If the woman is pregnant, it increases the risk of premature birth.
The study in PLoS Biology showed how a common type of bacteria found in the mouth that is linked with gum disease and dental plaque may support BV.
They did experiments in human vaginal specimens and in mice to look at bacterial behaviour.
The mouth bacterium, Fusobacterium nucleatum, appeared to aid the growth of other bacteria implicated in BV.
The researchers, Dr Amanda Lewis from the University of California and colleagues, say the findings show how oral sex might contribute to some cases of BV.
Experts already know that BV can be triggered by sex, including between women.
Prof Claudia Estcourt, spokesperson for the British Association for Sexual Health and HIV, said research such as this was important to add to the understanding of BV.
“We know BV is a really complex entity with lots of contributing factors.”
She said oral sex could pass on sexually transmitted infections and other bacteria that may or may not be important in other health conditions.