Oral sex can cause a vaginal problem called bacterial vaginosis or BV, according to research in the journal PLoS Biology.
BV is not a sexually transmitted infection.
This is an imbalance of bacteria commonly found in the vagina.
Women who experience it may have no symptoms, but some have a strong-smelling discharge.
The researchers investigated what effect oral bacteria have on the microbes that live and grow in the vagina.
BV is usually not serious, but it must be treated because BV makes women more susceptible to contracting sexually transmitted diseases and urinary tract infections.
If the woman is pregnant, it increases the risk of preterm birth.
This is very common and women experiencing it may have an unusual discharge that has a strong fishy smell.
You may notice a change in the color and consistency of your fluid, such as turning grayish-white, watery, and watery.
Your GP or sexual health clinic can arrange a swab to see if you have BV.
If the result is positive, it can be treated with antibiotic tablets, gel or cream.
Women without BV tend to have lots of “good” bacteria called lactobacilli which keep the vagina more acidic, with a lower pH.
Sometimes this healthy balance can change and cause other vaginal micro-organisms to develop.
The oral bacterium, Fusobacterium nucleatum, appears to aid the growth of other bacteria involved in BV.
The researchers, Dr Amanda Lewis of the University of California and colleagues, said the findings show how oral sex can 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 like this is important to increase understanding of BV.
He said oral sex can transmit sexually transmitted infections and other bacteria that may or may not be essential in other health conditions.
