The vaginal microbiome

A self regulating bacterial community that does a great deal of protective work. Understanding how it functions explains why most products aimed at cleaning or balancing it make things worse.

What is normally there

The vagina hosts a bacterial community that in most healthy people of reproductive age is dominated by species of lactobacillus. These bacteria metabolise glycogen from the vaginal lining, which is deposited there under the influence of oestrogen, and produce lactic acid as a result. The pH of the healthy vagina is consequently acidic, commonly in the range of about 3.8 to 4.5.

That acidity is the defence mechanism. It suppresses the growth of the mixed anaerobic bacteria associated with bacterial vaginosis, and it makes the environment less hospitable to some other organisms. Certain lactobacilli also produce hydrogen peroxide and other compounds with antimicrobial effects, and the whole community occupies the surface in a way that leaves less room for anything else to establish.

Not every healthy person has a lactobacillus dominant community. Research on population differences has found that a substantial proportion of entirely healthy women have more diverse communities, with variation associated with ancestry among other factors. That finding matters because it undermines the idea of a single correct composition that everyone should be aiming at.

What changes it, normally

• The menstrual cycle. Oestrogen levels change glycogen availability, and the composition shifts across the month. Blood raises pH during a period.
• Pregnancy, when the community typically becomes more stable and more lactobacillus dominant.
• Breastfeeding, when oestrogen is low and the environment resembles the postmenopausal state.
• Menopause. Falling oestrogen reduces glycogen, lactobacilli decline, and pH rises. This is part of genitourinary syndrome of menopause, and it explains why urinary infections become more common at the same time as vaginal dryness.
• Sex. Semen is alkaline, and sexual activity temporarily raises pH and introduces other organisms. This is normal and is one reason bacterial vaginosis symptoms are often more noticeable afterwards.

What disturbs it unhelpfully

Douching is the clearest example, because it physically flushes out the bacterial population and the acidic fluid together. Antiseptic washes and any internal cleaning product do a smaller version of the same thing. Antibiotics taken for something else reduce lactobacilli along with their target, which is why thrush often follows a course of antibiotics.

Smoking is associated with a less lactobacillus dominant community. Frequent use of scented products causes irritation of the surrounding skin, which is a separate problem often confused with this one.

When the balance shifts

Two consequences are common. Bacterial vaginosis occurs when lactobacilli fall and mixed anaerobes increase; the characteristic sign is odour rather than itching. Thrush is a yeast overgrowth and produces itching with a thick discharge. Both are described on the page covering thrush and bacterial vaginosis.

A raised vaginal pH is also linked with a higher risk of acquiring some sexually transmitted infections and, in pregnancy, bacterial vaginosis is associated with an increased risk of preterm birth. Those associations are the reason the subject receives research attention rather than being treated as a cosmetic matter.

Probiotics and the products sold around this

This is where marketing has run ahead of evidence. Products containing lactobacilli, taken orally or used vaginally, are widely sold on the promise of restoring balance.

What can be said fairly: some studies suggest that certain lactobacillus strains, used alongside standard antibiotic treatment, may reduce recurrence of bacterial vaginosis in some people. The trials vary considerably in strain, dose, route and quality, results are not consistent, and guidance from most major bodies does not currently recommend probiotics as a standard treatment. They are not a substitute for diagnosis and they are not a treatment for an active infection.

What can be said with more confidence: nothing needs to be added to a healthy vagina. Products claiming to balance pH, cleanse internally, or restore freshness are addressing a problem that, in the absence of symptoms, does not exist. Yoghurt applied internally is not a treatment and can irritate.

Diet is often mentioned. There is no reliable evidence that any particular food or supplement alters the vaginal microbiome usefully in a person with no symptoms, and claims about sugar in particular are not well supported for anyone whose blood glucose is normal. Poorly controlled diabetes is a genuine risk factor for recurrent thrush and is a different matter.

The short version

The community regulates itself when it is left alone and when oestrogen is present. After menopause, treating the tissue with local oestrogen restores the conditions that support it, which is a more direct intervention than anything sold as a balance product. Where symptoms exist, the correct step is diagnosis. Where symptoms do not exist, no step is needed at all.

Last reviewed: September 2026