What Is the Main Cause of BV? Triggers, Symptoms and Why It Returns

Bacterial vaginosis (BV) has no single cause. It happens when the lactobacilli that normally dominate the vagina, holding its pH at about 3.5 to 4, lose ground to a mixed population of anaerobic bacteria. The measured triggers for that shift are a period (BV onset roughly doubled in the day after menstruation), a new or additional partner (about 1.6 times the risk), douching (about 1.2 times, and slower clearance), a copper coil (about 1.3 times) and smoking; hormonal contraception and condoms lower the risk. BV affects about one woman in four, it is not an STI and it is not a hygiene failure.^{1-4, 7-12} The treatment is a course of antibiotics from a GP or sexual health clinic; the difficult part is that BV returns in more than half of women within a year, so this guide also covers the recurrence trials and when to be seen.^{1, 14}

What is bacterial vaginosis?

Bacterial vaginosis is the commonest cause of unusual vaginal discharge in women of childbearing age. The NHS describes a thin, grey-white discharge with a fishy smell, often stronger after sex, and notes that about half of women with BV notice nothing at all.¹ It is not an infection in the everyday sense, one germ arriving and multiplying, but a change in which bacteria are in charge. How common is BV? A 2019 meta-analysis put it at 23% of women in Europe and Central Asia, and a 2025 review in Nature Reviews Disease Primers describes it as affecting roughly one woman in four worldwide, with recurrence after standard treatment above 50% within three to six months.^{2, 3}

What is the main cause of BV?

The main cause is the loss of lactobacillus dominance. In a healthy vagina, lactobacilli make lactic acid and, measured without exposure to air, hold the pH at about 3.5 to 3.8, more acidic than the 4.5 most leaflets quote.^{5, 6} At that acidity the anaerobes present in small numbers in most vaginas (Gardnerella, Prevotella, Atopobium and others) cannot multiply. When lactobacilli fall, the anaerobes expand, the pH rises above 4.5, and the amines they release are the fishy smell. The 2025 primer is candid that nobody has identified a single organism that starts the sequence; research has measured the exposures that make the shift more likely, tabled below.³ One correction to a common phrasing: a healthy vagina is not a "balance" of good and bad bacteria but dominance by one or two Lactobacillus species. What is intimate flora? goes further into the species.

What actually changes in the vagina?

The founding 2011 study sequenced the vaginal bacteria of 396 symptom-free women and found five community types: four dominated by a single Lactobacillus species (L. crispatus, L. iners, L. gasseri or L. jensenii) and one, in about a quarter of these healthy women, with few lactobacilli and more anaerobes.⁴ Which type you sit in matters: L. crispatus dominance is the most protective pattern, and in a study that swabbed 100 women daily for ten weeks, an L. iners-dominated state more than doubled the chance of BV developing soon after (adjusted hazard ratio 2.25).⁷ This is why the same habits produce BV in one woman and nothing in her friend, and why you can get BV without any of the triggers below: the starting community is part of the cause.

How did I get BV all of a sudden?

Usually because something in this table changed in the week or two before.

So "why did I get BV out of nowhere?" usually has an ordinary answer: a period, sex (especially unprotected, or with a new partner), a recent coil, or a change in how you wash. Semen is alkaline and periods raise vaginal pH for days; in a vagina close to the tipping point, that is enough. The same list worsens BV once started: douching slows clearance and smoking keeps lactobacilli down. The coil finding is a conversation with the clinic that fitted it, not a reason to have it removed. You can get BV without ever having had sex; who is most likely to get BV? has the figures.

Is BV caused by being dirty, and how should I wash?

No. BV is not a hygiene failure; the one washing habit measured to raise the risk is washing too much: douching, or putting soap, water or any product inside the vagina. In the study that tracked underwear, tampons, pads, liners, wipes, sprays and powders over time, none was associated with BV; douching was.^{9, 10} The vagina cleans itself with its own lactic acid; rinsing the bacteria out removes the protection. The smell is not dirt but the amines the anaerobes release, so washing harder makes it worse.

The NHS advice: wash the vulva (the outside) with water and plain, unperfumed soap, nothing inside, and shower rather than bathe; dryness is a different problem, covered on vaginal dryness. Avoid perfumed soaps, bubble bath, vaginal deodorants, intimate washes, douches, antiseptic in the bath and strong detergents on underwear, and do not smoke.¹ Forum advice to use intimate washes, scented wipes or a peroxide or vinegar douche runs against that NHS list.

Is BV an STI, and did my partner cheat?

BV is not a sexually transmitted infection, and the NHS says so plainly, but sex can trigger it and the bacteria involved pass between partners.¹ A new or additional partner raises the risk (about 1.6 times), and so does unprotected sex with the same long-term partner: in a 450-woman Australian trial, sex with the same partner after treatment nearly doubled recurrence (adjusted hazard ratio 1.9), as did inconsistent condom use.^{8, 15} So BV after sex with a partner of many years is evidence of nothing but sex. Female partners carry about twice the risk because they share the same bacteria.⁸ Men do not get BV, but carry the bacteria on penile skin, which is why partner treatment is back in the research, below. With a new partner, get an STI test too, since the symptoms overlap. Can BV mean STD? and Can a man give a woman BV? take the partner questions one at a time; Can BV cause a positive chlamydia test? covers the overlap.

What are the symptoms of BV?

A thin, watery discharge, grey or white, often heavier than usual; a fishy smell, strongest after sex and around a period; and, in about half of women, nothing at all.¹ BV does not usually cause itching down there, soreness or a thick white discharge (thrush), pain passing urine (cystitis), or fever or pelvic pain, which need a GP the same day. See What does BV discharge look like?, what your discharge means and Does BV always have a smell?, and for the conditions it is mistaken for, vaginal thrush (contagion) and cystitis.

How is BV treated in the UK?

With antibiotics, prescribed after a swab or examination by a GP or sexual health clinic: a short course of tablets, or a vaginal gel or cream, using metronidazole or clindamycin; the gel too is prescription-only.¹ A sexual health clinic is the fastest route: no referral, often same-day diagnosis. Whether it clears on its own, and how long it lasts, is on can BV go away on its own; what does untreated BV lead to? sets out the risks.¹ Nothing else is an established treatment: garlic and vaginal yoghurt have only small studies, tea tree oil and peroxide or vinegar douches none we found, and several irritate; boric acid is separate. The high-dose vitamin D trial found no change in recurrence (hazard ratio 1.11); What supplements can I take for BV? grades vaginal vitamin C and the rest.²⁰

Why does BV come back, and what helps?

Because antibiotics clear the anaerobes but do not by themselves restore L. crispatus dominance, and because the triggers carry on. In the best-known cohort, 139 women given a standard course of oral antibiotics were followed for a year: BV returned in 58%.¹⁴ "Chronic BV" means recurrence, which these steps are measured to change.

1. Finish the course, and go back if it has not worked. No better three days in, or still symptomatic a week after starting: return to the clinic. It may need a different antibiotic, or be thrush or trichomoniasis.
2. Change the triggers you can. In the 450-woman trial, the same partner (adjusted hazard ratio 1.9) and inconsistent condom use (1.9) raised recurrence, and oestrogen-containing contraception halved it (0.5).¹⁵ Condoms for a few months are a fair test of whether sex is your trigger. Stop douching and perfumed products, stop smoking, and raise the coil with your clinic.
3. Ask about partner treatment. A 2025 trial in the New England Journal of Medicine randomised 164 couples: when the male partner took a week of oral antibiotics plus an antibiotic cream on the penis while the woman had her own course, BV returned in 35% of women within 12 weeks against 63% when she alone was treated.¹⁶ Earlier trials of oral antibiotics alone for men found no benefit, as did a 2016 Cochrane review of seven of them, so the NHS page still names only partners who are women; ask at a sexual health clinic.¹⁷
4. More than four episodes a year. The NHS says women in this position may be given a vaginal antibiotic gel to use for several months.¹
5. Live cultures after the course, as the trials describe. In a 2022 meta-analysis of ten randomised trials with 1,234 women, those who took live cultures (oral or vaginal) after antibiotics had a BV recurrence rate of 14.8%, against 25.5% without them (relative risk 0.55); route, dose, duration and number of species made no measurable difference.¹⁸ Every one of those trials gave the live cultures after an antibiotic course, not instead of one. Can I get rid of BV by taking probiotics? sets out those trials, how long live cultures take to work covers the trials' 8 to 60 days, and how many to take a day covers the counts. How to take them, reading a live-culture label, how to choose a women's live-culture supplement, what to expect, odour, vaginal use and how they help.

The full plan is Why do I keep getting BV?

When should I see a GP or sexual health clinic?

For a first episode, because only a swab separates BV from thrush, trichomoniasis and other infections, as how BV is tested explains. For any unusual discharge in pregnancy. For fever, pelvic pain or pain during sex. Burning when you pee is a urinary infection, not BV; see self-care for a UTI and why UTIs keep coming back. If you are no better three days into treatment, or still symptomatic a week after starting it. For more than four episodes a year, or thrush more than twice in six months. With a new partner (ask for an STI test). For a first episode under 16 or over 50, or any bleeding between periods or after sex, sores or skin changes. Sexual health clinics need no referral.¹

Does BV matter in pregnancy?

It can, so any unusual discharge in pregnancy is for the midwife or GP, who prescribe antibiotics for BV found in pregnancy; in the Cochrane review, antibiotics cleared it (relative risk 0.42) and reduced late miscarriage (0.20) but did not reduce preterm birth overall (0.88), and metronidazole was not linked to birth defects.¹⁹ Live cultures are not a substitute: in a UK trial of 304 pregnant women taking an oral two-strain capsule daily until delivery, BV rates were not significantly different from placebo, so the trials report no safety problems and no benefit in pregnancy.²² BV, thrush and UTIs in pregnancy has the rest.

Frequently asked questions

Can stress cause BV?

Stress has not been measured as a trigger in the cohort studies that measured periods, partners, douching and the coil. If episodes cluster around stressful weeks, look at what else changes then: sex, smoking and your cycle.

Can masturbation cause BV?

No study has linked it to BV; the measured sexual risk is a partner's bacteria.

What is Gardnerella, and is it the same as BV?

No. Gardnerella vaginalis is one of the anaerobes that overgrows in BV and the one most often named on a swab report, but the 1983 study that defined BV found it in half of all 397 students, most without BV, so finding it is not the diagnosis.²³ BV is the shift to a mixed anaerobic community of which Gardnerella is one member; it is not an STI.^{1, 3}

Can a vitamin or home remedy help with BV?

No oral vitamin has cut BV recurrence in a trial we found; vaginal vitamin C has one trial.²⁰ Antibiotics are the treatment; live cultures afterwards have the ten-trial evidence above. Period pain, inositol, PCOS, libido, rooibos and D-mannose are graded separately.

Is BV permanent, or incurable?

Neither. Each course clears BV in most women; the five steps above reduce recurrence, and the 2025 partner-treatment trial roughly halved it.^{14, 16}

Sources

1. NHS. Bacterial vaginosis. nhs.uk/conditions/bacterial-vaginosis.
2. Peebles K, et al. High global burden and costs of bacterial vaginosis: a systematic review and meta-analysis. Sexually Transmitted Diseases, 2019. PubMed 30624309.
3. Bradshaw CS, et al. Bacterial vaginosis. Nature Reviews Disease Primers, 2025. PubMed 40537474.
4. Ravel J, et al. Vaginal microbiome of reproductive-age women. Proceedings of the National Academy of Sciences, 2011. PubMed 20534435.
5. O'Hanlon DE, et al. Vaginal pH and microbicidal lactic acid when lactobacilli dominate the microbiota. PLoS One, 2013. PubMed 24223212.
6. O'Hanlon DE, et al. Vaginal pH measured in vivo: lactobacilli determine pH and lactic acid concentration. BMC Microbiology, 2019. PubMed 30642259.
7. Tamarelle J, et al. Factors associated with incidence and spontaneous clearance of molecular-bacterial vaginosis: results from a longitudinal frequent-sampling observational study. Sexually Transmitted Diseases, 2022. PubMed 35969846.
8. Fethers KA, et al. Sexual risk factors and bacterial vaginosis: a systematic review and meta-analysis. Clinical Infectious Diseases, 2008. PubMed 18947329.
9. Brotman RM, et al. A longitudinal study of vaginal douching and bacterial vaginosis: a marginal structural modeling analysis. American Journal of Epidemiology, 2008. PubMed 18503038.
10. Klebanoff MA, et al. Personal hygienic behaviors and bacterial vaginosis. Sexually Transmitted Diseases, 2010. PubMed 19823112.
11. Vodstrcil LA, et al. Hormonal contraception is associated with a reduced risk of bacterial vaginosis: a systematic review and meta-analysis. PLoS One, 2013. PubMed 24023807.
12. Peebles K, et al. Elevated risk of bacterial vaginosis among users of the copper intrauterine device: a prospective longitudinal cohort study. Clinical Infectious Diseases, 2021. PubMed 32505132.
13. Brotman RM, et al. Association between cigarette smoking and the vaginal microbiota: a pilot study. BMC Infectious Diseases, 2014. PubMed 25169082.
14. Bradshaw CS, et al. High recurrence rates of bacterial vaginosis over the course of 12 months after oral metronidazole therapy and factors associated with recurrence. Journal of Infectious Diseases, 2006. PubMed 16652274.
15. Bradshaw CS, et al. Recurrence of bacterial vaginosis is significantly associated with posttreatment sexual activities and hormonal contraceptive use. Clinical Infectious Diseases, 2013. PubMed 23243173.
16. Vodstrcil LA, et al. Male-partner treatment and recurrence of bacterial vaginosis: a randomised open-label trial. New England Journal of Medicine, 2025. PubMed 40043236.
17. Amaya-Guio J, et al. Antibiotic treatment for the sexual partners of women with bacterial vaginosis. Cochrane Database of Systematic Reviews, 2016. PubMed 27696372.
18. Chieng WK, et al. Probiotics, a promising therapy to reduce the recurrence of bacterial vaginosis in women? A systematic review and meta-analysis of randomized controlled trials. Frontiers in Nutrition, 2022. PubMed 36204368.
19. Brocklehurst P, et al. Antibiotics for bacterial vaginosis in pregnancy. Cochrane Database of Systematic Reviews, 2013. PubMed 23440777.
20. Turner AN, et al. A blinded, randomized controlled trial of high-dose vitamin D supplementation to reduce recurrence of bacterial vaginosis. American Journal of Obstetrics and Gynecology, 2014. PubMed 24949544.
21. Neggers YH, et al. Dietary intake of selected nutrients affects bacterial vaginosis in women. Journal of Nutrition, 2007. PubMed 17709453.
22. Husain S, et al. Effects of oral probiotic supplements on vaginal microbiota during pregnancy: a randomised, double-blind, placebo-controlled trial with microbiome analysis. BJOG, 2020. PubMed 30932317.
23. Amsel R, et al. Nonspecific vaginitis: diagnostic criteria and microbial and epidemiologic associations. American Journal of Medicine, 1983. PubMed 6600371.

This article is general information, not medical advice. See a GP or sexual health clinic for a first episode, for any discharge in pregnancy, and whenever treatment has not worked.