Bacterial vaginosis (BV) is the most common vaginal infection in women of reproductive age. It develops when the normal balance of vaginal bacteria shifts: protective Lactobacillus species decrease, and anaerobic bacteria (particularly Gardnerella vaginalis) overgrow. The classic clinical sign is a thin grey-white discharge with a strong fishy odour, especially after sex.
Medically reviewed by Dr. Qiao Yufei, MD
This guide covers what BV is, the Amsel diagnostic criteria, metronidazole treatment, why recurrence is so common, and pregnancy implications. BV pairs clinically with trichomoniasis (similar fishy odour) and candidiasis (yeast infection, the differential diagnosis). It is one of several conditions covered in our broader STDs in Singapore overview.
Bacterial vaginosis is a vaginal microbiome imbalance rather than a single-pathogen infection. The healthy vagina is dominated by Lactobacillus species that produce lactic acid, keep the vaginal pH below 4.5, and protect against pathogens. In BV, this balance is disrupted: Lactobacillus decreases, vaginal pH rises, and anaerobic bacteria including Gardnerella vaginalis, Atopobium vaginae, Mobiluncus, and Prevotella overgrow.
According to the US Centers for Disease Control and Prevention (CDC), BV affects roughly 1 in 3 women aged 15 to 44 in the US, making it the most common vaginal infection in this age group. Singapore prevalence is similar.
No, not in the strict sense. Women who have never been sexually active can still develop BV, and unlike chlamydia or gonorrhoea, BV is not transmitted from a single infected partner. However, BV is sexually associated: new or multiple partners, unprotected sex, and douching all disrupt the vaginal microbiome and increase risk.
Anything that disrupts the vaginal microbiome can tip the balance towards BV. Common contributors include:
About half of women with BV have no symptoms at all. When symptoms occur, they are typically distinctive:
Consider testing for chlamydia, gonorrhoea, or trichomoniasis simultaneously: these can co-occur with BV, and symptoms overlap. Routine screening is also reasonable for any new vaginal symptoms in sexually active women.
BV is diagnosed clinically using the Amsel criteria: at least 3 of 4 must be present for a diagnosis of BV.
Coats the vaginal walls evenly, unlike the curd-like discharge of yeast.
Tested with a pH strip on a sample of vaginal fluid. Normal vaginal pH is 3.8–4.5.
Adding 10% potassium hydroxide (KOH) to a sample releases a strong fishy amine smell.
Vaginal epithelial cells coated with bacteria, giving them a stippled "fuzzy" border under the microscope.
For research and difficult cases, the lab gold standard is Nugent scoring on Gram stain: a numerical score (0–10) based on relative quantities of Lactobacillus, Gardnerella/Bacteroides, and Mobiluncus morphotypes. A score of 7 or higher = BV.
Modern NAAT panels (such as Affirm VPIII or BD MAX Vaginal Panel) can detect Gardnerella, Atopobium, and Megasphaera together with simultaneous detection of trichomonas and yeast, providing a definitive lab diagnosis. At Mediway Medical Centre, the standard initial assessment uses Amsel criteria with reflex NAAT for unclear cases.
BV is treated with antibiotics that target the overgrowth of anaerobic bacteria. Several options are equally effective; choice depends on patient preference, pregnancy status, and prior recurrence:
Oral metronidazole and tinidazole interact with alcohol, causing a disulfiram-like reaction (severe nausea, vomiting, headache). Avoid alcohol during treatment and for at least 24 hours after metronidazole, or 72 hours after tinidazole. Vaginal preparations (gel, cream) carry less alcohol risk but caution is still recommended.
Routine treatment of male partners is not currently recommended: BV is not transmitted as a single-pathogen STI, and partner treatment has not been shown to reduce recurrence in trials. However, in same-sex female partners, both may benefit from concurrent treatment because they share vaginal microbiomes.
Recurrence is the most challenging aspect of BV. About 50% of women treated for BV develop recurrence within 12 months. The underlying issue: antibiotics kill the overgrown anaerobes but do not always restore the protective Lactobacillus community.
Recurrent vaginal symptoms warrant medical assessment. Your doctor can confirm BV with Amsel criteria, exclude other causes (yeast, trichomoniasis, chlamydia, gonorrhoea), and discuss suppressive treatment options if needed.
Many BV cases resolve spontaneously without treatment, but untreated symptomatic or recurrent BV can cause:
BV is straightforward to treat with antibiotics. The main challenge is preventing recurrence: lifestyle changes plus, where needed, suppressive maintenance therapy.
The most common vaginal infection in women aged 15 to 44. It develops when the normal balance of vaginal bacteria shifts: protective Lactobacillus species decrease and anaerobic bacteria, particularly Gardnerella vaginalis along with Atopobium, Mobiluncus, and Prevotella, overgrow. BV is a microbiome imbalance rather than a single-pathogen infection.
BV is not classically a sexually transmitted infection: women who have never been sexually active can develop it, and BV is not transmitted from a single partner the way chlamydia or gonorrhoea is. However, BV is sexually associated: new or multiple partners, douching, and unprotected sex all increase risk by disrupting the vaginal microbiome. BV also raises the risk of acquiring other STIs.
Yes. BV is curable with antibiotics, typically metronidazole 500 mg orally twice daily for 7 days, metronidazole vaginal gel for 5 days, or clindamycin vaginal cream for 7 days. Symptoms usually resolve within a few days. However, recurrence is common: about 50% of women have BV again within 12 months.
The classic symptoms are a thin, grey-white vaginal discharge with a strong fishy odour, particularly noticeable after sex. Vaginal itching, burning, and discomfort can occur but are less prominent than with thrush (yeast infection) or trichomoniasis. About half of women with BV have no symptoms at all.
Using the Amsel criteria: at least 3 of 4 must be present: (1) thin homogeneous grey-white discharge; (2) vaginal pH greater than 4.5; (3) fishy amine odour when 10% potassium hydroxide is added to a sample ("whiff test"); (4) clue cells (vaginal epithelial cells coated in bacteria) on microscopy. Nugent scoring on Gram stain is the lab gold standard. NAAT testing is increasingly available for definitive identification.
Recurrence is common because antibiotics kill the overgrown anaerobes but do not always restore the protective Lactobacillus dominance. About 50% of women have BV recurrence within 12 months. Strategies for recurrent BV: longer or twice-weekly suppressive metronidazole gel, lifestyle changes (avoid douching, use unscented products, consider partner reassessment), and increasingly, probiotic or vaginal microbiome restoration approaches.
Yes. Untreated BV in pregnancy is associated with preterm labour, premature rupture of membranes, low birth weight, and postpartum endometritis. Symptomatic BV in pregnancy should be treated with metronidazole or clindamycin. Asymptomatic BV screening is not routinely recommended in low-risk pregnancies but may be considered for women with prior preterm birth.
References: CDC: BV treatment guidelines · WHO: STIs fact sheet · HealthHub Singapore
This page is for educational purposes only and does not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional for diagnosis and treatment of suspected bacterial vaginosis.