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Bacterial vaginosis


Bacterial vaginosis (BV) is a common yet poorly understood condition in which the balance of bacteria inside the vagina becomes disrupted.

Around one in three women will experience at least once episode of BV at some point.

This imbalance often triggers a change to the usual vaginal discharge, which results in a fishy smelling, greyish discharge from the vagina. However, half of women with bacterial vaginosis have no symptoms. 

See your GP if you notice any abnormal discharge from your vagina, especially if you are pregnant. It is important to get this type of symptom diagnosed quickly to rule out other infections and prevent complications (see below).

If you do not have symptoms, there is no need to get tested as bacterial vaginosis without symptoms does not pose a threat to health or to pregnancy.

Read more about the symptoms of bacterial vaginosis.

What causes bacterial vaginosis?

The vagina contains a mix of so-called "good" bacteria, which can help protect against infection, and a smaller amount of "bad" bacteria, which can cause infection.

In cases of BV, the bad bacteria begin to outnumber the good bacteria, leading to inflammation inside the vagina, which can result in the fishy discharge.

What leads to this imbalance is still unclear. It is not classed as a typical sexually transmitted infection (STI), but it can develop after having sex with a new partner.

There is no evidence that a woman with BV can pass on any type of infection to her male sexual partner. But the same may not be true for women who have sex with other women.

You can also get bacterial vaginosis if you:

  • use scented soaps or bubble baths
  • have an intrauterine device (IUD)
  • smoke
  • use vaginal deodorant

Read more about the possible causes of bacterial vaginosis.

Treating bacterial vaginosis

BV can usually be successfully treated using a course of antibiotic tablets or gel.

However, it is common for BV to reoccur. Over half of women successfully treated with BV will have a reoccurrence of symptoms, usually within six months.

Women who have very frequent episodes of BV may be referred to a gynaecologist.

Read more about treating bacterial vaginosis.


BV is a concern if it develops in pregnant women as it increases the risk of pregnancy-related complications, such as premature birth or miscarriage. However, the increase in risk is small.

As a precaution, you should contact your GP if you are pregnant and you begin to have vaginal discharge (although discharges can be a normal part of pregnancy).

Bacterial vaginosis can also increase your risk or getting some STIs.

Read more about the complications of bacterial vaginosis.

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The main symptom of bacterial vaginosis (BV) is a change in your usual vaginal discharge.

Your discharge may:

  • become thin and watery
  • change to a white or grey colour
  • develop a strong, unpleasant, fishy smell, particularly after sexual intercourse

Other symptoms of BV can include:

  • pain during intercourse
  • pain when passing urine
  • some light bleeding from your vagina

BV does not usually cause itching or irritation.

Around half of all women with BV do not have any symptoms. This is known as asymptomatic BV.

If you do not have symptoms, there is no need to get tested to find out if you have BV as asymptomatic bacterial vaginosis does not pose a threat to health or pregnancy.

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Bacterial vaginosis (BV) occurs when there is a change in the natural balance of bacteria in your vagina.

Your vagina should contain bacteria called lactobacilli. These bacteria produce lactic acid. This makes the vagina slightly acidic, which prevents other bacteria from growing there.

However, if you have BV, you have less lactobacilli, which means your vagina is not as acidic as it should be. This allows other types of bacteria to grow.

It is still unclear what causes the change in the balance of bacteria, although the risk is increased if you:

  • are pregnant
  • have a new sexual partner
  • have multiple sexual partners
  • use an intrauterine device (IUD), a contraceptive device that fits inside the womb
  • smoke
  • use scented soaps or bubble bath
  • put antiseptic liquids in the bath
  • wash or clean out your vagina with water or other fluids
  • use vaginal deodorant
  • use strong detergents to wash your underwear
  • have previously been infected with the human papilloma virus (HPV) – a group of viruses that can cause genital warts and, less commonly, cervical cancer
  • eat a diet high in carbohydrates, such as lots of white bread, baked potatoes and processed food

For reasons that are unclear, BV is more common in black women than in other ethnic groups.

Is bacterial vaginosis an STI?

It is unclear whether BV is a sexually transmitted infection (STI) as there is conflicting evidence on this issue.

Evidence that suggests BV could be an STI includes:

  • rates of BV are higher in women who have multiple sexual partners
  • rates of BV are lower in women who use a condom during sex

There is also evidence that women with BV can pass the condition to women they have sex with, although exactly how this happens is still unclear.

Evidence that suggests BV may not be an STI includes:

  • treating male partners with antibiotics does not prevent the reoccurrence of BV
  • rates of BV can vary significantly in different ethnic groups, which cannot be explained by sexual activity alone
  • BV can occur in women who are not sexually active

Many experts think that sexual activity plays a role in BV, but other factors are probably also responsible for the condition.

It may also be possible that an initial episode of BV is caused by some type of sexual infection, but further episodes are caused by other factors.

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See your GP as soon as possible if you have any abnormal discharge from your vagina. It is important to get bacterial vaginosis (BV) diagnosed to rule out other infections, such as trichomonas vaginalis or gonorrhoea. These can both cause abnormal vaginal discharge.

If you prefer, you could visit a sexual health clinic for a diagnosis, instead of your GP. Sexual health clinics are sometimes known as genito-urinary medicine (GUM) clinics. They are usually located in a hospital or are part of another health centre. Find your nearest sexual health service.


Your GP or healthcare professional may diagnose BV from a description of your symptoms and by examining your vagina. In particular, they will look for:

  • a thin, greyish discharge
  • an unpleasant smell

In some cases, this may be enough to confirm your diagnosis. However, if you are sexually active and it is possible that you may have a sexually transmitted infection (STI) instead of BV, you may need to have some further tests. 


Your GP or healthcare professional will use a swab or a small plastic loop to collect a sample of cells from the walls of your vagina. A swab looks a bit like a cotton bud but is smaller, soft and rounded.

The swab or loop is used to pick up samples of discharge and cells. It only takes a few seconds and is not usually painful, although it may be slightly uncomfortable for a moment.

The samples will be examined to check for BV. The result may be available immediately or the sample may be sent to a laboratory, in which case the result will usually be available within a week.

The pH (alkaline/acid balance) of your vagina may be measured. A swab will be taken from inside your vagina and wiped over a piece of specially treated paper. The paper changes colour depending on the pH level. A pH level higher than 4.5 is an indication that you may have BV.

These tests and all sexual health services are free on the NHS. They are available to everyone, regardless of age.

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Bacterial vaginosis (BV) can be treated with antibiotics. If used correctly, treatment is effective in around 9 out of 10 cases.

There is currently no evidence that probiotics, such as those found in live yoghurt, are of any benefit in treating or preventing BV.


Metronidazole is the most common and preferred antibiotic treatment for BV. It is available in three forms:

  • as tablets to be swallowed twice a day for seven days
  • as tablets to be taken as a single larger dose that you take only once
  • as a gel that you apply to your vagina once a day for five days

The most effective way to take metronidazole is in tablet form over seven days. This is also the preferred treatment if you are pregnant or breastfeeding.

As a precaution, the use of metronidazole (and most other types of medication) is not normally recommended during pregnancy. But an exception is made in the case of BV, as not treating it poses a far bigger risk to the pregnancy than taking the medication.

More women find that their BV is more likely to return if they take the single larger dose. 

Warning on alcohol

It is recommended that you do not drink alcohol while taking metronidazole and for 48 hours afterwards. This is because metronidazole can react with alcohol, causing you to feel very unwell.

If metronidazole is taken as directed, it is very rare for you to have side effects.

After treatment

If your BV symptoms disappear after treatment, you will not need to be tested for BV again to confirm that the treatment has worked. However, you will need to be tested if:

  • your signs and symptoms do not go away
  • your signs and symptoms return
  • you are treated for BV while you are pregnant

Further treatment

The first course of treatment may not be effective in around 1 in 10 people.

If your treatment has been unsuccessful, your GP will need to check that you took the medicine correctly. If you did and it did not work, you may be prescribed one of the different options described above.

Referral to a specialist

If you have repeated episodes of BV in a short space of time, your GP may recommend that you are referred to a gynaecologist (a specialist in treating conditions of the female reproductive system) for further investigation.

If you are pregnant, you may be referred to an obstetrician (a specialist in pregnancies). They will be able to discuss further treatment options with you. 

Vaginal pH correction treatments

Vaginal pH correction treatments are a relatively new way of treating bacterial vaginosis. This involves placing a tablet inside your vagina which will then dissolve. As it dissolves, it changes the acid balance inside your vagina, making it a less hospitable environment for harmful bacteria. Acidity is measured using a scale known as the pH scale.

Most vaginal pH correction treatments are available over-the-counter from pharmacists.

Your pharmacist should be able to advise you if vaginal pH correction treatments are suitable for you.

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Pregnant women with untreated bacterial vaginosis (BV) that is causing symptoms have a higher risk of developing complications that can affect their pregnancy.

These include:

  • premature birth (where the baby is born before the 37th week of pregnancy) – women with BV are twice as likely to have a premature birth than other women, and it is thought that BV is responsible for one in three of all premature births in the UK
  • miscarriage (the loss of pregnancy at some point during the first 23 weeks) – women with BV are six times more likely to have a miscarriage than other women
  • the amniotic sac breaking open too early – the amniotic sac is the bag of fluid where the unborn baby develops 
  • chorioamnionitis – an infection of the chorion and amnion membranes (the membranes that make up the amniotic sac) and the amniotic fluid (the fluid that surrounds the foetus)
  • postpartum endometritis – irritation or inflammation of the lining of the womb after giving birth

If you are pregnant and have symptoms of BV, see your GP immediately, although there is no cause for concern. BV can be safely treated when you are pregnant, which will reduce the risk of all of these complications.

Routine testing for BV is not recommended for pregnant women without any symptoms of BV (asymptomatic BV). This is because there is no evidence that asymptomatic BV increases the risk of complications in pregnancy.

Sexually transmitted infections (STIs)

There is evidence that having BV can make you more at risk of catching STIs. This is possibly because the drop in "friendly" bacteria inside your vagina reduces their protective effects against infection.

If you have BV, you may be at more risk of:

Other health conditions

BV has been linked to several other conditions. It is thought that BV may contribute to:

  • pelvic inflammatory disease (PID), a condition that involves infection and inflammation (swelling) of the upper female genital tract, including the womb, fallopian tubes and ovaries
  • tubal factor infertility, caused by damage to the fallopian tubes, which connect the ovaries to the womb

In-vitro fertilisation

Women who have BV and are using in vitro fertilisation (IVF) may have:

  • a lower success rate
  • an increased risk of early miscarriage

If you are having IVF and have symptoms of BV, see your GP or speak to your infertility specialist.

Recurrent BV

In around 20–25% of women who have been successfully treated, BV may return within one month. Over time, BV may reoccur in up to 80% of women.

If your BV reoccurs, return to your GP to discuss further treatment options.

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