Bacterial vaginosis, thrush and trichomoniasis

Changes in vaginal discharge, smell or itch are among the most common reasons people seek sexual health care. Three conditions account for most cases, and self-diagnosis is unreliable — a 2002 Ferris study found only about 34% of women self-treating for thrush actually had it.

Bacterial vaginosis (BV)

An imbalance of the vaginal microbiome in which protective lactobacilli are outnumbered by anaerobes such as Gardnerella vaginalis. It is the most common cause of abnormal discharge in people of reproductive age, affecting roughly 23–29% globally (Peebles et al., 2019 meta-analysis).

• Signs: thin grey-white discharge with a fishy odour, often stronger after sex or during menstruation. About half of cases are symptomless. Itch and soreness are usually absent — that points away from BV.
• Diagnosis: Amsel criteria or Nugent scoring of a vaginal smear; pH above 4.5.
• Treatment: metronidazole 400–500 mg twice daily for 7 days, or intravaginal metronidazole or clindamycin (BASHH 2012, updated; CDC 2021). Recurrence is the main problem — roughly 50% within 12 months.
• Why it matters: BV is associated with increased risk of acquiring HIV, chlamydia, gonorrhoea and HSV-2, and with preterm birth.
• BV is not classed as an STI, but sexual activity clearly influences it. The 2025 StepUp RCT (Vodstrcil et al., NEJM) found that treating male partners alongside the patient cut recurrence from 63% to 35% — the first strong evidence that partner treatment helps.

What does not help: douching (it makes BV more likely and is not recommended by any guideline), scented washes, and over-the-counter thrush treatments.

Vulvovaginal candidiasis (thrush)

Overgrowth of Candida, usually C. albicans. Around 75% of people with a vagina have at least one episode in their lifetime; 5–8% have recurrent thrush (four or more episodes a year).

• Signs: itch and soreness, thick white discharge that does not smell, splitting or redness of the vulva, pain on sex or urination.
• Triggers: antibiotics, uncontrolled diabetes, pregnancy, immunosuppression. It is not caused by poor hygiene and is not sexually transmitted, although it can be passed between partners occasionally.
• Treatment: a single dose of oral fluconazole 150 mg, or clotrimazole pessary/cream. In pregnancy use topical treatment only — oral fluconazole is avoided.
• Recurrent thrush needs a confirmed diagnosis by swab and an induction-plus-maintenance regimen over six months (BASHH 2019); self-treating repeatedly without a swab is how vulval dermatoses and lichen sclerosus get missed for years.

Trichomoniasis

Caused by the parasite Trichomonas vaginalis, and this one is a sexually transmitted infection. WHO estimated 156 million new infections globally in 2020, making it the most common curable STI worldwide.

• Signs: frothy yellow-green discharge, odour, itch, pain on urination; up to 70% of infections cause no symptoms, and most infected men are asymptomatic.
• Diagnosis: NAAT is far more sensitive than microscopy (which misses 30–50%).
• Treatment: metronidazole. CDC 2021 changed the recommendation for women to 500 mg twice daily for 7 days after a randomised trial showed lower repeat infection than single-dose 2 g; single-dose 2 g remains standard for men.
• Partners must be treated and sex avoided for 7 days. Re-test at 3 months.
• Trichomoniasis is associated with preterm birth and increased HIV acquisition risk.

Quick comparison

When to see a clinician

Discharge with pelvic pain or fever, discharge in pregnancy, symptoms that fail over-the-counter treatment, recurrent episodes, bleeding after sex, or any first episode you have not had diagnosed before.

Sources

• CDC, STI Treatment Guidelines, 2021.
• BASHH, BV guideline 2012 (rev.); vulvovaginal candidiasis guideline 2019; trichomoniasis guideline 2021.
• Peebles K et al., Sexually Transmitted Diseases, 2019 (global BV prevalence).
• Vodstrcil LA et al., New England Journal of Medicine, 2025 (StepUp partner-treatment trial).
• Kissinger P et al., Lancet Infectious Diseases, 2018 (7-day metronidazole trial).
• WHO, global STI incidence estimates, 2020.