A grey or yellow discharge with a noticeably fishy smell — especially one that's stronger after your period or after sex — is most often bacterial vaginosis (BV), an imbalance in the normal vaginal bacteria rather than an infection you caught from a partner. It's extremely common, easily confirmed with a simple test, and straightforward to treat, but it's worth distinguishing from other causes before assuming it will clear on its own.
What usually causes this
- Bacterial vaginosis (BV) is the most common cause of a fishy-smelling grey or yellow-grey discharge. It happens when the normal balance of vaginal bacteria shifts, not from poor hygiene and not necessarily from a sexually transmitted infection
- Menstrual blood and hormone shifts around your period can temporarily change vaginal pH, which is why BV-related smell often becomes more noticeable right after a period ends, even if the underlying imbalance was already there
- Semen is alkaline and can temporarily raise vaginal pH after sex, which is part of why a fishy smell linked to BV is often most noticeable after intercourse
- Hormonal contraceptives, including emergency contraception, can shift vaginal pH and bacterial balance for some women, which is why a new or worsened discharge pattern sometimes follows taking the morning-after pill — though this is less common than the period- or sex-linked pattern
How to tell BV from other causes
| Pattern | Usual cause |
|---|---|
| Thin, grey or yellow-grey, fishy smell, worse after sex or period | Bacterial vaginosis |
| Thick, white, cottage-cheese-like, itchy, little odour | Yeast infection |
| Yellow or green, frothy, with itching or soreness | Trichomoniasis — needs testing |
| Any discharge with pelvic pain, fever, or pain during sex | See your doctor promptly, don't wait to self-diagnose |
Smell and colour alone are a useful first clue, not a diagnosis — a vaginal swab is a quick way to confirm which of these it actually is, especially the first time it happens.
Why it keeps coming back for some women
BV has a real tendency to recur, even after a course of antibiotics clears it. Triggers that disturb the vaginal bacterial balance again include a new sexual partner, douching, scented soaps or washes near the vaginal area, and sometimes the hormonal shift around your period itself. Recurrence doesn't mean the first treatment failed or that you're doing something wrong — it reflects how easily this particular balance can tip, and repeated episodes are a reason to discuss a longer or different treatment approach with your doctor rather than just repeating the same short course each time.
Frequently asked
Is bacterial vaginosis a sexually transmitted infection?
No. BV is an imbalance of bacteria that are normally present in the vagina — it isn't classified as an STI, although sexual activity can be one of several factors that disturb the balance. It can occur in women who aren't sexually active too.
Do I need my partner treated as well?
Generally no. BV treatment is usually just for the affected partner, unlike some other infections. Your doctor will tell you if your specific situation calls for anything different.
Can the morning-after pill actually cause this kind of discharge?
It can be one contributing factor for some women, through the hormonal shift it causes, but it's a less common trigger than the period- or sex-related pattern. If a new discharge appears after taking emergency contraception, it's still worth getting checked rather than assuming the pill is the only explanation.
How is this actually diagnosed and treated?
A vaginal swab checked under a microscope or with a simple pH test usually confirms BV quickly. Treatment is typically a short course of an antibiotic, either as tablets or a vaginal gel/cream, prescribed after the swab confirms the diagnosis.