A fishy vaginal odor — especially one that gets stronger after sex — most often points to bacterial vaginosis (BV) or the STI trichomoniasis. BV isn't sexually transmitted, but trich is. A yeast infection typically doesn't smell fishy, and a strong odor can also come from something as simple as a forgotten tampon. Only a test settles which one it is.
thin grey discharge, fishy odor (not an STI)
frothy, itchy discharge with an odor
| Item | Value |
|---|---|
| Bacterial vaginosis (BV) | curable — thin grey discharge, fishy odor (not an STI) |
| Trichomoniasis | curable — frothy, itchy discharge with an odor |
Which conditions actually cause an unusual vaginal odor
The odor itself is a clue, not a diagnosis. The most reliable signal is a fish-like smell, which clinicians link to a shift in vaginal bacteria and to the parasite that causes trich. Both conditions can change the discharge and irritate the tissue, and both are frequently silent — so the odor you notice may be the only thing nudging you toward testing.
Bacterial vaginosis (BV)
BV happens when the protective Lactobacillus bacteria that normally dominate the vagina get crowded out by anaerobic bacteria — microbes that thrive without much oxygen. As those anaerobes multiply, they release compounds (amines) that produce a strong fish-like smell, which is why the odor often spikes after sex: semen is alkaline and amplifies the reaction. BV is the most common vaginal condition in people ages 15–44 CDC, BV.
The classic pattern is a thin white or gray discharge with that fishy odor, sometimes with itching, burning, or burning when you urinate. Plenty of people have no symptoms at all. BV isn't passed between partners the way an STI is, but having a new or multiple partners raises the odds of the bacterial shift that triggers it. For the full symptom picture, see our guide to bv symptoms.
Trichomoniasis (trich)
Trichomoniasis is caused by a single-celled parasite, Trichomonas vaginalis, and it's the most common curable STI CDC, trich. Because it's a true infection passed during sex, a positive result means your recent partners need treatment too — otherwise you'll pass it back and forth.
About 70% of people with trich have no signs or symptoms at all. When women do have them, the picture overlaps heavily with BV: itching, burning, redness or soreness, discomfort urinating, and a clear, white, yellowish, or greenish discharge that can carry a fishy smell. Men usually notice nothing, though some get itching inside the penis, burning after urinating or ejaculating, or discharge. Symptoms, when they show up, tend to appear 5 to 28 days after exposure — but they can surface much later, which is one reason timing alone won't tell you when you caught it.
When the odor is NOT an STI
Not every unusual smell means an infection at all, and chasing the wrong cause wastes time. A few common non-STI explanations:
- A forgotten tampon is a classic culprit — a retained tampon produces a strong, foul odor within days, and the fix is simply removing it (see a clinician if you can't).
- Normal day-to-day variation in scent happens with your cycle, sweat, and diet, and a mild musky smell is healthy, not a problem.
- A yeast infection usually does NOT cause a fishy smell — it more often brings a thick, cottage-cheese-like discharge and intense itching, so a fishy odor points away from yeast.
How to tell them apart
Here's the honest part: you can't reliably tell these apart by sight or smell alone. BV and trich share the fishy odor, the discharge changes, and the itching, and both are commonly silent — which is exactly why self-diagnosis fails so often CDC trich guidelines, 2021. Clinicians lean on a few discriminating clues, but they confirm with testing:
- Vaginal pH: BV and trich both push pH above the normal range, while yeast usually keeps pH normal — a pH strip helps separate yeast from the rest.
- The whiff test: adding a drop of potassium hydroxide to a sample releases that fishy amine odor in BV (and sometimes trich), a quick bedside clue.
- Discharge character: BV tends toward thin and gray; trich can run frothy and yellow-green; yeast is thick and white — but these overlap enough that they only point a direction.
- Itching and soreness that's prominent leans more toward trich or yeast than uncomplicated BV.
Because these features overlap so much, a test — not the symptom — is what tells you which one (if any) you have. The practical bottom line: overlapping symptoms are precisely why a lab result turns a guess into an answer.
Side-by-side comparison
| Feature | Bacterial vaginosis (BV) | Trichomoniasis | Yeast infection |
|---|---|---|---|
| STI? | No (bacterial imbalance) | Yes (parasite) | No (fungal overgrowth) |
| Odor | Strong fishy, worse after sex | Can be fishy | Usually none / not fishy |
| Discharge | Thin, white or gray | Clear, white, yellow, or greenish | Thick, white, cottage-cheese-like |
| Itching/soreness | Sometimes | Common when symptomatic | Prominent |
| Vaginal pH | Above normal | Above normal | Usually normal |
| Often silent? | Yes | Yes (~70% have no symptoms) | Less so |
How it's tested
BV is diagnosed using the Amsel criteria (three or more of: thin discharge, clue cells under the microscope, vaginal pH above 4.5, and a positive whiff test), a Nugent score on a Gram-stained slide (the reference standard), or an FDA-cleared molecular test CDC BV guidelines, 2021. Trich is best detected with a NAAT (nucleic acid amplification test) such as the Aptima assay, which has very high sensitivity and runs on a vaginal swab or a urine sample. In practice, testing means a urine cup, a self-collected swab, or a quick exam, and results usually come back within a few days — often free or low-cost at health departments, Planned Parenthood, and Title X clinics. See the full how-to and what to bring on our get tested page, and the deeper dive on trichomoniasis testing & diagnosis.
What to do next
Don't reach for a leftover yeast cream or an over-the-counter "odor" wash — if it's BV or trich, that won't fix it and can delay real treatment. Get tested, and if a partner is involved, plan for them to be treated too (especially with trich, which bounces back and forth). Both conditions are curable with prescription medication; for the specific regimens and what recovery looks like, see our treatment guidance via the get tested hub.
Red flags — when to get seen urgently
Most odor changes are not emergencies, but see a clinician promptly if you notice any of these:
- Fever, chills, or lower-belly or pelvic pain along with the discharge — possible spread to the upper reproductive tract.
- A foul smell you suspect is from a tampon or object you can't remove yourself.
- You're pregnant — BV and trich in pregnancy are linked to complications and warrant prompt care.
- Heavy bleeding, severe pain, or sores along with the odor.
- Symptoms that don't clear after treatment, which may mean the wrong cause was treated or a partner reinfected you.