Trichomoniasis is a sexually transmitted infection caused by a parasite, while bacterial vaginosis (BV) is an overgrowth of the vagina's own bacteria — not an STI in the traditional sense. Both can cause a thin discharge and a fishy odor, so symptoms alone won't tell them apart. Only a test can confirm which one you have, and they need different treatments.
most common curable STI; metronidazole
not an STI; recurs in up to 60%
| Item | Value |
|---|---|
| Trichomoniasis | curable — most common curable STI; metronidazole |
| Bacterial vaginosis (BV) | curable — not an STI; recurs in up to 60% |
What each one is
Trichomoniasis
Trichomoniasis is caused by a single-celled parasite called Trichomonas vaginalis, which is passed during sex. It's the most common curable STI, and in the US there are an estimated 2.6 million infections at any time Sex Transm Dis, 2018. The burden falls heavily on women, who account for over 80% of cases. The catch is how quiet it is — about 70% of infected people have no symptoms at all CDC, so it spreads unnoticed for long stretches.
Bacterial vaginosis (BV)
BV isn't an infection you catch from one person in the usual sense — it's a shift in the vaginal ecosystem. Normally, protective Lactobacillus bacteria keep the vagina mildly acidic. In BV, those lactobacilli are crowded out by anaerobic bacteria (organisms that thrive without oxygen), which changes the smell and chemistry of the vagina. It's the most common vaginal condition in women ages 15–44 CDC. Like trichomoniasis, it's frequently silent — many people with BV notice nothing at all. You can read more on the full picture of bv symptoms.
Symptoms compared
The overlap is real, and it's the reason these two get confused. Both can produce a thin discharge and a fish-like odor, especially after sex. Where they tend to diverge is in the irritation and the discharge color.
Trichomoniasis in women tends to cause genital itching, burning, redness or soreness, discomfort with urination, and a discharge that can run clear, white, yellowish, or frankly greenish — often with a fishy smell. In men it usually causes nothing, though some notice itching inside the penis, burning after urinating or ejaculating, or a discharge.
BV classically produces a thin white or gray discharge and a strong fish-like odor that's most noticeable after sex. Some people also get itching, burning, or burning when they urinate — but the irritation is often milder than with trichomoniasis, and there's no greenish discharge.
How to tell them apart
Here's the honest answer: you usually can't tell these apart by feel. The symptoms overlap enough that even experienced clinicians don't diagnose on appearance alone — a test settles it. That said, a few features push the odds one way or the other:
- A greenish or yellow, sometimes frothy discharge with significant itching and soreness leans toward trichomoniasis.
- A thin white-to-gray discharge with a strong odor after sex but little irritation leans toward BV.
- A positive partner history or other STI exposure raises suspicion for trichomoniasis, which is sexually transmitted; BV is not classically passed between partners.
- Vaginal pH is a useful clue clinicians use: BV typically pushes pH above the normal range, and trichomoniasis can too — which is exactly why pH can't separate them on its own.
Misdiagnosing one as the other matters because the treatments aren't interchangeable. Treating presumed BV with a topical gel will do nothing for a parasite, and you'd stay infected and contagious. When in doubt, test rather than guess.
Side-by-side comparison
| Feature | Trichomoniasis | Bacterial vaginosis (BV) |
|---|---|---|
| Cause | Parasite (Trichomonas vaginalis) | Imbalance of vaginal bacteria |
| Sexually transmitted? | Yes | Not classically |
| Discharge color | Clear, white, yellow, or greenish | Thin white or gray |
| Odor | Fishy | Strong fishy, worse after sex |
| Itching/soreness | Common, often pronounced | Possible, usually milder |
| Often symptomless? | Yes (~70% have no symptoms) | Yes (many have no symptoms) |
| Preferred test | NAAT (molecular) | Amsel criteria, Nugent score, or molecular test |
| First-line treatment | Oral metronidazole (or tinidazole) | Oral metronidazole, or vaginal gel/cream |
Testing
For trichomoniasis, a NAAT (nucleic acid amplification test) is the preferred method — it's highly sensitive, in the range of 95–100%, and can run on a vaginal swab or a female urine sample. That's far more reliable than the older microscopy 'wet mount,' which misses many cases. For the full walkthrough, see trichomoniasis testing & diagnosis.
BV is diagnosed differently. Clinicians use the Amsel criteria (three or more of: thin discharge, clue cells on microscopy, elevated vaginal pH, and a positive 'whiff' fishy-odor test), a Nugent score from a Gram-stained slide (the lab reference standard), or an FDA-cleared molecular test. Because the diagnostic paths differ, the right test depends on what's suspected.
In practice, testing is straightforward: a urine sample, a self-collected swab, or a quick exam depending on which condition is on the table. It's free or low-cost at health departments, Planned Parenthood, and Title X clinics — and you can also get tested without a long wait. If you're testing after a specific encounter, it helps to know when to test after exposure so you don't test too early.
Treatment compared
Both respond to the antibiotic metronidazole, but the regimens differ in important ways.
For trichomoniasis in women, the recommended treatment is metronidazole 500 mg orally twice daily for 7 days CDC, 2021. The 2021 guidelines moved women off the old single 2 g dose because the multi-day course works better: in a randomized trial, about 19% of women on the single dose were still infected at follow-up versus 11% on the 7-day course Muzny et al.. Men are still treated with metronidazole 2 g orally as a single dose, and tinidazole 2 g as a single dose is an alternative for either. Sex partners should be treated too, since trichomoniasis ping-pongs between partners.
For BV, the recommended options are metronidazole 500 mg orally twice daily for 7 days, metronidazole 0.75% gel as one 5 g applicator intravaginally daily for 5 days, or clindamycin 2% cream intravaginally at bedtime for 7 days CDC, 2021. Routinely treating male partners isn't part of standard BV care, which is one more way the two conditions differ in management.
One thing worth bracing for: BV is the great recurrer. Standard antibiotics cure 80 to 90% of acute episodes, but it comes back in up to 60% of women within a year SASGOG. Recurrent BV — three or more episodes a year — often calls for a months-long maintenance regimen rather than just another single course. Trichomoniasis, by contrast, is curable with the right course, though reinfection from an untreated partner is common.
Can you have more than one at once?
Yes. BV and trichomoniasis frequently coexist, partly because both involve a loss of the protective Lactobacillus environment. That's another argument for proper testing — if you only treat the BV and the trichomoniasis is missed, you stay infected and contagious. Both conditions also matter beyond discomfort: BV's disruption of the vaginal environment is linked to a higher risk of acquiring HIV, with a large meta-analysis of more than 30,000 women finding roughly a 60% increase in risk Atashili et al.. Trichomoniasis raises HIV risk as well, which is why neither should be brushed off as a minor nuisance.
When to see a clinician
See a clinician if you have an unusual discharge, a persistent fishy odor, genital itching or soreness, or pain with urination — and especially if symptoms don't clear after self-treatment or keep returning. Get tested if a partner was diagnosed with trichomoniasis, if you have a new partner, or if you're pregnant, since both conditions can affect pregnancy. Don't self-diagnose from a search and reach for whatever's in the cabinet; the wrong product wastes time and leaves you contagious.