Common Causes (Ranked by Likelihood)
BV is the dominant cause of fishy period smell. The chemistry is straightforward: menstrual blood (pH 7.4) dramatically raises vaginal pH from its normally acidic state. Acidic conditions (pH 3.8-4.5) maintained by Lactobacillus bacteria suppress anaerobic bacteria. When pH rises during menstruation, these anaerobes multiply rapidly and produce trimethylamine — the compound responsible for the smell of rotting fish. The 'whiff test' used by gynecologists (adding potassium hydroxide to vaginal discharge produces a strong fishy odor) works because BV amines smell exactly like fish at alkaline pH — which is exactly what menstrual blood creates. BV affects about 29% of reproductive-age people in the US, many undiagnosed.
Trichomoniasis (T. vaginalis) produces a fishy, musty, or foul smell from frothy yellow-green discharge. Like BV, this smell intensifies during menstruation because the pH change favors the parasite's activity. About 70% of trichomoniasis infections are asymptomatic — menstruation may be when the smell first becomes noticeable. The difference from BV: trichomoniasis discharge tends to be frothy and yellow-green rather than thin and gray. Testing is necessary to distinguish the two.
Some people have a chronic, low-grade BV or sub-clinical vaginal microbiome imbalance that doesn't produce strong symptoms most of the month — but becomes symptomatic during menstruation when pH is disrupted. The fishy smell during the period may be the only indication that the microbiome is shifted toward BV-associated bacteria. This is why many people describe 'my periods have always smelled this way' — there's been a longstanding microbiome imbalance that's only evident during the monthly pH disruption.
A retained tampon or forgotten menstrual cup is a different category of fishy smell — not the subtle-to-moderate fishy smell of BV, but a severe, rotting, genuinely foul odor that is unmistakably abnormal. This can cause toxic shock syndrome if left too long and must be taken seriously. If the fishy smell is dramatically worse than anything you've experienced before and accompanied by unusual dark discharge, fever, or feeling unwell — this must be ruled out first.
How to Fix It
Get BV Diagnosed and Treated
$20-100BV is the most likely cause and it's easily, reliably treated.
- 1See a gynecologist or primary care doctor for a vaginal swab — BV is diagnosed in minutes with a simple pH test, whiff test, and microscopy.
- 2Standard treatment: metronidazole gel (vaginal, applied daily for 5 days) or oral metronidazole (500mg twice daily for 7 days). Both have ~85-90% cure rates.
- 3OTC option if you want to try first: boric acid vaginal suppositories (600mg, used during your period) restore acidic pH and are evidence-based for BV treatment.
- 4After treatment, take oral Lactobacillus probiotics (rhamnosus and reuteri strains) to help restore protective bacteria.
- 5To prevent recurrence at next period: some gynecologists recommend preventive boric acid suppositories on the first 2-3 days of your period to maintain acidic pH.
Prevent BV from Flaring During Your Period
$5-20For recurring fishy periods despite treatment — maintaining pH is the key.
- 1Use RepHresh vaginal gel (OTC, pH-lowering) or boric acid suppositories on the first day of your period to counteract the alkaline pH shift from blood.
- 2Avoid menstrual cups and pads if you're already BV-prone — cups concentrate blood (raising pH), and pads allow blood to contact air and bacteria. Tampons are better for BV prevention.
- 3Don't use scented menstrual products — these disrupt vaginal flora.
- 4After your period, use a Lactobacillus vaginal suppository or probiotic to help restore the microbiome.
- 5Consider whether semen exposure (alkaline) is compounding the issue — consistent condom use significantly reduces BV recurrence for sexually active people.
Test for Trichomoniasis
$30-80If BV treatment doesn't fully resolve the fishy smell or discharge is yellow-green.
- 1Ask your doctor for a trichomoniasis test — a swab or urine test confirms or rules it out.
- 2At-home STI tests (LetsGetChecked, Everlywell) include trichomoniasis testing.
- 3Trichomoniasis is treated with a single dose of metronidazole (2g) or tinidazole — very effective.
- 4Sexual partners must also be treated to prevent reinfection.
- 5If treated for BV but symptoms recur, trichomoniasis or resistant Candida should be tested before retreating with BV medication.
When to Call a Professional
Severely foul, rotting smell — dramatically worse than previous periods — rule out a forgotten tampon immediately; seek medical attention same day.
Fishy smell with fever, pelvic pain, or feeling unwell — possible pelvic inflammatory disease (PID); go to urgent care.
BV treated multiple times but still recurs every period — see a gynecologist specializing in vaginal health; extended therapy or maintenance protocol may be needed.
Yellow-green frothy discharge with fishy smell and recent new partner — get tested for trichomoniasis.
How to Prevent It
Use pH-lowering products at the start of your period — boric acid suppositories or RepHresh counteract alkaline menstrual blood pH.
Never douche — disrupts protective Lactobacillus and worsens BV.
Use condoms consistently — semen pH disruption is a major BV trigger.
Take Lactobacillus probiotics after each period to restore protective vaginal flora.
Change menstrual products frequently — reduces bacterial proliferation in stagnant blood.
Treat BV fully — stopping treatment early before symptoms resolve leads to recurrence.
Why Fishy Period Smell Is Specifically a BV Sign
The fishy smell during periods is so reliably associated with BV that gynecologists use it as a diagnostic clue. The mechanism is precise: BV-associated anaerobic bacteria produce amines (trimethylamine, putrescine, cadaverine) that are volatile at alkaline pH. During the rest of the menstrual cycle, the vagina's normal acidic pH (3.8-4.5) suppresses amine volatilization — the amines are present but trapped in their non-volatile form. When menstrual blood raises pH to near-neutral, the amines convert to their volatile form and become intensely odorous.
This is exactly what the 'whiff test' exploits: gynecologists add a drop of 10% potassium hydroxide (alkaline) to vaginal discharge — if BV is present, the amines volatilize instantly, producing a strong fishy odor. The person's own menstrual blood is performing this same chemical reaction every month, which is why the fishy smell is so characteristic of BV during periods.
Importantly, a fishy period smell doesn't mean poor hygiene or that something is fundamentally wrong — it means the vaginal microbiome has shifted, which is a medical condition, not a personal failing. BV is the most common vaginal condition in reproductive-age people and is treated with a short antibiotic course or boric acid.
Frequently Asked Questions
The fishy smell is specific — it's caused by trimethylamine from BV bacteria becoming volatile when menstrual blood raises vaginal pH. 'Just bad' period smell is typically metallic or musky (iron oxidation from blood). Distinctly fishy is almost always BV. The two smells are chemically different and diagnostically significant.
Sometimes — BV occasionally resolves on its own when the vaginal pH returns to normal after the period ends and Lactobacillus bacteria recover. But BV that recurs every period usually means the microbiome imbalance persists between cycles and just becomes symptomatic during the monthly pH disruption. Treatment followed by preventive measures is more reliable than waiting.
Yes — older blood that has had more time to interact with vaginal bacteria tends to smell stronger. At the end of the period, the blood is darker and more concentrated, and has been in contact with vaginal bacteria longer, which amplifies BV-related fishy odors. This end-of-period intensification is consistent with BV.
Not necessarily — BV is not a sexually transmitted infection (though sexual activity influences vaginal pH). The fishy smell is most commonly BV, which can develop in people who have never been sexually active. However, trichomoniasis (an STI) also causes a fishy smell during menstruation and can only be distinguished from BV with testing. If there's any possibility of STI exposure, testing is the right approach.