Fishy smell after unprotected sex: why, and what to do

Author: Aunt Vadge
Qualified Naturopath | BHSc(N)

Last reviewed: June 30, 2026

Hi Aunt Vadge,

I’m almost at the end of my period and had unprotected rough sex. Now I’ve noticed a fishy smell in the remaining brownish discharge. Is this an infection or something serious? When do I need to see a doctor?

Yours,
Concerned
Age 41, Canada

Dear Concerned,

A fishy smell after unprotected sex is most often bacterial vaginosis (or its cousin aerobic vaginitis) — a microbial imbalance rather than a true infection — and it’s extremely common, so try not to panic. But because it followed unprotected sex, it’s worth a quick test to rule out an STI first, since trichomonas and gonorrhoea can also smell fishy; rule those out, then treat what’s left.

The timing tells a neat little pH story. Your healthy lactobacilli keep the vagina acidic (pH around 3.8–4.5), which holds disruptive bacteria in check, and two things just pushed your pH up: menstrual blood (alkaline) and semen (alkaline, around pH 7–8).

Stack those together at the tail end of your period and you get a brief, less-acidic window where the wrong bacteria can bloom — hence the smell turning up right now. The brownish discharge is simply the end of your period, nothing sinister.

And this rarely comes out of nowhere; there’s usually a flora imbalance already simmering, and the period-plus-semen combo just tips it over.

The part the standard advice leaves out is that he’s part of this. Men carry BV and AV microbes on the penis, almost always without symptoms, and pass them between partners during unprotected sex, ejaculation or not — so a recurring ‘fishy vagina’ is often a two-person problem.

It’s worth a frank chat with your partner, and if this keeps happening, treating him too is what finally breaks the cycle; it’s not gross and it’s not your fault, just microbiology nobody tells men about.

Practically: get a swab to rule out an STI (because if it is one, home treatment won’t fix it), don’t use any vaginal treatment for 48 hours before the swab or you risk a false result, grab some pH strips to keep an eye on things (above 4.5 points to BV or AV), and if it’s not an STI, our free Killing BV guide and the tools for a non-infected smelly vagina have your home options.

Most likely it’s common, very treatable BV — just test first to be sure, and don’t forget your partner in the equation.

Warmest regards,
Aunt Vadge

This is general information based on current research and our clinical experience, not a substitute for personalised medical advice.