Can Semen Cause a Fishy Smell the Next Day?
The short answer is yes — and it doesn't always mean infection. Here's exactly what's happening and how to tell if it's something more.
Last updated: 2026-06-08
The pH chemistry
Semen has a pH of 7.2–8.0. Vaginal flora maintains an acidic environment (pH 3.8–4.5) that suppresses the bacteria responsible for BV odor. When semen raises that pH, anaerobic bacteria briefly overgrow and release trimethylamine — the same fishy-smelling compound that causes BV odor.
How long does it last?
For women with healthy vaginal flora, the pH self-corrects within 6–24 hours. The smell should be mostly gone by morning. If the smell persists beyond 36–48 hours, or returns every single time after unprotected sex, it may indicate that BV bacteria were already present and the semen-pH rise is simply unmasking them.
The BV-unmask pattern
If you always get a fishy smell after unprotected sex that resolves in a day, you may have a subclinical BV that only becomes detectable when pH rises. A vaginal swab taken within 24 hours of unprotected sex (when pH is still elevated) is more likely to test positive if this is the case.