Note to readers: This post was updated on August 08, 2026 to lead with a direct diagnosis by smell type, separate sewer gas from mold/hard water causes, and add a link to our drain odor guide. We refreshed the details below so the advice reflects how we approach this work today.
A shower that smells bad right after you run it usually means one of four things: a dry P-trap letting sewer gas through, biofilm buildup on the showerhead or drain, hard water mineral scum feeding bacteria, or mold in the grout and curtain. The rotten-egg smell points to sewer gas. A musty, damp smell points to mold or biofilm. Figure out which one you’ve got before you reach for a cleaner.
Does it smell like rotten eggs? That’s sewer gas.
Every drain has a P-trap — the U-shaped section of pipe that holds a bit of standing water and blocks sewer gas from coming back up through the drain. Showers that don’t get used often (a guest bathroom, for example) can dry out that trap, and once it’s dry, gas has a clear path into your bathroom.
How to fix a dry trap
Run water in the drain for 30 seconds to refill the trap. If the smell comes right back within a day or two, the trap may be draining too fast because of a vent pipe issue, or there’s a crack somewhere letting the water evaporate faster than normal. A blocked or damaged vent stack is the other common cause of sewer gas smell — vents are what let air pressure equalize in the system so traps hold water properly. That’s a job for a plumber with a camera, not a DIY fix.
Does it smell musty or sour instead? Check for biofilm and mold.
Biofilm is the slimy layer that builds up inside showerheads and along drain walls in a warm, wet environment — it’s normal, but it smells bad once it accumulates. You’ll often notice reduced water pressure from the showerhead at the same time, since the same buildup narrows the holes.
Clearing biofilm and mold
- Soak the showerhead in equal parts water and white vinegar for a few hours, then scrub with an old toothbrush.
- Wipe down tile, grout, and the shower curtain or liner with a vinegar-water mix weekly; replace a liner once it shows black spots that don’t scrub out.
- Run the bathroom fan or crack a window during and after showers so surfaces dry instead of staying damp all day.
Mold and mildew need moisture to keep growing. A bathroom that never fully dries out between uses will keep producing the smell no matter how often you clean, so ventilation matters as much as the cleaning itself.
Is your water hard? That changes what’s growing in there.
Hard water leaves mineral scum on shower walls and fixtures when it mixes with soap. That scum traps bacteria, and the bacteria breaking down organic matter in the scum is what produces the odor — not the minerals themselves. If you’re also seeing gray or white residue that’s hard to scrub off, hard water is likely part of the problem. A water softener addresses this at the source; regular vinegar cleaning manages it in the meantime.
When is it more than a cleaning problem?
If you’ve cleaned the showerhead, checked the trap, and the smell is still there after a few days, the cause is probably downstream — a partial clog in the branch line trapping wastewater and gas, or a vent pipe problem. Persistent drain odor that doesn’t respond to cleaning is worth a professional look before it gets worse.
Call a plumber if: the smell returns within a day of cleaning, you notice slow drainage along with the odor, more than one fixture in the house smells the same way, or you smell gas rather than mildew. Multiple fixtures smelling at once usually points to a main line issue rather than something local to the shower.
If your shower still smells strange after running water and cleaning hasn’t fixed it, call Western Rooter & Plumbing at (626) 448-6455 or schedule online. We’ll run a camera if needed to find out what’s actually going on in the line.
