Why Does My Bathtub Drain Smell Bad? Causes by Smell, and Fixes

A quick test separates them. The Minnesota Department of Health guide to hydrogen sulfide in water suggests smelling the water from the hot and cold faucets after you have been away from home for a few hours. Fill a glass of each from the tub spout, step away from the drain and smell them. If the water smells, it is a water problem. If the water is fine and the smell is at the drain, it is a drain problem.

Graphic: summary of this guide, made by our team from the cited sources.

Match the smell to the cause

What you notice	Likely cause	First thing to try
Musty or rotten smell right at the drain, slow draining	Hair and soap buildup in the stopper and drain	Pull the stopper and clean it
Sewer smell after the tub sat unused	Trap water evaporated	Run water for a minute
Sewer smell plus gurgling when other fixtures drain	Blocked vent pulling the trap seal	Check the roof vent; call a plumber
Rotten-egg smell from hot water only	Water heater anode reaction	See the water heater section below
Rotten-egg smell from hot and cold water	Hydrogen sulfide in the water supply	Test the water

Sources: water heater and supply causes from NDSU and the Minnesota Department of Health; traps and vents from the Wisconsin Department of Health Services sewer gas page.

Cause 1: Gunk in the drain and stopper

Hair, soap and skin cells collect on the stopper linkage and the first few inches of pipe, and bacteria grow on it. You will often see slower draining too.

1. Remove the stopper. Many lift-and-turn and push-pull stoppers unscrew. Trip-lever stoppers usually come out through the overflow opening once you remove the cover plate screws. Check your tub maker's instructions if you are not sure which type you have.
2. Pull out the hair and slime with gloves, or use a plastic drain hair tool.
3. Scrub the stopper and the drain opening with an old toothbrush and dish soap.
4. Run hot water for a minute and check the smell.

Cause 2: A dry or damaged trap

Every tub has a trap below it that holds water to block sewer gas. The International Plumbing Code requires each fixture trap to hold a water seal of at least 2 inches and no more than 4 inches (Section 1002.4 of the International Plumbing Code, Chapter 10). Local codes may differ. In a guest bath that isn't used for weeks, that water can evaporate. The City of North Royalton says normal use keeps traps full and recommends running water through drains you don't use often (City of North Royalton sewer odor tips).

If the smell comes back soon after you refill the trap, the trap or drain pipe may be leaking, or the drain may be losing its seal to a vent problem. The same idea applies under a sink: our guide on how to clean a smelly P-trap under the kitchen sink shows how to take a trap apart and clean it.

Cause 3: A vent problem

Drains need air from the plumbing vent on your roof. If the vent is blocked, water rushing down another drain can suck the water out of the tub trap. The sign is gurgling in the tub when a toilet flushes or a sink drains. The Wisconsin Department of Health Services lists a leaking or blocked roof plumbing vent as one way sewer gas enters a home, and suggests checking the vent occasionally for leaves or bird nests. Roof work is risky, so a plumber is the safer choice for clearing a vent.

Cause 4: The water heater or the water supply

If only the hot water smells like rotten eggs, NDSU says that points to sulfates in the water reacting with the water heater's magnesium anode rod. Its suggested fix is to remove the anode or replace it with an aluminum one, and to have the water tested first. The Minnesota Department of Health recommends having a plumber or water system professional do this work unless you are very familiar with water heaters. If both hot and cold water smell and you are on a well, the Minnesota health department lists the groundwater, the well or the plumbing system as possible sources, so test the water before you buy treatment.

If the water heater has other problems, our guide to a water heater that keeps tripping the breaker covers electric units.

When the smell is a safety issue

A strong, persistent sewer smell is not just unpleasant. The Wisconsin Department of Health Services says hydrogen sulfide can cause eye and breathing irritation, headaches, dizziness and nausea, and at high concentrations it can interfere with your sense of smell, so you can no longer rely on the odor as a warning. It advises leaving the building and calling the fire department if you believe sewer gas levels are high in an enclosed space.

If more than one drain smells, or the smell comes with slow drains throughout the house, see why a drain keeps clogging and consider a plumber.