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Bathroom Sink Smells? The Overflow Hole Is the Usual Culprit

The overflow channel never gets rinsed, so slime builds up there and in the drain. Scrub it out with a long thin brush and the smell goes.

By Jamilur Rahman Updated November 5, 2026 · 6 min read

Time
20 min
Cost
Under $20
Difficulty
Easy
A white bathroom sink overflow hole near the rim with a faint dark film inside it
In this guide
  1. Quick answer
  2. Free vs. permanent fix
  3. What you'll need
  4. Why it happens
  5. Try this first (free)
  6. When free fixes aren't enough
  7. Buy the fix
  8. Why does my bathroom sink drain smell bad? Check three places, in order
  9. What to skip
  10. FAQ
  11. Related problems

Quick answer

Why does my bathroom sink smell? Most often it is slime growing in the overflow hole and on the drain walls, not a sewer problem. Clean the stopper, flush the overflow with baking soda and vinegar, and scrub the channel with a long thin brush. If it smells like sewage instead of slime, check for a dry trap or a vent problem.

Free fix vs. permanent fix

Free fix · 20 minutes

Baking soda, vinegar and hot water

  • Wipe the stopper and drain opening clean
  • Pour baking soda, then vinegar, into the overflow and the drain
  • Wait 15 minutes, then rinse with very hot tap water
  • Run water in rarely used sinks every week or two

How long it lasts: A few weeks. The fizz loosens some film, but slime that clings inside the overflow channel is still there.

Most readers choose Permanent fix
Overflow cleaning brush set (illustration)
Illustration

An overflow cleaning brush set

The overflow channel is a hidden tube that water almost never flushes. A thin, flexible brush reaches in and scrubs the walls where slime and toothpaste collect, which is the cause for most sinks. A fizzing cleaner cannot reach all of it.

What to get: Overflow cleaning brush set

How long it lasts: Reusable. A quick pass every month stops the slime from building up again.

Check price on Amazon

As an Amazon Associate I earn from qualifying purchases.

What you'll need

  • Baking soda and white vinegar

    Loosens the film in the overflow and the drain

    You have it

  • Old toothbrush

    For the stopper and the drain opening

    You have it

  • Rubber gloves

    For handling slime

    You have it
  • Our pick
    Overflow cleaning brush set (illustration)
    Illustration

    Overflow cleaning brush set

    Long, thin and flexible, for the hidden channel

    Check price
  • Enzyme drain gel (illustration)
    Illustration

    Enzyme drain gel

    Digests the film inside the pipe

    Check price

Why it happens

1

Slime in the overflow hole

It is a hidden tube that almost never gets flushed. Soap, toothpaste and skin oil collect there, and bacteria turn it into a sour, mildew or rotten-egg smell.

2

Slime on the drain wall and stopper

The same film builds up under the stopper and inside the pipe. You smell it every time the water runs.

3

A dry P-trap

The U-bend under the sink holds water that blocks sewer gas. In a rarely used sink, that water evaporates and the smell comes up.

4

A loose trap connection or a blocked vent

A leaking joint lets gas out, and a blocked vent can make the water in the trap get pulled out. Gurgling is the clue.

5

Hot water only

If only the hot water smells like rotten eggs, the source is usually the water heater, not the sink. See the related page on shower water.

Try this first (free)

Do these in order, starting with the simplest.

  1. 1. Find the source

    Sniff the overflow hole, the drain opening and the stopper separately. A smell that is strongest at the overflow means slime. The drain opening points to the trap. Hot water only points to the water heater.

  2. 2. Clean the stopper and drain opening

    Tool used: Old toothbrush

    Lift the stopper, wipe off the slime with a toothbrush and soapy water, and scrub inside the drain as far as you can reach.

  3. 3. Flush the overflow

    Tool used: Baking soda and white vinegar

    Pour a quarter cup of baking soda into the overflow hole, then half a cup of white vinegar. Let it fizz 15 minutes, then pour a pitcher of very hot tap water through the overflow.

  4. 4. Scrub the overflow channel

    Tool used: Overflow cleaning brush set

    Push the thin brush down the overflow hole and work it back and forth along the walls. Rinse with hot water. The brush pulls out slime that fizzing leaves behind.

    Overflow cleaning brush set (illustration)
    Illustration

    Tool for this step

    Overflow cleaning brush set · Long, thin and flexible, for the hidden channel

    Check price on Amazon

  5. 5. Treat the drain overnight

    Tool used: Enzyme drain gel

    At bedtime, pour an enzyme gel down the drain and leave it overnight. Rinse in the morning. This handles the film inside the pipe that the brush cannot reach.

    Enzyme drain gel (illustration)
    Illustration

    Tool for this step

    Enzyme drain gel · Digests the film inside the pipe

    Check price on Amazon

  6. 6. Refill the trap

    Run water in rarely used sinks for 30 seconds every week or two. A full P-trap blocks sewer gas.

When free fixes aren't enough

If the smell returns within a couple of weeks, the slime in the overflow channel is thicker than baking soda can reach, and a brush is the fix. If it smells like sewage or rotten eggs from the drain and not the overflow, the trap may be dry, loose or siphoned, or the vent may be blocked and the sink gurgles when it drains. A strong, lasting sewer smell, or anyone feeling unwell in the room, means open windows and call a plumber. A rotten-egg smell from hot water only usually points to the water heater.

Buy the fix

Still stuck after the free fixes? These are the products we would look at.

Our picks compared
PickProductFixesWho it's forBuy
Best pick
Overflow cleaning brush set (illustration)
Illustration
Overflow cleaning brush set
the slime in the overflow channel that causes most sink smells Anyone whose sink smells like mildew or rotten eggs and is not blocked. Check price
Upgrade pick
Enzyme drain gel (illustration)
Illustration
Enzyme drain gel
the slime inside the drain pipe People whose drain opening still smells after the overflow is clean. Check price
Best pick
Overflow cleaning brush set (illustration)
Illustration

Overflow cleaning brush set

Fixes: the slime in the overflow channel that causes most sink smells

Look for: Long, thin and flexible, so it bends down the overflow channel; Several sizes to match different sinks; Nylon bristles, no metal.

Who it's for
Anyone whose sink smells like mildew or rotten eggs and is not blocked.
Honest downside
Your sink has no overflow hole, or the smell is clearly sewer gas from the trap.
Check price on Amazon
Upgrade pick
Enzyme drain gel (illustration)
Illustration

Enzyme drain gel

Fixes: the slime inside the drain pipe

Look for: Enzymes or bacteria, listed on the label; A gel that clings to the pipe walls; Safe for your pipes and septic system, per the label.

Who it's for
People whose drain opening still smells after the overflow is clean.
Honest downside
You recently used a chemical drain opener. Flush well with water first.
Check price on Amazon

As an Amazon Associate I earn from qualifying purchases. We don't show prices because they change, so use the button to see today's price.

How we picked: How we picked: this page is based on plumbing and drain-care guides, advice on sewer gas and dry traps, product directions from brush and enzyme gel makers, and what owners report in reviews. We do not test products by hand.

Why does my bathroom sink drain smell bad? Check three places, in order

  1. The overflow hole: sniff it. A sour or musty smell here is slime in the channel.
  2. The stopper and drain opening: lift the stopper and sniff the slime on it.
  3. The trap and vent: a sewer smell with gurgling points here.

What to skip

Scented drain fresheners and bleach plus vinegar

A fragrance product hides the smell for a day while the slime keeps growing. Never pour bleach and vinegar together, or one right after the other, because the fumes are toxic.

Frequently asked questions

Why does my bathroom sink smell like sewer or sewage when I run the water?
The trap may be dry, loose or siphoned, or the vent may be blocked. Run water for 30 seconds to refill the trap, then check for gurgling and for a loose connection under the sink.
Why does my bathroom sink smell like rotten eggs or sulfur?
Bacteria in drain slime give off a sulfur smell. If only the hot water smells, the water heater is the likelier source. If the smell is strong or does not fade, it may be sewer gas, so ventilate and call a plumber.
Why does my bathroom sink smell like mildew or fish?
Slime and mold in the overflow and under the stopper smell musty or fishy. If a fishy or burning smell comes from near an outlet or the fan, switch off that circuit and have an electrician look.
Why does my bathroom sink smell but it is not blocked?
A smell does not need a clog. Slime in the overflow and on the pipe walls is enough, and it builds up even when water drains fast.
How do I stop my bathroom sink from smelling and growing mold?
Wipe the stopper weekly, brush the overflow monthly, run the fan after showers and keep the area dry. A weekly pass beats a monthly rescue.

As an Amazon Associate I earn from qualifying purchases. Affiliate disclosure

Overflow cleaning brush set (illustration)
Illustration

Our pick: Overflow cleaning brush set

Check price