Sewer gnats come from drain buildup and need both cleaning and drain treatment to disappear

Sewer gnats (also called drain flies or filter flies) live in the slime and organic matter that builds up inside your pipes. They lay eggs in that buildup, so killing the adults alone will not stop them—new ones will hatch within days. To get rid of them permanently, you need to clean the drain itself, then treat it to prevent eggs from hatching. The whole process takes about two weeks if you stay consistent.

The gnats are not a sign your home is dirty. They thrive in any drain that has standing water or decomposing material—even a rarely used shower drain or a kitchen sink that drains slowly. Once you break the breeding cycle, they disappear and do not come back unless the same conditions return.

Key Takeaways

  • Sewer gnats breed in the sludge inside your pipes, so drain cleaning is the only permanent fix.
  • A combination of boiling water, a drain brush or plumbing snake, and enzyme drain cleaner will remove the buildup where eggs hatch.
  • After cleaning, use a drain treatment like apple cider vinegar and baking soda or a commercial enzyme product once a week for two weeks to kill remaining larvae.
  • Covering drains with a plug or mesh screen at night stops adults from flying out while you treat the pipes.
  • If gnats return after two weeks of treatment, the drain is still holding moisture or buildup—repeat the cleaning process on that specific drain.

Step 1: Identify which drain is the source

Sewer gnats cluster around one or two drains, usually the kitchen sink, bathroom sink, or shower. Watch where you see the most gnats flying, and check that drain first. If you see them near multiple drains, start with the kitchen sink—it collects the most organic matter.

Pour a small amount of water down the drain and watch for gnats to emerge. If they swarm up, that is your main source. Mark it so you know where to focus your effort.

Step 2: Clean the drain physically

Boil a kettle of water and pour it slowly down the drain. This kills some larvae and loosens the buildup. Wait five minutes, then use a drain brush or plumbing snake to scrub the inside of the pipe as far as you can reach. Push down, twist, and pull back repeatedly to break up the sludge. You will feel resistance—that is the buildup. Keep going for at least two minutes.

If you do not have a drain brush, a straightened wire coat hanger with a small hook bent at the end works. Push it down the drain and hook the sludge, pulling it back up. It will be dark, slimy, and smell bad—that is exactly what you want to remove.

After scrubbing, pour another kettle of boiling water down the drain to flush out the loosened material. Do this once a day for three days in a row on the affected drain.

Step 3: Treat the drain with enzyme cleaner or vinegar

Once you have cleaned out the bulk of the buildup, use a drain treatment to kill any remaining eggs and larvae. You have two main options: a commercial enzyme drain cleaner (like Green Gobbler or Bio-Clean) or a homemade mixture of baking soda and apple cider vinegar.

For the homemade method, pour half a cup of baking soda down the drain, followed by one cup of apple cider vinegar. Cover the drain with a plug or wet cloth and let it sit for 30 minutes. The fizzing action breaks down remaining organic matter. Then flush with hot water. Repeat this treatment once a day for 7 to 10 days.

For a commercial enzyme product, follow the package instructions—most require you to pour the product down the drain, cover it, and leave it overnight. Use it once a day for 7 to 10 days. Enzyme cleaners work more slowly than boiling water but are gentler on older pipes.

Step 4: Cover the drain at night

While you are treating the drain, cover it with a plug or a piece of mesh screen at night. Adult gnats emerge from the drain in the evening and early morning. A straightforward drain plug or even a damp cloth stops them from flying out into your home. This does not kill them, but it keeps them contained while the drain treatment kills the eggs and larvae below.

You can remove the cover during the day when you are using the sink. The goal is to trap the adults so they cannot lay new eggs elsewhere in your home.

Step 5: Continue treatment for two full weeks

Sewer gnat eggs hatch in about 48 hours, and the larval stage lasts 8 to 10 days. To break the cycle completely, keep treating the drain for at least two weeks. This ensures you kill every generation of larvae before they become adults.

After two weeks, stop the daily treatment and monitor the drain. If you see gnats again within a few days, it means that drain still has moisture or buildup. Repeat the physical cleaning (boiling water and drain brush) and go back to daily treatments for another week.

What to do if gnats return after treatment

If sewer gnats come back after you have finished treatment, one of three things is happening: the drain is still wet and holding organic matter, a different drain is now the source, or the drain trap is damaged and allowing sewer gas and gnats to enter.

Check the drain by running water and watching how fast it drains. If it drains slowly, repeat the physical cleaning with boiling water and a drain brush. If it drains normally but gnats still appear, check other drains in your home—bathroom sinks, showers, and floor drains are common secondary sources. Treat each one the same way: boiling water, brush, and enzyme cleaner for two weeks.

If you have treated multiple drains and gnats persist, the drain trap (the U-shaped pipe under the sink) may be cracked or disconnected. This is rare but allows sewer gnats to enter directly from the sewer line. Call a plumber to inspect the trap.

Frequently Asked Questions

Can I use bleach to kill sewer gnats?

Bleach kills adult gnats and some larvae, but it does not remove the buildup where eggs are laid. Gnats will return within days because new eggs will hatch. Bleach also damages older pipes and is less effective than boiling water combined with physical scrubbing. Stick with boiling water, a drain brush, and enzyme cleaner for lasting results.

How long does it take to get rid of sewer gnats completely?

If you clean the drain thoroughly and treat it consistently, gnats should be gone within two to three weeks. The first week kills most adults and larvae. The second week catches any eggs that were laid after you started treatment. If you see gnats after two weeks, the drain still has buildup and needs another round of cleaning.

Will sewer gnats go away on their own?

No. Without treatment, they will keep breeding in the drain indefinitely. The population may shrink if the drain dries out completely, but any moisture or organic matter will restart the cycle. Active treatment is the only way to stop them.

Can I use a drain fly trap instead of cleaning the drain?

Drain traps (sticky traps placed over the drain opening) catch adult gnats but do not stop eggs from hatching inside the pipe. They are useful for monitoring whether treatment is working, but they cannot replace drain cleaning. Use a trap alongside boiling water and enzyme cleaner, not instead of them.

What if the gnat problem is in a floor drain or basement drain?

Floor drains and basement drains work the same way as sink drains. Pour boiling water down the drain, use a drain snake if you can access it, and treat with enzyme cleaner or vinegar for two weeks. If the drain is rarely used, it may dry out and crack, allowing gnats to enter from the sewer. Keep it wet by pouring water down it once a week, even after gnats are gone.