How to Get Rid of Drain Flies

To get rid of drain flies, you have to remove the slimy film inside the drain where their larvae live: find the drain they come from, scrub it out with a pipe brush, then use an enzyme drain cleaner. Repeat weekly for three to four weeks, because killing the adults on the wall does nothing while the breeding site is still there.

Make sure they are drain flies

Drain flies (also called moth flies or sewer gnats, family Psychodidae) are true flies that look like tiny moths:

• 1/16 to 1/5 inch long (2–5 mm), gray or tan, with the body and wings covered in fuzzy hairs.
• Wings held like a roof over the body when resting, giving a small triangle or heart shape.
• Weak, jerky fliers. They hop a few inches and land again, usually on the wall by the sink, tub or toilet.
• Active at night and most noticeable in bathrooms, basements and laundry rooms.

If the flies are tan with red eyes and hang around the fruit bowl, they are fruit flies. If they are slender and black and rise from houseplant soil, they are fungus gnats. Our gnats vs fruit flies guide covers all three side by side.

Step 1: find the exact drain (the tape test)

Don't clean everything blindly. Find the source first:

1. At night, put a strip of clear tape over each drain opening, sticky side down: sinks, tub, tub overflow, shower, floor drain. Leave a small gap for air.
2. In the morning, look at the sticky side. Flies that emerged overnight will be stuck over the drain they came from.
3. Repeat for 2–3 nights to see if there is more than one source.
4. No flies on any tape but still seeing them? Look beyond the drains (see below).

Alabama Extension and UF/IFAS both recommend this method because it beats guessing from where adults happen to rest. Drain flies fly poorly, so the nearest drain is usually the source, but not always. Two extra spots worth taping: the tub overflow plate and the seam where the toilet meets the floor (a failed wax seal lets flies breed under the toilet).

Step 2: scrub out the biofilm

Females lay eggs in masses of roughly 15 to 100 in the gelatinous film on pipe walls, a mix of bacteria, soap scum, hair and skin oils. Per Alabama Extension, eggs hatch in about 2 days, larvae feed for 8 to 15 days, pupae take 1 to 2 days, and the whole cycle runs 8 to 24 days depending on temperature. Larvae (thin, legless, grayish, up to about 3/8 inch) breathe through a short tube at the tail, which is why a quick flush of liquid doesn't drown them. Adults emerge ready to mate within hours. Remove the film and you remove the population.

1. Pull the P-trap under the sink if it comes off by hand (put a bucket underneath). Scrub the inside of the trap and the pipe with a narrow brush.
2. If the trap won't come off, run a stiff, flexible drain brush down the drain as far as it will go and work it back and forth.
3. Clean the tub overflow. The hole under the faucet is the most overlooked source. Unscrew the cover plate and brush inside.
4. Shower and floor drains: lift the grate, scrub the grate and the walls below it.
5. Don't forget the pieces: pop-up stopper, pivot rod, shower strainer and the underside of a garbage disposal's rubber splash guard collect the same slime. Clean them before reinstalling.
6. Finish with an enzyme or bacterial drain cleaner (gel or foam types cling to the pipe walls better than liquid). Use it at night when the drain won't be used for several hours. Texas A&M notes these products need repeat applications over several weeks; they work best after mechanical cleaning, not instead of it.
7. Repeat weekly for 3–4 weeks. The cycle is 8 to 24 days, so this catches anything that hatches from surviving eggs.

Kill the adults you can see with a fly swatter, a vacuum, or a basic flying-insect spray. That is cosmetic: it makes the room bearable while the cleaning works. A small sticky trap or a UV light trap near the sink is useful as a counter: if the daily catch keeps falling, the plan is working.

Drain fly killer products, compared

How dangerous are these products

The real fix for drain flies (a brush and an enzyme cleaner) is close to harmless. The risks come from the extras: spray in a small bathroom with no ventilation, bleach mixed with other cleaners, and foggers that coat every surface.

Comparison table

Enzyme / bacterial drain gel or foamActive ingredient: microbes, enzymes

CatsSafe

DogsSafe

Kids, pregnancy, allergiesSafe

keep the bottle out of reach; irritating if swallowed

Fish, birds, rodents, reptilesSafe

Bio foam with IGRActive ingredient: microbes + hydroprene (check the label)

CatsSafe

low toxicity; keep pets away until it settles

DogsSafe

Kids, pregnancy, allergiesCaution

keep kids out while applying

Fish, birds, rodents, reptilesCaution

don't use near the aquarium

Flying-insect sprayActive ingredient: pyrethrins, often with piperonyl butoxide

CatsKeep out

keep out until aired; cats lick it off paws and fur

DogsCaution

keep away while spraying

Kids, pregnancy, allergiesKeep out

out while spraying and airing; triggers asthma

Fish, birds, rodents, reptilesDanger

extremely toxic to fish; birds out of the room

Sticky and UV trapsActive ingredient: none

CatsCaution

keep sticky cards where pets can't reach

DogsCaution

Kids, pregnancy, allergiesSafe

Fish, birds, rodents, reptilesCaution

small animals can get stuck

BleachActive ingredient: sodium hypochlorite

CatsKeep out

fumes; keep away until rinsed and aired

DogsKeep out

same

Kids, pregnancy, allergiesKeep out

never mix with ammonia or acid cleaners: toxic gas

Fish, birds, rodents, reptilesDanger

birds are very sensitive to fumes

FoggersActive ingredient: pyrethroids + flammable propellant

CatsDanger

coats everything

DogsKeep out

evacuate

Kids, pregnancy, allergiesDanger

everyone out for the label time; fire risk

Fish, birds, rodents, reptilesDanger

deadly to fish and birds

Sprays and foggers. Pyrethrins are less toxic to cats than permethrin, but in a small bathroom the mist settles on the floor and litter box. Close the door, keep pets and kids out until aired, and never spray toward an aquarium. Bleach combined with ammonia or acidic drain products releases toxic gas: if you feel burning in the eyes, nose or chest, leave and get fresh air.

If something goes wrong: wash skin with soap and water; rinse eyes for 15 minutes; if someone swallowed a product or has trouble breathing after fumes, call Poison Control at 1-800-222-1222. For a pet that's drooling, trembling or vomiting, call your vet or ASPCA Animal Poison Control at (888) 426-4435. See also pet-safe pest control.

What doesn't work, and why

• Boiling water. It cools within seconds in the pipe and doesn't penetrate the slime. It can also crack PVC fittings and porcelain.
• Bleach. Flows straight past the biofilm. Larvae have water-repellent hairs and a breathing tube that lets them ride out a brief flush.
• Baking soda and vinegar. The fizz looks impressive but doesn't remove a film that's been building for months.
• Bug bombs and foggers. They kill some adults, leave residue on every surface, and never reach the larvae.
• Fly spray down the drain. Alabama Extension is blunt: never pour insecticides down drains. They aren't labeled for it and don't reach larvae inside the film.
• Mixing cleaners. Never combine bleach with ammonia or acidic cleaners in a drain; the fumes are dangerous.

The Utah State and Alabama extension services both list these methods as ineffective on their own, and Alabama adds that some can damage pipes. People who say "I've poured bleach down every day for a month" are almost always describing a drain that was never brushed.

Drain flies in the bathroom: the usual hiding spots

• Sink trap and the horizontal pipe behind it
• Tub overflow (very common)
• Shower drain below the strainer
• Toilet: a failed wax ring lets water and sewage seep under the base, creating a hidden breeding spot
• Rarely used guest bath sink or shower, where the water seal dries out

Run water in unused drains once a week so the trap stays full.

If the drains are clean and they still show up

Drain flies need wet organic matter, not necessarily a drain. Check for:

• Leaks under the kitchen sink, behind the dishwasher, or under a shower pan.
• Floor drains and sump pits in basements and garages.
• A cracked sewer line under the slab. If flies appear near a floor crack or baseboard with no drain around, this is a plumber's job. A camera inspection costs roughly $150–$400 (approximate, varies by region).
• Air conditioner condensate pans and drip lines.
• Outside sources like clogged gutters or a wet compost pile next to a window.

What people who dealt with it say

Threads on r/pestcontrol, r/HomeImprovement and r/Plumbing about drain flies follow a predictable arc, and the patterns are useful:

• Almost everyone starts with boiling water, bleach or vinegar, and almost everyone reports it did nothing. The posts that end with "they're gone" mention a brush, pulling the trap, or weeks of a foaming bio product.
• The source is often a drain nobody uses. Pest pros repeatedly point to basement floor drains next to the washer, guest showers whose traps have dried out, and tub overflows. One technician described a customer whose entire infestation came from a basement floor drain he had never looked into.
• Things get worse before they get better. One homeowner foamed the basement shower and sink with a bio product before a week's vacation and came back to a tape full of flies; pros replied that years of built-up film take repeated treatments every couple of weeks to break down, and that scrubbing the pipe speeds it up.
• Exterminator sprays rarely help. Several people paid for repeated spray visits that "just killed the adults." A pest pro with 11 years in the trade said he never uses insecticides for drain flies; he has customers treat every drain with an enzyme cleaner daily until the flies stop.
• When flies come out of a wall, a floor crack or an access panel, it's plumbing. The hardest cases on Reddit ended with a camera inspection, a moisture test or a small hole in the drywall that revealed a leaking pipe or a broken sewer line under the slab. Homeowners with 100+ flies a day from a slab bathroom were told pest control can't fix that.
• Check the non-drain gunk too: forgotten cereal bowls in a kid's room, a refrigerator drip pan, a dishwasher filter or wet sponges can breed the same small flies.

Typical timeline in these threads: a light problem fades in 2–3 weeks of brushing plus enzyme treatment; a long-ignored drain takes a month or more; a sewer-line problem doesn't end until the pipe is fixed.

When to call a pro

Call a plumber when the tape test points to a floor drain or an area with no visible plumbing, or when flies keep coming after a month of weekly cleaning. Pest control companies can treat drains with foaming biological products and insect growth regulators, but they can't fix a broken pipe; repeated spraying for adults is a sign the source hasn't been found. Expect a one-time drain fly service around $150–$300 (approximate).

Keeping them from coming back

• Brush and enzyme-treat busy drains once a month.
• Clean the tub overflow when you clean the tub.
• Fix drips and leaks promptly.
• Keep traps in rarely used drains filled with water (run them weekly).
• Clean the garbage disposal splash guard and dishwasher filter monthly.
• Keep the refrigerator and AC condensate pans clean and dry; a dehumidifier helps in damp basements.

Drain flies don't bite and aren't a sign of a dirty home. They're a sign of a dirty pipe, and that is a fixable problem.