How to Get Rid of Drain Flies (Sewer Flies) for Good

Illustration: a long flexible brush worked down a bathtub drain, with the drain cover set aside on the tub edge.

To get rid of drain flies, find the drain they breed in by taping it overnight, then scrub the slime out of that drain with a stiff brush and flush it with hot water or a bacterial drain gel. Bleach and bug spray won’t reach the larvae. You’re done when the tape stays clean for a week.

Cost: about $29–46 in supplies. Difficulty: moderate. Call a pro if every drain is scrubbed and the tape still catches flies after two weeks, or flies come up through floor cracks, loose tiles or a sewer backup; that’s a job for a plumber, not an exterminator.

This guide covers drain flies breeding in sinks, tubs, showers, toilets, floor drains and drip pans. It won’t sell you a spray that ends them: adults you kill are replaced every day until the slime is gone, and a broken pipe under the floor needs a plumber, not a product.

Saw drain flies tonight? Swat or vacuum the adults; they don’t bite. Before bed, lay a strip of clear tape across each sink, tub and shower drain, and check which one caught flies in the morning.

Drain flies are one of three common small flies in homes; our plan for getting rid of gnats covers all three.

Shopping for a drain fly killer? Buy for the larvae in the slime, not the adults. Start with a long, flexible drain brush; add a bacterial drain gel only if you want to, and pick one with “bacteria cultures” or “enzymes” on the label. Skip aerosol sprays and kits built around them: spray kills only the flies already out.

What Works and What Doesn’t Against Drain Flies

Do Drain Flies Bite or Make You Sick?

No, drain flies don’t bite or sting. They’re a nuisance, not a health threat. Iowa State Extension notes that because they come from drains they could in theory carry germs, but there’s no evidence they actually spread disease. They don’t damage your pipes either.

What they do is multiply. Drain flies, also called moth flies, sewer flies or filter flies, lay 30 to 100 eggs at a time that hatch in under 48 hours (Texas A&M AgriLife).

Drain Flies vs. Fruit Flies (and Fungus Gnats)

Before you buy a drain fly killer, make sure that’s what you have.

Adults are about 0.1 inch (2–3 mm) long (Virginia Cooperative Extension). They rest on walls by day, hover over drains at night and fly only a few feet at a time.

Fruit flies go away once you remove the fruit they breed in, and fungus gnats die when you let potting soil dry out between waterings. Neither responds to drain scrubbing. One catch: fungus gnats can breed in drains too, so the tape test below applies to them as well.

What Causes Drain Flies?

A slow or seldom-used drain. The larvae live in the gelatinous slime (biofilm) inside drainpipes and traps, feeding on its bacteria, algae and fungi. Any spot that stays wet for a week or more can host them:

• Sink, tub and shower drains, including the overflow hole under the rim, which rarely gets flushed.
• Toilets and tanks that aren’t used often, like a guest bathroom’s.
• Basement floor drains and sump pits.
• Drip pans under the refrigerator, dehumidifier or air conditioner, and icemaker lines.
• Under loose tiles or cracked grout in a shower, where water seeps and sits.

Back from vacation to a bathroom full of flies? These short infestations usually fade once water runs through the toilets and drains again.

Illustration: a sink overflow hole, the inside of a toilet tank, a drip pan under a refrigerator and a basement floor drain, each with a thin film of slime.

Step 1: Find the Drain With the Tape Test

Find the breeding drain first instead of treating every drain in the house.

1. Tape it before bed. Lay one strip of clear tape, sticky side down, across the middle of the drain, leaving a gap on each side. Don’t seal the whole opening: with no airflow, the flies won’t come up (Texas A&M).
2. Check in the morning. Flies stuck to the tape mean that drain is a breeding site.
3. Test every suspect. Include the tub overflow, floor drains and any toilet that sits unused.

Another option: an upside-down clear cup with a thin film of vegetable oil inside works the same way.

Illustration: clear tape across a shower drain with a few small flies stuck underneath (left), and an oiled clear cup turned upside down over a bathroom sink drain (right).

Step 2: Scrub the Slime Out of the Drain

This is how to kill drain flies for good: remove the slime and you remove the larvae and their food.

1. Pull the stopper or cover and clean the hair and gunk off it.
2. Scrub the pipe with a stiff, long, flexible drain brush as far down as it reaches, then the overflow hole.
3. Toilets: scrub under the rim and inside the tank, where a slime film forms at the water line.

Only for badly clogged drains: Virginia Tech says the U-shaped trap under a sink may need to come off for cleaning, and a shower clogged with soapy hair may need a plumber’s snake. Most people can skip both. Renters can ask maintenance to do the trap.

Step 3: Flush and Treat With a Bacterial Drain Gel

Flush. After scrubbing, pour boiling water down the drain to wash out what’s left. Maryland Extension just says hot water, so very hot tap water also works. Hot water without scrubbing gives only short-term control.

Treat. A bacterial drain gel digests the buildup the brush missed, and it’s the drain fly treatment for drains you can’t scrub. On the shelf, look for “bacteria cultures” or “enzymes” on the label. The labels of the gels we priced say:

• How much: 4 ounces per drain, poured around the edge to coat the sides. A quart covers 8 doses.
• When: at bedtime, so the drain sits unused overnight. Don’t rinse it right after. You can shower as usual by day.
• How long: every night for 5–7 days, then every 1–2 weeks to keep it clean.

The gel is optional: if the tape stays clean after scrubbing, skip it. With a septic system, check that the label says it’s compatible.

Kids and pets: these gels contain bacteria and enzymes, not insecticide, but the labels still say to keep the bottle out of reach of children. Never pour insecticide down a drain, and never mix bleach with ammonia or pour it after a caustic (clog-dissolving) drain cleaner; Texas A&M warns that combination can release chlorine gas.

Why Sprays and Quick Pours Don’t Last

Bleach, vinegar and other pours meant to kill sewer gnats reach only a few larvae on the surface of the slime; Virginia Tech says they won’t penetrate the buildup. The larvae are hard to drown, too: they trap air bubbles and survive a day or more underwater. Any of the drain fly home remedies poured without scrubbing leaves the slime and its larvae in place.

Bug spray kills adults only, so new flies keep emerging every day.

How Long Does It Take to Get Rid of Drain Flies?

Expect a week or two after the breeding drain is cleaned. The whole life cycle takes about 10 to 15 days at 70°F (University of Kentucky), so a few already-hatched adults may linger.

Retape the drain every few nights. If it still catches flies a week after scrubbing, treat again or look for another source.

Why Do Drain Flies Keep Coming Back?

Usually because a breeding spot was missed. Check these before you scrub the same sink again:

• The overflow hole of a sink or tub you already scrubbed.
• A drain you never use, like a basement floor drain, laundry sink or guest shower. Run water through it regularly.
• Appliance drip pans and condensate lines.
• Outdoors. Drain flies breed in wet, shady spots where the AC condensate drains, in clogged gutters and near faulty septic lines, then fly in through windows and doors. Kentucky says to look outside if nothing shows up on the tape indoors.

After that, a regular brush and hot-water flush keeps the slime from building back up.

When Drain Flies Mean a Plumbing Problem

If every drain is clean and the tape stays empty but flies keep appearing, the source may be under the floor. The University of Maryland lists sewer leaks and backups as breeding sites. Suspect one when flies come up through cracks in a slab or basement floor, or from under loose tiles.

That’s a job for a plumber, not an exterminator: no spray fixes a broken pipe. For flies from under shower tiles, regrouting so water can’t seep behind them stops the source.

Renting? Tell your landlord in writing, with photos of the flies on the tape and where they came from, especially for a floor drain, a leak, loose tiles or a trap that needs removing.

Which Plan Fits Your Situation

What It Might Still Cost

DIY: U.S. list prices at Walmart (brushes) and DoMyOwn (gels), October 1, 2026, before tax. Pro: Fixr (updated January 2025). One homeowner on Reddit was quoted $600 to treat drain flies.

Watch out for ready-made drain fly kits: one we priced at $73.15 includes a $35.74 can of aerosol spray, which kills only the adults. The brush and the gel do the real work.

Before You Start: A Quick Checklist

• Tape every suspect drain before bed, overflow holes included.
• Buy a long, flexible brush before any gel or spray.
• Pull stoppers and covers so you can reach the pipe.
• Read the gel label for “bacteria cultures” or “enzymes”, and septic compatibility.
• Never mix bleach with ammonia or a caustic drain cleaner.
• Run water through rarely used drains and toilets every week.
• Retape the drain every few nights for two weeks.

The Bottom Line

Drain flies come from slime, so the fix is a brush, not a spray. Tape each drain to find the one they breed in, scrub it and its overflow, flush with hot water and add a bacterial gel only for drains you can’t reach. Expect a week or two of stragglers. If every drain is clean and flies still come up from the floor, call a plumber.