Those small, fuzzy, moth-like flies hovering near your kitchen or bathroom sink are almost certainly drain flies — also called moth flies or sewer gnats. They breed in the slimy organic biofilm that accumulates inside drain pipes, and they can appear suddenly in large numbers. While they’re not dangerous, they indicate a sanitation issue in your plumbing that won’t resolve on its own.

What Causes Drain Fly Infestations

Drain flies lay their eggs directly in the gelatinous buildup that coats the inside of drain pipes. This biofilm consists of decomposing organic matter — food particles, soap residue, hair, grease, and bacteria. Even drains that appear clean on the surface can harbor thick biofilm deposits further down the pipe. Infrequently used drains are the most common culprits because the standing water and undisturbed biofilm create ideal breeding conditions. Moisture-driven pest problems tend to cluster in the same areas of your home. If you’re seeing drain flies in the bathroom, there’s a good chance conditions are also favorable for silverfish — learn how to get rid of silverfish in bathrooms as part of the same remediation effort.

Confirming the Source

Before treating, confirm which drain is producing the flies. Place a strip of clear tape over the drain opening at night, sticky side down, leaving gaps for air flow. In the morning, check the tape for trapped flies. Repeat this test on every drain in the affected room, including floor drains, overflow drains, and utility sink drains. The drain with the most trapped flies is your primary breeding site.

Removing the Biofilm — The Only Permanent Fix

Chemical drain cleaners and bleach provide only temporary relief because they don’t remove the entire biofilm layer. The most effective approach is mechanical cleaning followed by enzymatic treatment.

1. Mechanical Cleaning

Use a stiff drain brush (a long, narrow bottle brush works well) to physically scrub the inside walls of the drain pipe. Push the brush down as far as you can reach and rotate it to dislodge the biofilm. You’ll notice dark, slimy material coming off the pipe walls. Flush with hot water while scrubbing.

2. Enzyme Drain Treatment

After mechanical cleaning, pour an enzyme-based drain cleaner (not a chemical cleaner) into the drain. Enzyme treatments contain bacteria that consume the remaining organic material over several hours. Apply the treatment before bed so it sits in the pipe overnight. Repeat every night for five to seven consecutive nights to fully break down the biofilm.

3. Boiling Water Flush

Between enzyme treatments, pour a pot of boiling water slowly down the drain once daily. The heat helps loosen residual biofilm and kills any larvae near the drain opening. Use caution with older PVC pipes — a near-boiling temperature of around 200°F is safer than a full rolling boil for plastic plumbing.

Preventing Recurrence

Once the biofilm is eliminated, prevent rebuilding by running water through all drains at least once per week — including guest bathrooms, laundry sinks, and floor drains that don’t get daily use. Install mesh drain covers to catch hair and food debris before they enter the pipe. Flush kitchen drains with hot water after doing dishes. Good drain maintenance is just one component of whole-home pest prevention. Food debris in drains and around sinks also attracts rodents — see how to keep mice out of kitchen cabinets for complementary strategies that address the same underlying cleanliness issues.

Prevention Tips

• Run water through every drain in your home weekly, including guest baths
• Install mesh drain covers on all sinks and shower drains
• Flush kitchen drains with hot water after washing dishes
• Apply enzyme drain cleaner monthly as preventive maintenance
• Fix leaking pipes and faucets promptly to eliminate moisture sources
• Clean garbage disposal units weekly with ice cubes and citrus peels

Drain flies are a symptom of biofilm buildup, not a standalone pest problem. Address the root cause — the organic material inside your pipes — and the flies disappear permanently. And while you’re improving your home’s sanitation, consider a broader approach: prevent pantry moth infestations covers another common kitchen pest that responds to the same cleanliness-first strategy.