Drain flies don't come from your drain. They come from what's growing in it.

Biofilm — the slimy microbial layer of grease, soap scum, hair, and food residue that builds up on the inside walls of your drainpipes. That film is what drain fly larvae eat and what adults lay eggs in. If you have drain flies, you have a drain that has grown a food source. The flies are the symptom; the buildup is the cause.

Because they don't remove the biofilm. Boiling water cools rapidly when it hits cold pipe and runs over the film. Baking soda and vinegar produce a short foaming reaction with almost no scrubbing effect. The eggs and larvae in the biofilm stay put and hatch within days — which is why the flies appear to come back.

Pure B.S. is a bio surfactant concentrate made with surfactin produced by Bacillus subtilis. Poured in overnight, it helps wet, lift, and release the organic film on the pipe wall so it rinses away with normal water flow. That film is where drain-fly larvae feed and adults lay eggs, so removing it removes the breeding site. Pure B.S. does not kill flies; it removes the buildup that supports them.

Treat each affected drain once a week for four weeks: 1 oz poured in at night, left undisturbed 8+ hours, then rinsed with hot water. Eggs are already in the pipe when you start, and the drain fly life cycle runs 8 to 24 days, so eggs laid before your first treatment can still hatch. Four weekly treatments cover the full cycle. After that, treat once a month.

No. Pure B.S. is a cleaner for organic buildup — grease, food residue, soap film, and the grime where odors and drain flies start. It is not a disinfectant or sanitizer. If your application requires sanitizing, clean first with Pure B.S. and sanitize separately with a registered product.

Yes. Pure B.S. is septic-safe, non-hazardous, and non-corrosive, and it is safe on PVC pipe. That is a real difference from bleach and caustic drain openers, which kill the bacteria a septic tank needs to break down waste — so clearing a drain problem with them can create a tank problem.

For a residential infestation coming from indoor drains, usually not. Pest-control sprays target the adult flies rather than the breeding site, so they treat the symptom. Call a professional if the source is outside the building — a broken sewer line, a cracked septic tank, standing water on the property — if the infestation persists after four weeks of weekly treatment of every drain, or if you are in a regulated facility that needs compliance documentation.

Yes. The organic buildup that feeds drain flies is the same buildup that slows drains and produces odor. Pure B.S. helps break down fats, oils, grease, and sugar, which is why the same overnight treatment is used in garbage disposals and commercial floor drains.

Yes, diluted. For floors and utility areas, mix 4 oz with 1 gallon of clean water and mop, scrub, or foam-clean. For kitchens, bathrooms, and other hard surfaces, add 5 oz to a 32 oz spray bottle, fill with clean water, spray, let it soak a few minutes, then wipe clean. Test in an inconspicuous area first, and rinse food-contact surfaces with clean water after use.

Not for a household drain. Poured in at night, Pure B.S. settles into the P-trap under your sink. The trap and low points stay wet continuously, which is why treatment placed there keeps working overnight. What a pour cannot do is keep the vertical tailpiece above the trap water line wet, or reach the sink overflow channel — foam fills and clings to those above-water surfaces, which is why Virginia Tech Extension recommends a microbial foam for larvae in spaces brushes and gels cannot reach. We do not sell a foaming applicator and will not recommend a consumer device, because useful foam depends on the equipment producing it as much as the chemistry. The free fix for the same gap: after pouring, run a stiff bottle brush down the tailpiece and scrub the overflow opening. Trap gets the soak, tailpiece gets the brush. Our drain fly guide walks through it in detail.

Replacing a section of pipe removes the biofilm in that section only. The breeding site is usually in something that was kept: the strainer body, tailpiece, P-trap, sink overflow, disposal connection, or the branch line above or below the new work. Identify the drain actually producing flies by taping clear packing tape over each suspect drain overnight with a small air gap, then inspect and scrub the retained parts. If that comes up empty, check sources outside the repaired line — HVAC condensate pan and line, sump pit, floor drains, refrigerator drip tray, or a leak keeping an area damp. A new line can also re-colonize if a fixture upstream keeps supplying organic material.

It works on organic buildup — biofilm, fats, oils, grease, sugar, soap film. It is not a drain opener and will not clear a physical obstruction: hair mass, wipes, mineral scale, tree roots, or a sagged or collapsed pipe. It also cannot correct a plumbing defect. If a drain is slow because the P-trap has dried out, a vent is blocked, or the line is improperly pitched, treatment will not change that — refill the trap or get the plumbing looked at. Recurring backups, gurgling, multiple slow fixtures at once, or sewage odor all point to a plumbing problem rather than a buildup problem.