TL;DR
A kitchen drain smells because a layer of grease, food, and bacteria -- biofilm -- is coating the inside of the pipe just below the strainer. That is the source about eighty percent of the time. The other common causes are a dry or clogged P-trap, a garbage disposal, or a blocked plumbing vent, and each one has a different fix.
Where the smell is actually coming from
Every time you rinse a plate, a thin film of fat, starch, soap, and food particles coats the inside of the drain. Bacteria colonize that film and produce sulfur compounds as they break it down.
The layer is usually in the first six to twelve inches of pipe, right where you cannot see it and right where water runs past too quickly to scour it. That is why the smell comes back a day after you pour something down the drain.
Two clues point at biofilm specifically. The smell is worse when the water has been off for a few hours, and it is worse when hot water runs, because heat volatilizes the compounds and sends them up.
How do I tell where the smell is coming from?
Run cold water for thirty seconds and sniff, then run hot and sniff again. If hot makes it worse, that is biofilm in the pipe. If the smell is there with no water running, check the P-trap. If it is strongest with the disposal running, the disposal is the source. If it smells like sewage, suspect the vent.
Fixing the most common cause: biofilm
Start mechanically. Push a long bottle brush or a flexible drain brush down through the strainer and scrub the pipe wall as far as it reaches, then flush with hot water. The brown sludge that comes back up is your answer.
Follow with an enzyme drain treatment. Enzyme and bacterial products digest the organic film over several applications instead of blasting it, and they are gentler on pipes than caustic cleaners. Apply at night when nobody will run water for several hours, since dwell time is what makes them work.
Skip the chemical drain openers for an odor problem. They are designed to punch through a physical clog, they do little to biofilm on the walls, and they are hazardous. Critically, never pour two different drain products into the same drain, and never combine any of them with bleach. Those combinations can react violently.
Source: plumbing maintenance and drain care guidance
The baking soda and vinegar question
Pouring baking soda then vinegar down a drain fizzes impressively. The reaction neutralizes itself in seconds and produces mostly water and carbon dioxide, so it is not a powerful cleaner.
It is not useless. The foam gives brief agitation, and a follow-up flush of very hot water carries loosened material away. Treat it as a light maintenance rinse, not a repair. One caution: boiling water can soften PVC joints and stress an adhesive fitting, so very hot tap water is the safer choice.
When the P-trap is the problem
The P-trap is the U-shaped bend under the sink, and the water sitting in that bend is the seal that blocks sewer gas from entering your home.
If a sink goes unused for weeks, that water evaporates and the seal disappears. The fix is comically simple: run water for thirty seconds. If the smell vanishes, that was it, and a monthly run in guest bathrooms and rarely used sinks prevents it.
A trap can also be full of trapped debris. If a sink drains slowly and smells, the trap likely holds a slug of grease and food. It unscrews with slip nuts by hand or with pliers -- put a bucket under it first, and expect a mess.
Disposals, dishwashers, and other suspects
A garbage disposal collects food under the rubber splash guard, on the underside of the flange, and in the grind chamber. That is a separate job, covered in a smelly garbage disposal.
A dishwasher drains into the same pipe, and a dirty filter or a sagging drain hose can push odor back into the kitchen. If the smell is worst right after a cycle, start with a dishwasher that smells.
An air gap on the counter next to the faucet can also clog and smell. Pop the chrome cap off, remove the plastic cover underneath, and clean out the sludge.
Sewer smell means stop and call someone
A drain that smells organic and slightly sour is a cleaning problem. A drain that smells like raw sewage, especially in more than one fixture, is not.
That points to a blocked plumbing vent, a broken trap, or a main line issue -- plumbing repairs rather than cleaning tasks. Gurgling drains, bubbling toilets, and slow drainage across the house point the same direction. Persistent musty odors not tied to a drain are a different investigation, covered in getting rid of a musty smell, and visible mold is remediation work for a specialist.
Keeping the drain from smelling again
Do not pour grease down the sink. Wipe greasy pans with a paper towel before washing, and pour cooled fat into a container for the trash.
Run cold water while grinding in a disposal and for thirty seconds afterward, so material actually leaves the pipe instead of settling in the first bend.
Flush the drain weekly with hot tap water and a squirt of dish soap. Once a month, run an enzyme treatment overnight. That routine, plus keeping the sink clean daily, prevents nearly every kitchen drain odor before it starts.
FAQ
Why does my kitchen sink smell worse when I run hot water?
Heat vaporizes the sulfur compounds produced by bacteria in the biofilm coating the pipe, so hot water carries the smell up into the room. It is a reliable sign that the problem is organic buildup on the pipe wall rather than a dry trap or a vent issue.
Is it safe to pour bleach down a kitchen drain?
It is not recommended. Bleach does little to biofilm, it can damage some pipes and seals, and it is extremely dangerous if it meets another drain product, an acid, or ammonia already in the line. Use an enzyme treatment instead, which digests the buildup that causes the odor.
Can I pour boiling water down the drain?
Very hot tap water is safer. Boiling water can soften PVC pipe and stress glued joints, especially in older plumbing, and it can also set grease further down the line if it cools before flushing through. Hot tap water plus dish soap does the same job with less risk.
Why does my sink smell only when I have not used it in a while?
The water in the P-trap has evaporated, breaking the seal that keeps sewer gas out of the room. Run the tap for thirty seconds to refill it. In sinks that go unused for months, a small amount of mineral oil poured in after refilling slows evaporation.
Does a smelly drain mean my pipes are clogged?
Not necessarily. Odor comes from buildup on the pipe walls, which can smell badly long before it restricts flow. If the sink also drains slowly, gurgles, or backs up, then you have both problems and the clog should be addressed first.
Should I use a chemical drain cleaner for odor?
No. Caustic drain openers are formulated to dissolve a physical blockage, not to remove the thin film that causes smells, and they carry real burn and fume hazards. Never mix two drain products or combine one with bleach. Mechanical brushing plus enzymes is the better approach.
If the wider house needs the same attention, that is what house cleaning services cover.
Drains are one of those problems that gets solved for a week and returns, usually because the routine around them slipped. If you would rather have the kitchen kept up consistently, every home is different, so the real number depends on size, condition, and what you want covered. Get a quote and we will walk you through it.