Bathroom Sink Drain Smells Like Rotten Eggs: Here's Why

Top TLDR:

When a bathroom sink drain smells like rotten eggs, the cause is almost always hydrogen sulfide gas — produced by a dry P-trap, hair-and-soap-scum biofilm, a partial clog, or a blocked sewer vent. Polk County homeowners can fix most cases immediately by running water to refill the trap, cleaning the stopper, and flushing with baking soda and hot water. Call S&S Waterworks if the rotten egg smell returns within days or affects multiple fixtures at once.

That sulfur smell coming from your bathroom sink isn't random. It has a specific source, and it won't go away on its own. The compound behind it is hydrogen sulfide — a gas produced when bacteria break down organic material in an oxygen-deprived environment, or when sewer gases travel into your home through a compromised water seal. Both situations are fixable. The key is knowing which one you're dealing with.

This guide covers every reason a bathroom sink drain smells like rotten eggs, what each cause looks and smells like in practice, and the exact steps to eliminate it. For a broader look at drain odors throughout the home, see our complete guide to eliminating drain odors and finding the source.

What Rotten Egg Smell from a Drain Actually Means

The rotten egg odor is almost always hydrogen sulfide (H₂S). It's the same compound produced naturally in swamps, sulfur springs, and sewage systems, and it has a distinct, unmistakable smell even at very low concentrations. In a bathroom sink, H₂S reaches you through one of two pathways: bacteria inside the drain pipe are producing it as they digest organic material, or it's traveling in sewer gas from further down the line through a broken water seal.

Distinguishing between the two pathways matters because the fixes are different. Bacterially produced H₂S from biofilm or a clog is a drain-cleaning problem. Sewer gas entering through a failed or dry water seal is a plumbing integrity problem. The sections below cover both.

Cause 1: A Dry P-Trap — The Most Common Culprit

The P-trap is the curved section of pipe beneath the sink. It's designed to hold a small reservoir of water at all times, and that water does one critical job: it creates a physical barrier between your bathroom and the sewer gas in the drain system below. When that water evaporates, sewer gas — which contains hydrogen sulfide along with other compounds — travels freely up through the drain opening and into the room.

Dry P-traps are the leading cause of rotten egg smell in bathroom sinks, particularly in guest bathrooms, vacation homes, or any sink that hasn't been used in a week or more. The smell appears suddenly and is strongest right at the drain. It's full-on sewer odor — not musty or sour, but raw and sulfurous.

The fix: Run the cold water tap for 30 to 60 seconds. The trap refills and the smell stops almost immediately. For sinks used infrequently, pour a tablespoon of mineral oil down the drain after refilling the trap — it floats on the water surface and significantly slows evaporation. Make it a habit to run water through every sink in the house at least once a week, including that guest bathroom no one uses. If the smell returns within a day or two after refilling the trap, the trap may be siphoning dry under pressure from an improperly vented drain line, which requires a professional assessment.

Cause 2: Hair and Soap Scum Buildup at the Stopper

Every bathroom sink has a drain stopper — the mechanism that opens and closes to hold or release water. Hair, soap scum, toothpaste residue, and skin cells collect on the underside of the stopper and in the drain opening just below it with every use. In a warm, moist environment, the bacteria that colonize this material produce hydrogen sulfide as they break it down. The result is a persistent rotten egg or sewage smell that tends to worsen in warmer months.

This is one of the most overlooked sources of bathroom drain odor because the visible top of the drain stopper looks clean while the underside is heavily fouled. It's also one of the easiest to fix.

The fix: Pull the stopper out — most lift straight up, others require a slight counterclockwise turn. Look at the underside. What you'll typically find is a dark, slimy coating of decomposing organic material. Scrub it thoroughly with a stiff brush and dish soap, clean the drain opening it sits in, and run hot water for a minute before replacing it. In many cases, this single step eliminates a rotten egg smell that seemed serious. Clean the stopper monthly to prevent recurrence.

Cause 3: Biofilm Inside the Drain Pipe

If cleaning the stopper improves things but the rotten egg smell persists, biofilm inside the pipe is the likely remaining cause. Biofilm is a thin but continuously growing layer of bacteria, soap scum, and organic matter that coats the interior walls of drain pipes. The same bacterial activity that produces the smell in the stopper buildup happens at greater scale inside the pipe — and it's not reachable by simply cleaning what you can see.

Bathroom sink biofilm is typically located in the first 12 to 18 inches of pipe below the drain, where organic material accumulates before the water velocity carries it further down the line. Warm temperatures accelerate bacterial growth, which is why biofilm-related drain odors in Lakeland and the greater Polk County area tend to peak during the summer months and can seem worse after the home has been closed up with the air conditioning running.

The fix: Pour half a cup of baking soda into the drain, follow with half a cup of white vinegar, and let it foam and work for 15 minutes. Flush with the hottest water your tap produces for two to three minutes. This loosens surface biofilm and neutralizes odors. For established biofilm, follow up with an enzymatic drain cleaner applied before bed — these biological products digest organic material inside the pipe safely and without the pipe wear that chemical cleaners cause over time. Repeat weekly for three to four weeks for significant buildup. As noted in our DIY sewer maintenance guide, enzymatic cleaners are among the safest and most effective tools for routine drain maintenance between professional cleanings.

Cause 4: The Overflow Drain Opening

Most bathroom sinks have an overflow drain — a small oval or round opening near the top rim of the basin that prevents the sink from flooding if the main drain is blocked and the tap is left running. This opening connects to a small channel that routes water down to the main drain pipe. Almost no one ever cleans it, and over time it accumulates the same biofilm and organic material as the main drain — sometimes more severely because water flows through it only in overflow situations, leaving material sitting in the channel indefinitely.

The overflow channel is a surprisingly frequent source of persistent bathroom sink odors. Because it's inside the rim of the sink and not visually obvious, many homeowners clean every other part of the sink while leaving this channel completely untouched for years.

The fix: Insert a small flexible brush or pipe cleaner into the overflow opening and scrub out what you can reach. Use a small funnel or a syringe to flush a solution of baking soda dissolved in hot water directly into the opening. Follow with a vinegar flush. Commercial drain cleaning foam products that expand inside the drain work especially well for overflow channels because the foam reaches into the curve of the channel. Incorporate overflow cleaning into your monthly bathroom maintenance routine.

Cause 5: A Partial Clog Trapping Stagnant Water

A partial clog — one that slows drainage without blocking it completely — creates a pocket of standing water inside the pipe. That stagnant water becomes an ideal environment for anaerobic bacteria, the microorganisms that produce hydrogen sulfide in the absence of oxygen. The rotten egg smell from a partial clog is often accompanied by noticeably slower drainage and may intensify after the sink has sat unused for several hours.

In bathroom sinks, partial clogs are almost always composed of hair and soap scum that accumulates at or just below the drain stopper. The hair binds to the stopper mechanism and traps additional material until the restriction becomes significant enough to cause slow drainage.

The fix: Remove the stopper and clear any hair and buildup you can see and reach. A drain snake — even a basic hand-operated model — can reach into the pipe to break up and retrieve material that's below the visible portion of the drain. As covered in our DIY sewer maintenance guide, manual snakes are effective and safe for hair clogs in bathroom sinks and are appropriate DIY tools for this type of problem. Install a mesh drain screen that catches hair before it enters the pipe — this simple, inexpensive step prevents the majority of bathroom sink clogs from forming in the first place.

Cause 6: A Blocked or Damaged Sewer Vent Pipe

Your home's drain-waste-vent system includes pipes that run through the interior walls and up through the roof, venting sewer gases outside and equalizing pressure in the drain lines. When these vent pipes are blocked — by debris, a bird nest, or leaf accumulation from Polk County's oak and pine-heavy neighborhoods — the system can't equalize properly. Sewer gas that would normally escape outside instead backs up into the drain lines and enters the home through the nearest water seal it can overcome.

A sewer vent problem produces a rotten egg or sewage gas smell that comes and goes, sometimes seems to worsen when wind conditions change, and may be accompanied by a gurgling sound in the drain when water runs — the gurgle being the sound of air being pulled through the partially water-sealed P-trap as pressure tries to equalize.

If the rotten egg smell is present even when the P-trap is full, doesn't respond to any cleaning of the drain itself, and is possibly occurring at other fixtures as well, a vent blockage or damage is the likely cause. This is not a DIY repair. Contact the team at S&S Waterworks to diagnose the vent system — our plumbers serve Lakeland, Winter Haven, Auburndale, Mulberry, and Bartow and can identify vent issues quickly, often using camera inspection technology to pinpoint the blockage without guesswork.

Cause 7: The Problem Is in the Main Sewer Line

When a bathroom sink drain smells like rotten eggs and the odor is also noticeable at other drains or throughout the house, the source may not be in the sink itself at all. Significant main sewer line problems — particularly tree root intrusion, which is common in Polk County's mature residential neighborhoods — can create conditions that allow sewer gas to migrate back up through multiple fixtures simultaneously. You may also notice gurgling at toilets when sinks drain, or slow drainage at multiple fixtures at once.

Main sewer line issues won't respond to any fixture-level cleaning. They require professional assessment, video camera inspection of the line, and appropriate treatment — whether that's cable cleaning to break up root intrusion, hydro jetting to scour the line completely clean, or another method based on what the inspection reveals. Our main sewer line cleaning service addresses these root causes and prevents them from escalating into sewage backups and property damage.

Step-by-Step: What to Do First When Your Bathroom Sink Drain Smells Like Rotten Eggs

Work through these steps in order. Each addresses the most common cause before moving to the more complex ones.

Step 1: Run the water for 60 seconds to refill the P-trap. Wait 15 minutes. If the smell is gone, you had a dry trap — done.

Step 2: Remove the stopper and clean it thoroughly, along with the visible drain opening. Run hot water for two minutes. If the smell improves significantly, stopper buildup was the source or a major contributor.

Step 3: Clean the overflow drain opening with a brush, pipe cleaner, or foam drain cleaner. Flush with baking soda and hot water.

Step 4: Apply the baking soda and vinegar treatment to the main drain opening. Follow with an enzymatic cleaner before bed.

Step 5: If you notice slow drainage at any point, use a hand snake to clear the pipe of hair and soap scum below the stopper.

Step 6: If the smell persists after all of the above, or returns within a few days, or is present at multiple fixtures, contact S&S Waterworks for professional diagnosis. A single video camera inspection of the drain line eliminates all remaining guesswork about what's happening inside the pipe.

When to Call a Professional

Some situations genuinely require professional tools and training to resolve. Call a plumber when the rotten egg smell returns quickly despite thorough cleaning, when it's present in multiple bathrooms or at other fixtures at the same time, when it's accompanied by gurgling sounds at the toilet or other drains, or when it's strongest away from the drain itself rather than directly at it.

A professional specialized drain cleaning can include video camera inspection to see exactly what's inside the pipe, motorized cable snaking to clear any material a hand snake can't reach, and hydro jetting to scour the line completely clean if years of biofilm are the root cause. For Polk County homes with older plumbing — cast iron or clay drain lines in particular — professional cleaning removes the heavy scale and buildup that accumulates in rougher pipe interiors and that DIY methods can't adequately address. A full overview of your home's plumbing maintenance needs is available in our complete plumbing solutions guide for Polk County homeowners.

Preventing the Rotten Egg Smell from Coming Back

Run water through every sink in the house at least once a week — including guest bathrooms, basement sinks, and any other rarely used fixtures. This single habit prevents P-trap evaporation, the most common cause of rotten egg drain odors in homes throughout Polk County.

Install a quality mesh drain screen in every bathroom sink to catch hair before it enters the drain. Clean it every few days. Clean the drain stopper and overflow opening monthly. Use an enzymatic drain cleaner monthly to keep biofilm from establishing inside the pipe. And schedule professional drain cleaning annually — regular professional maintenance removes the slow accumulation of material that eventually produces persistent odors and clogs, and it catches developing problems before they become expensive ones.

S&S Waterworks provides professional drain cleaning, hydro jetting, camera inspection, and full plumbing services throughout Lakeland, Winter Haven, Auburndale, Mulberry, and Bartow. Upfront pricing, same-day availability, and a 100% satisfaction guarantee on all work. Book an appointment online or call (863) 362-1119.

Bottom TLDR:

A bathroom sink drain that smells like rotten eggs is caused by a dry P-trap, stopper buildup, drain biofilm, a partial hair clog, a fouled overflow channel, a blocked sewer vent, or a main line issue — in that order of likelihood. Refill the P-trap first, then clean the stopper and overflow opening, then apply baking soda and an enzymatic cleaner; if the rotten egg smell persists or returns fast, call S&S Waterworks at (863) 362-1119 for professional diagnosis.