Something smells off in your shower, and you’ve already ruled out the toilet, scrubbed the grout, and lit every candle in the linen closet. Before you start tearing into your walls, read about what happened to Tim and Kathy.

The couple found their dream home in the Atlanta suburbs about a decade ago. The house was around 10 years old when they moved in, which felt like the sweet spot: old enough that any construction quirks had been sorted out, but new enough to avoid serious structural headaches. A year after settling in, though, their bathroom started putting off a gas-like, sulfur odor that nobody could explain.

It wasn’t their first weird bathroom smell. A few months before the sulfur issue showed up, they tracked down a sewage odor to mold growing inside the toilet tank. That one had a clear fix. The shower smell was different. Two separate plumbers came through, checked the pipes, checked the P-trap, and found absolutely nothing wrong. Tim and Kathy turned to a Reddit forum and found dozens of homeowners dealing with the exact same mystery.

The forum comments told the whole story. “I was convinced something died in the walls,” wrote one person. Another said, “My husband and the plumbers that came to my home didn’t smell it, but it drove me mad.” Kathy jumped in with a half-joking theory: “Maybe it’s not a gasket or P-trap, maybe it’s the smell of the stuff you’re washing down the drain coming back up to haunt you.”

She was right.

After two years of chasing the problem, the answer turned out to be sitting on the shower shelf the whole time. “It was freakin shampoo,” Kathy wrote. “Dandruff shampoo at that. Like Head & Shoulders, Selsun Blue, whatever medicated shampoo.”

Plumber Jordan Benjamin said this kind of call isn’t unusual at all. “This problem is actually more common than you think,” he told the Family Handyman. “I’ve had more than a few service calls to fix this exact issue.”

So what’s the science here? Dandruff and other medicated shampoos are loaded with surfactants, oils, and stabilizers. These ingredients don’t dissolve well in water, so every time you rinse your hair, a small amount of residue coats the inside of your drainpipes. The problem compounds fast. Those pipes are already home to soap scum, loose hair, and organic matter, all of which form what plumbers call biofilm. Each shower adds another layer. Over time, that biofilm gets thick enough to start producing the sulfur-like odor Kathy described.

Kathy had tried nearly everything before landing on the real answer. She worked through bleach, vinegar, baking soda, Dawn dish soap, and Tide laundry detergent. None of it stuck. She tried an enzyme cleaner next, which helped, but the smell came back the moment someone washed their hair with medicated shampoo again.

The fix that actually worked was simpler than she expected. Switching away from medicated shampoo stopped the buildup at the source, and a proper enzyme-based drain cleaner cleared out what had already collected in the pipes. Enzyme cleaners work by breaking down the organic material in biofilm rather than just masking or temporarily displacing the odor. Unlike bleach, they don’t just push the gunk further down the line.

If your family relies on dandruff shampoo for a real scalp condition, don’t panic and toss it out. You can reduce the drain buildup significantly by rinsing your hair for a full extra 30 seconds to push residue past the drain opening, using a mesh drain cover to catch hair before it can feed the biofilm, and doing a monthly enzyme cleaner treatment as basic maintenance. The Environmental Protection Agency’s Safer Choice program has a searchable database of cleaning products, including drain cleaners, that meet safety standards for indoor use, which is worth checking if you have young kids in the house.

For anyone starting from scratch with a stubborn drain smell, the Plumbing Manufacturers International offers homeowner resources on drain maintenance and water quality that can help you figure out whether your issue is a simple biofilm problem or something that needs a licensed plumber.

Tim and Kathy’s ordeal stretched across two years and multiple service calls before a Reddit forum pointed them toward an answer that cost almost nothing to fix. If your bathroom has been putting off a smell you can’t pin down, take a look at what you’re washing down the drain before you assume the worst.