A drain can smell because of organic buildup in the pipe, a dry trap, or sewer gas entering through a seal or venting problem. The right fix depends on where the odor starts.
Identify which drain smells and when
Notice whether the odor appears after days of nonuse, while water is running, or when another fixture drains. That pattern helps separate a dry trap from buildup or a venting problem.
Refill a dry trap first
For a rarely used sink, shower, or floor drain, run water to restore the water seal. If the odor disappears and returns only after weeks of nonuse, evaporation was likely the cause.
Clean accessible buildup
Remove hair, biofilm, and debris from strainers, stoppers, and overflow passages. A brush and detergent are usually more useful than mixing reactive household chemicals.
Persistent sewer-gas odor deserves attention
If the odor affects several drains, you hear gurgling, the trap loses water, or you see leaks, there may be a vent, seal, or pipe problem. Do not simply mask it with fragrance.