The bathroom sink can have a bad smell because mildew and mold can grow in the sink’s overflow drain compartment.
This overflow area connects to the drain because of the double wall built to protect from an over-running sink. The overflow connects to the drain in a double walled area that, by design, retains some standing water. The area sometimes gets water, allowing bacteria to grow and stink like sewer gas, a garbage disposal or the toilet.
I use Zep Drain Care; it is an enzyme-drain-buildup-remover. The enzymes eat the organic growths and live off the accumulated material. This removes the source of the smell to cleanse this area.
Mix a scoop of the powder with a pint of warm water: not hot, not cold. Funnel the solution into the overflow hole. Do not fret if of some of the product over-pours into the sink, it will only help clean the trap and drain.
Allow to set overnight, then flush the sink the following day with warm water. Repeat again the next night until the odor is gone.
Do not use any of the caustic drain cleaner. Some people try vinegar, bleach or salt water.
But we at found an enzyme-based formula breaks down years of drain-clogging build-up to prevent clogs before they form clogs in sinks, toilets and showers. Prevention is better than a cure, which supports the use on a regular schedule before problems start.