The bathroom sink gets less abuse than the kitchen sink, which is constantly fed with grease, food waste, and other kitchen debris. Bathroom sinks and tub drains are typically treated more gently, but they still see their fair share of soap scum, hair, and dead skin. The gunk that gets trapped in the drains can start rotting over time and generate a foul odor.
You should minimize the amount of material that enters the drains in order to prevent bad odors. You should also make a habit of manually removing any debris that you see accumulating in the drain. If the bathroom sink odor persists after you clean out the drain, you may have a bigger problem on your hands.
If the cause of the stench is a clogged overflow pipe, you can try pouring a cleaning solution into the drain. Some household cleaning products are effective in dissolving the debris that causes clogged bathtubs and sinks. One common solution is a mixture of baking soda and vinegar. You can also try oxygen bleach. Do not use chlorine if your drain connects to a septic system because chlorine will destroy the good bacteria in the septic tank.
If the odor fails to go away, examine the P-trap in the bathroom fixtures. The P-trap is located underneath the bathroom sink and the bathtub. It is designed to catch small objects and keep them from entering the septic tank or sewer. It also prevents sewer gasses from entering your home. The water in the trap may be suctioned out by bad drain cleaning or a clogged pipe. It may also evaporate if the sink or bathtub is not used often. The absence of water in the P-trap will allow sewer gases to enter your home. You can pour water into the trap and restore the water seal to prevent the noxious gases from entering your house.