A smelly bathroom sink drain is a common problem, as odour, residue and bacteria accumulate regularly in the drain. Luckily, keeping the drain clean and odour-free is a fairly easy task, and requires only a couple of inexpensive household ingredients. Avoid expensive and chemically-based drain cleaners sold at supermarkets and home improvement stores; they are often ineffective and use chemicals that you do not want around your home or family.
Pour ½ cup baking soda into the drain opening.
Pour ½ cup white vinegar into the drain. The vinegar disinfects and foams when mixed with the baking soda to work through the pipes and clean the walls. Let the foam work for about 30 minutes.
Pour 1 gallon boiling water down the drain to flush out leftover residue.
Repeat every one to two weeks to prevent odour from returning.