Bathroom drains can take on a foul smell over time, particularly if they haven't been cleaned regularly. This is due to several factors. Hair and old skin are typically going down the drains. Add in bits of soap, natural human oils and dirt, and you've got the recipe for a smelly mess. Clean drains immediately after noticing the odour and then about once a month. You may not need to use a commercial drain cleaner to remedy a smelly drain.
Determine the cause of the smell. If the odour is simply a foul odour but does not smell like sewage, proceed with the remaining steps. If the smell is very strong and does smell like sewage, you may have an issue with a sewer line backing up in your bathroom. In this instance, you may need to call in a professional to fix the issue.
Remove any hair from the drain trap. Long hair can become tangled in the drain trap, making it difficult for the drain treatment to reach the source of the smell. Pull out any visible hair.
Shake about a half cup of baking soda into the drain. If you have difficulty getting the baking soda down the drain, remove the drain trap. Typically, there will be one screw in the centre of the trap. If there is no screw, do not attempt to remove the trap.
Pour a cup of white vinegar down the drain. This will create a chemical reaction that will scrub the inside of the drain and pipe to remove any debris causing odours.
Run hot water down the drain for several minutes. This will help flush out any remaining debris that was loosened by the vinegar and baking soda but not completely removed.
Repeat as necessary. If the smell remains after the initial treatment, repeat the process until the drain is completely clean and odour-free. Pouring essential oils down the drain can be helpful if the smell lingers, but in reality this is simply masking the odour instead of removing it.
You may need to use a commercial chemical drain cleaner to get the drain completely clean.