If you have an odour coming from your drains chances are it is because of sulphur. To be more specific hydrogen sulphide gas is caused by bacteria in the pipes. Although it is good to note that this isn't the only reason for smelly drains it is a very common reason. In order to fix the problem, you need to disinfect the pipes.This can be done by pouring half a cup of bleach down your drain or alternatively, you can use half a cup of baking soda followed by a cup of vinegar. Keep in mind that if you use the latter of the two there will be a mini chemical reaction that will cause foam to come back up the pipe, it is advised that you block the drain after pouring the vinegar to help force the foam through the pipe.After either method, you should then wash warm to hot water down the drain. If this does not work then you should contact your local plumber.