Bad odors from drains, whether in your tub or in a small bath vanity, are generally caused by some sort of blockage in the P-trap (or lack of a P-trap) in a drain. A P-trap is a U shaped pipe under the drain. Its purpose is to hold water in the bottom of the trap to keep sewer gas from escaping through the drain and into the house. If the P-trap is installed in your drain, you may have something caught in the trap. This is actually more common of a problem than many people realize and easily fixed. We talked about removing hair clogs from tub drains in a previous post and generally, that’s what you’ll find. You can remove these by using a drain snake or some sort of checmical (liquid lumber, etc.) to chemically eat away the clog. After this, you should have no problems. If you do, you may not have a P-trap installed at all, and would need to install one to stop the odor.