My bathroom drain stinks! The smell gets worse after you run the water. My husband cleaned the P-trap and I cleaned the drain itself. I used baking soda and vinegar about a month ago and it worked, but last week I noticed the smell coming back so I used Drano just in case. Now the smell is back full force :/ the drain looks great! I don't have an overflow so I know it's not that. What else could it be if the P-trap has been cleaned and the drain is clear as well?
Foul odors are usually due to the water seal in the P-trap failing for some reason. Sometimes the water simply evaporates in little used fixtures. Probably not the case here. The other cause for failure is the lack of proper venting. Without a proper vent, the water seal gets siphoned out of the P-trap after the fixture is used.
If you have an air admittance valve serving as a vent, it could be failing to open properly, or it could be stuck open, allowing sewer gases to enter the room via that path rather than the P-trap. These devices should be accessible. Simply manipulating the valve disc by hand can dislodge it from it's stuck position and allow it to work again. Or the device might have failed and needs replacement.
You quite possibly do not have an AAV, but the more common vent through the roof. This time of year in Northern climes, the vent could become obstructed by ice or snow. Simply clearing the blockage will solve the problem. It's also possible a bird decided the vent would make a nice warm nesting site. Other vent problems could be more endemic of the installation itself, being either undersized or even non-existent or otherwise ineffective, or an alternative, less effective, trap arrangement such as S-traps or drum traps. If you've lived there for a while and the odors just started happening, these endemic issues are probably unlikely.