Free standing shoiwer drain has a bad odor all the time.

Shine a strong flashlight down into the drain and see if you have any build up of the walls of the pipe. If you do this could be the source of the odor, try to scrape the walls of the pipe just a little at a time and then wash the shower out to get rid of the stuff you have scraped to stop up the drain line.

Try this, mix 1/2 cup of baking soda with 1/2 cup table salt and dump into the drain. Now add a cup of vinegar and watch the reaction. Allow to soak over night and then flush with several buckets of very hot water. That should clean and freshen the drain completely.

I agree with Bob and Milo, good advices,

Most bad odors in bathroom drains come from rotting hair mixed with grease from the soap. Try this. Remove the two screws holding the strainer. (TIP; put the screws in the soap dish before they get lost down the drain.) Shine a light down there and with a bent coathanger fish out any hair that may be hiding down there. Then pour a 1/2 gal. of bleach down into the trap and let it set over night. Next morning flush the mess down with 2 large pans of boiling water, (this is important). The bleach will dissolve the hair and make it slippery and the boiling water will melt the grease and get the mess out of your drain. Smell better now? Cheers TOM