Shower-drain odor usually comes from hair and biofilm near the drain, a dry trap in a rarely used shower, or sewer gas entering because of a drain problem. Start with accessible cleaning rather than aggressive chemical mixtures.
Remove hair and debris at the grate
Wear gloves and clean the underside of the cover as well. Damp organic debris can smell even when water still drains normally.
Brush the accessible drain throat
A narrow brush with detergent helps remove the sticky film of soap, body oils, and microorganisms. Rinse with hot tap water, not boiling water.
If it is rarely used, refill the trap
Run water for a minute or two. If the odor disappears and returns after a long period of nonuse, trap evaporation is a likely explanation.
Odor plus gurgling is not just grime
If trap water disappears, other drains gurgle, or the odor is strong and persistent, have the venting and drain system checked professionally.