"No effect" seems overly strong. From what I've read, pro/pre-biotics do help, but it's transitive and fades with time after a treatment. Troublesome, of course, as a company would much rather sell a transitive treatment than something once and done.
I’ve never understood the point of taking a probiotic unless you’d been taking antibiotics first. The various ecological niches in your gut biome would already be populated.
There is pretty minimal labeling of probiotic capsules in the grocery store (grr, Orrin Hatch) but then again nobody really knows anyway. I had my gut cleaned out due to a heavy course of antibiotics last summer and just repopulated it by kissing my partner and eating unpasteurized joghurt. Not especially scientific either, but it worked.
For example, covid wreaks havoc in the gut which might cause tilt towards bacteria producing ammonia which then leaks to blood stream, causing the brain fog. Supplementing prebiotics like lactulose can help to suppress this effect.
