Dr Russell Jaffee, who runs a company that formulates and sells nutritional supplements, says oral quercetin is not absorbed into the bloodstream well enough to have any effect. Fortunately quercetin dihydride is widely available (at least in the US) and is absorbed much better.
I tried quercetin (to prevent “mast-cell degranulation”, nothing to do with covid) 3 or 4 times over the years and never noticed any effect. Then I tried quercetin dihydride and noticed the desired effect.
I mean quercetin dihydrate. (I current cannot edit my contributions to LW because software is complicated.)
Peperine (black pepper extract) can help make quercetin more bioavailable. They are co-administered in many studies on the neuroprotective effects of quercetin: https://scholar.google.com/scholar?hl=en&as_sdt=0,22&q=piperine+quercetin