NOW Foods, one of our recommended supplement companies, recently tested 24 quercetin brands. Twenty out of the 24 brands tested were found to have less than 90 percent of the potency listed on the label; 14 of the 24 brands had less than 50 percent of the labeled potency. Two of the 24 brands advertised vegetarian capsules, but were found to contain gelatin. One product had no detectable quercetin. Read more.