I haven't tried it, but supposedly you can make your own Quercetin by boiling down citrus peels. In herbalism they call it making a decoction -- it's not complicated. You take grapefruit, lemon, and orange peels and cover them with water, then boil them for a while until it makes a very strong tea. Then you let the mixture cool, strain it, and drink a couple swigs every few hours to combat whatever's got you feeling poorly. The peels can be fresh or dry. Some people have been putting the peels through espresso makers. If I do it, I'll just put the peels in my pressure cooker.