Serpentwyne is the result of long years of experiments and research by people like Kessler. Serpentwyne is extracted from the dangerous Silver Serpent Venom and removes all of the ill effects. Instead, drinking it will now restore a person to full health. The secret of producing the Serpentwyne, together with one of the last Silver Serpents, was entrusted to the Brotherhood of the Rose, so that they produce it for all the ones in need.
Prior to Ultima IX, the serpent was kidnapped and the Serpentwyne storage of the Cathedral of Love pludered by Aria and her helpers, meaning that no more Serpentwyne could be produced, even after the serpent was returned (production needs time). The bottles were scattered all over the world and were hard to find.