A PDE-5 inhibitor was detected and isolated from a health supplement claimed to be a preparation of fresh oyster extracts and be able to promote and support healthy sexual function and endurance, etc. The structure of this PDE-5 inhibitor was elucidated using LC-UV, LC-TOF-MS, MS-MS, IR spectroscopy, and 2D NMR. It was characterized as 8-(2-(4-(hydroxymethyl)piperidin-1-yl)benzylamino)-3-ethyl-1H-imidazo[4,5-g]quinazoline-2(3H)-thione, a compound reported to be a PDE-5 inhibitor.