Ge X, Li L, Koh HL, Low MY. J Pharm Biomed Anal. 2011 Nov 1;56(3):491-6. A sildenafil analogue was detected and isolated from a health supplement claimed for human use. The structure of this new analogue was elucidated using LC-UV, LC-Orbitrap-MS, IR spectroscopy, 1D and 2D NMR. It was characterized as dithio-desmethylcarbodenafil containing 2 thiocarbonyl groups instead of 2 carbonyl groups, and 4-methyl substitution, on the piperazine ring, rather than 4-ethyl substitution, when compared to sildenafil.