Compounds elucidated as new tadalafil analogues were found to be adulterated in a dietary supplement and detected during routine analysis by HPLC with photodiode array detection. The UV spectra of these compounds were found to be almost identical to that of tadalafil. The unknown compounds were isolated by preparative HPLC and elucidated by quadrupole-time-of-flight MS and NMR spectroscopy. Two compounds were identified as cyclopentyltadalafil and trans-cyclopentyltadalafil, from the substitution of the tadalafil N-methyl group with a cyclopentyl group. Another analogue was a dimeric form of tadalafil with an N-cyclopentyl group. This represents the first report of the elucidation of compounds used as adulterants in a dietary supplement.