Sildenafil and related compounds, vardenafil, tadalafil and hydroxyhomosildenafil, in dietary supplements were characterized using thin layer chromatography (TLC), infrared (IR) spectroscopy, gas chromatography/mass spectrometry (GC/MS), high performance liquid chromatography (HPLC) and liquid chromatography/mass spectrometry (LC/MS). TLC with Dragendorff reagent gave a lower detection limit than 1 μg for all 4 compounds. IR spectra of the compounds were so characteristic as to discriminate one from another easily. After TMS derivatization, all the peaks of these compounds appeared and well-separated on the GC/MS chromatograms at a high column temperature (320°C), although hydroxyhomosildenafil was not detected without derivatization. In HPLC and LC/MS analyses, an ODS column with a mobile phase composed of a mixture of 20 mM ammonium acetate (pH 5.0) and methanol (40:60) allowed separation of the 4 compounds. Simple electrospray-ionization mass spectra with only protonated molecules were obtained except for tadalafil, which was cleaved easier than the other compounds, so cone voltage should be lower for tadalafil. Quantitative analysis of the 4 compounds was carried out using HPLC (UV at 230 nm). The lower limit of detection for each compound was 50 ng/ml at a 10-μl injection volume (0.5 ng on the column, S/N>3). A calibration curve was linear in the range of 0.01-1.0 mg/ml for each compound ( r ²>0.997). Our methods were applied to casework samples (confiscated tablets), and disclosed that those tablets contained varying amounts of sildenafil and/or tadalafil and were counterfeit products.