Sildenafil, the active ingredient in Viagra (Pfizer), is a prescription medicine used for erectile dysfunction. Compounds with chemical structures similar to that of sildenafil were isolated and purified during the analysis of some herbal products marketed for treatment of erectile dysfunction. Structural elucidation using liquid chromatography-diode array detection, infrared spectroscopy, liquid chromatography-tandem mass spectrometry, and nuclear magnetic resonance spectroscopy confirmed that the compounds were homosildenafil, hydroxyhomosildenafil, and acetildenafil. The implications of adulteration by compounds structurally related to prescription drugs are discussed. Unlike established drugs, the efficacy and safety of such analogues are largely unknown. This poses a great challenge for safety and health administrators to detect these modified structures and to regulate them. Consumers who use such adulterated products are at risk of developing serious adverse reactions, potentially leading to death. Greater collaboration and exchange of information between various health authorities, health professionals, academics, researchers, and industry, as well as public education, are key steps in the efforts to stem the growing trend of adulteration of herbal products by analogues of prescription drugs.