With aging population, sedentary lifestyle, physical and psychological stresses from everyday life and work, increasing prevalence of both communicable and non-communicable diseases and increasing healthcare costs, it is timely to explore the use of Traditional Chinese Medicine and other medicinal plants for health promotion, disease prevention and treatment purposes. Quality, safety and efficacy of herbs and herbal products are closely inter-related. The quality control and safety considerations of herbs and complementary health products are of paramount importance. A brief overview of the factors affecting the quality of herbs and herbal products will be given. In particular, the presence of undeclared drugs and drug analogues pose a challenge to health regulators and healthcare professionals. Unknowing users are at risks of adverse events associated with the use of such illegal and poor quality preparations. The issues and challenges related to the adulteration of complementary health products with phosphodiesterase Type 5 inhibitors and their analogues will be discussed. Concerted efforts to screen for undeclared drug adulterants and the adverse events associated with complementary health products from 1998 to 2016 in Singapore will be shared.