Ayurveda emphasizes the importance of authentic medicinal substances (Dravya) in achieving therapeutic success. Due to ecological imbalance, overexploitation, seasonal variation, and geographical limitations, several classical drugs have become scarce. The Ayurvedic concept of Abhava Pratinidhi Dravya provides a scientific framework for substitution of unavailable drugs with pharmacologically similar alternatives. In contrast, Apamishrana Dravya refers to adulteration, which compromises drug quality and safety. This article elaborates classical references with original shlokas, need, types, clinical relevance, differences between substitution and adulteration, and quality control measures.