Understanding the importance of drug interaction is fundamental for patient-centered care and safe use. Assessment of pharmacological profiles is an interesting strategy for new drug development and associations, treatment optimization, prevention of adverse effects, pharmacovigilance, and personalized medicine. A rational and unified system of classifying drugs and their interaction remains challenging due to confounding variables such as pathological conditions, genetic factors and microenvironments, associations of drugs, and non-drug entities (herbs, vitamins, nutrients, etc.). In this chapter, we focus on chemical classes, clinical indications, pharmacological interaction, and targets of drugs that are associated with their therapeutic or adverse outcomes. The knowledge of drugs and interaction attributes could inspire innovative pharmaceutical research and rational associations of drugs.