Active pharmaceutical ingredients (APIs) are the essential, active ingredients of pharmaceutical dosage forms and drug delivery systems. This chapter provides a comprehensive and systematic coverage of pharmaceutical and biomedical applications of pyrrolidone and caprolactam‐based materials in API applications. Antibiotics and antimicrobials are indispensable drugs for human and animal health and well‐being. The chapter also provides an explanation of the mechanism of action for the excipients in dosage forms ranging from conventional solids and liquids to novel solids and colloidal drug delivery systems. The modulation of solubility of poorly soluble drugs is a viable and safe approach for enhancing the drug's bioavailability. The drug development within the pharmaceutical industry has undergone a significant transformation. Prior to the late 1980s, drug candidates were primarily established by empirically‐based screening programs, where the leading candidates were mostly selected from chemical compounds that had already undergone significant technical investigation.