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Excipients are conventionally described through intrinsic material properties, pharmacopeial specifications, and functional labels such as binder, disintegrant, solubiliser, stabiliser, or release modifier. This vocabulary has supported pharmaceutical development for decades because it simplifies excipient selection and links material identity to expected product performance. Yet the same vocabulary becomes unstable when dosage forms are compositionally dense, structurally heterogeneous, and highly dependent on manufacturing history. The central problem is that excipient performance in complex dosage forms often deviates from what would be predicted by isolated material tests. A polymer that stabilises supersaturation in one amorphous solid dispersion may fail in another, while a lipid excipient that improves solubilisation under one digestion condition may promote precipitation under another. Such behaviour suggests that excipient functionality is not merely carried by the excipient molecule, but is produced within the dosage form system. This article proposes a theoretical reframing of excipient functionality as a system property. In this view, functionality emerges from the combined effects of formulation composition, spatial architecture, and processing history. The purpose is not to replace molecular or compendial characterisation, but to relocate those measurements within a broader systems framework. The proposed theory defines excipient functionality as an emergent outcome of interactions among drugs, excipients, process energy, phase behaviour, and microstructural organisation. It explains why apparently similar formulations can display different dissolution, supersaturation, release, or stability behaviours when their processing route or internal architecture differs. Three tables are used to contrast the reductionist and system-property paradigms, map overlooked interactions, and identify design implications. Adopting a system-property view would shift pharmaceutical development from selecting excipients as isolated ingredients toward designing excipient functions as relational outcomes. It would encourage formulation scientists to evaluate not only what an excipient is, but what it becomes within a particular dosage form. This perspective offers a conceptual basis for more predictive, adaptive, and robust pharmaceutical product design.

Adaptive drug delivery has often been imagined as a technologically advanced system in which sensors, software, power sources, and feedback algorithms continuously monitor biological conditions and adjust therapy. This vision has stimulated important innovation, but it has also encouraged the assumption that adaptation requires electronic intelligence. In many pharmaceutical contexts, this assumption may unnecessarily increase complexity, cost, and technical fragility. A different design logic is possible. Dosage forms can respond to physiological environments through the intrinsic behavior of materials rather than through real-time electronic sensing. Such systems do not measure, calculate, or transmit information digitally; instead, they translate local biological conditions into physical or chemical changes that modulate drug release. This article develops a theory-driven framework for passive adaptive dosage forms. It distinguishes passive responsiveness from active sensor-driven feedback and defines adaptation as an emergent property of material–environment interaction. The framework is intended for non-electronic dosage forms that use physiological cues such as pH, enzymes, glucose, redox gradients, temperature, or mechanical stress to regulate release. The article synthesizes evidence and concepts from stimuli-responsive polymers, hydrogels, molecularly imprinted polymers, shape-memory systems, glucose-responsive platforms, and self-oscillating gels. It does not present new empirical data. Instead, it uses existing literature to clarify the design principles needed to treat passive responsiveness as a deliberate pharmaceutical strategy. Passive adaptive dosage forms offer a simpler and potentially more translatable route to adaptive therapy. Their promise lies not in replacing all electronic systems, but in expanding the adaptive delivery paradigm beyond sensors and circuits. By foregrounding material-based triggering and release behavior design, the article positions passive responsiveness as a distinct and underdeveloped class of pharmaceutical system design.