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Drug–device combination products occupy a technically demanding position between pharmaceutical formulation, engineered device performance, and real-world user interaction. Their development requires the simultaneous control of drug product quality, delivery-system reliability, and safe administration by intended users. Yet these domains are still frequently treated as separable workstreams rather than as mutually shaping elements of one system. This conceptual framework article addresses the limitations of sequential development models in which formulation is stabilised first, device selection follows, and user validation is deferred until late-stage development. Such sequencing may appear efficient during early development, but it can conceal incompatibilities that only emerge during device verification, usability testing, clinical bridging, or regulatory review. The result is often redesign, delayed translation, or unresolved uncertainty about whether the final product can perform reliably under intended conditions of use. The objective of this article is to propose and defend the Formulation–Device–User Triangle as a unified design logic for drug–device combination products. The triangle positions formulation, device, and user as co-equal vertices that continuously constrain and enable one another. It is intended not as a replacement for existing quality, design-control, or human-factors processes, but as an integrating framework that makes their interdependence explicit. The proposed triangle reframes combination product design as a system-level co-development problem. It argues that a product is not ready for translation simply because its formulation is stable, its device is functional, or its users can pass a summative test. Readiness depends on whether the formulation tolerates device action, the device accommodates formulation variability, and the user interface supports reliable administration across real-world conditions.