Abstract for: The impact of market composition on resilience under supply-side disruptions in generic pharmaceutical supply chains

Ensuring continuous access to essential medicines is critical for a well-functioning healthcare system. Yet drug shortages are increasing worldwide and impose significant clinical, economic, and humanistic burdens. Pharmaceutical supply chains (PSCs) are complex and vulnerable systems where multiple actors interact under delays, feedback loops, and regulatory constraints. In the Netherlands, this vulnerability is amplified by a strong reliance on low-margin generic drugs, cost-driven procurement practices, and a preference policy that concentrates demand on a limited number of suppliers. This study develops a system dynamics (SD) model to analyze how market composition affects the resilience of the Dutch generic PSC under supply-side disruptions. The model captures multi-echelon interactions, substitution between generic brands, and pre- and post-shortage dynamics through endogenous feedback loops. System resilience is evaluated using metrics such as time-to-survive, time-to-recover, shortage duration, substitution volume, and alternative treatments. The results show that highly concentrated markets amplify disruption impacts and lead to longer shortages. In contrast, even moderate diversification significantly improves system performance by enabling substitution and distributing inventory across suppliers. The model further reveals how substitution dynamics, capacity constraints, and quota limitations interact and generate cascading effects across the supply chain. These findings highlight the critical role of market structure in shaping supply chain resilience and suggest that policies promoting diversification can mitigate shortage risks. Future work will extend the model to include demand-side and operational disruptions and use the resulting comprehensive stress-test insights to design and assess targeted policy interventions. Only for language refinement, including translation, grammar, and improving readability