Mastering particle size analysis: lessons, challenges, and future directions from the FDA-CRCG workshop.

Particle-size distribution (PSD) measurement is a cornerstone of pharmaceutical development and regulatory assessment, influencing formulation stability, dissolution, product quality, and performance. Yet the seemingly straightforward nature of "size measurements" often conceals substantial scientific and practical complexity. To strengthen shared understanding and advance good measurement practices, the U.S. Food and Drug Administration (FDA) and the Center for Research on Complex Generics (CRCG) co-hosted a two-day hybrid workshop, "Mastering Particle Size Analysis: A Step-by-Step Illustration of Techniques and Best Practices," held on September 23-24, 2025. The workshop brought together scientists from regulatory agencies, industry, academia, and instrument manufacturers to examine the principles, data interpretation, and analytical procedure robustness of two widely used techniques-Dynamic Light Scattering (DLS) and Laser Diffraction (LD). The program integrated scientific lectures, vendor-led demonstrations, and structured small-group discussions organized around real-world case studies. Prior to the workshop, participating vendors conducted independent characterization of five pre-workshop materials representing diverse formulation types, providing a shared baseline for comparison. In total, 2823 participants registered for the workshop, including 127 for in-person attendance and 2696 for virtual attendance from across the global pharmaceutical community. Of those that registered, 119 participants attended in-person, and 1464 attended virtually during the two days. Discussions throughout the workshop emphasized that PSD data cannot be interpreted in isolation because what is measured depends on the model assumption, sample condition, and analytical purpose. Participants concluded that reliable characterization demands "fit-for-purpose" analytical procedure design, transparent reporting, and a clear linkage between measurements and material function. Major outcomes included (i) recognition that methodological transparency outweighs pursuit of a single "true" particle size; (ii) identification of the dominant sources of inter-laboratory variability; and (iii) broad consensus on the need for harmonized terminology, validation criteria, and reporting expectations. The workshop closed with collective commitments to prepare a community white paper and to initiate an inter-laboratory study aimed at improving comparability of PSD data across techniques, platforms, and laboratories-steps that will help ensure measurement integrity and regulatory confidence in complex drug development. Supplementary Information: The online version contains supplementary material available at 10.1186/s41120-026-00148-4.