Erim Bešić, Davor Šakić
Background/Objectives: Electron paramagnetic resonance (EPR) spectroscopy is a direct and highly specific method for detecting paramagnetic species, including free radicals, transition-metal centers, spin labels and spin probes. In pharmaceutical research, it is particularly valuable in systems involving radical chemistry, oxidative degradation, drug-membrane interactions, drug delivery carriers, metallodrugs and oxygen-sensitive microenvironments. This review aims to summarize current applications of and emerging opportunities for EPR spectroscopy in pharmaceutical research, with primary emphasis on continuous-wave (CW) EPR, the mode most widely used for routine pharmaceutical measurements. Methods: This review is organized around the main CW-EPR approaches relevant to pharmaceutical research, including direct detection of paramagnetic species, spin trapping, spin labeling and spin-probe analysis. The discussion emphasizes how spectral parameters such as the g-value, hyperfine splitting, signal intensity and linewidth can provide structural, kinetic and microenvironmental information. Results: CW-EPR supports the study of radical-mediated drug activity and toxicity, antioxidant properties, formulation stability, membrane interactions, carrier structure, metallodrug behavior and oxygenation. Its main strength is the ability to connect molecular-level radicals and paramagnetic processes with broader pharmaceutical questions related to drug efficacy, safety, stability and delivery. Conclusions: CW-EPR is a specialized but versatile analytical tool at the interface of radical chemistry, pharmaceutical technology and biomedical research. Recent developments in compact instrumentation, selective probes and spectral simulation may further expand its use in pharmaceutical development, stability assessment and drug delivery research.