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High-dose intravenous vitamin C (HDIVC) has been investigated as a potential adjunctive therapy in critical illness, including sepsis, acute respiratory distress syndrome (ARDS), and COVID-19. Despite a strong mechanistic rationale, clinical trials have yielded inconsistent results. From a clinical pharmacology perspective, this variability may reflect, at least in part, differences in pharmacokinetic exposure, timing of administration, and patient selection rather than a lack of biological activity. Intravenous administration enables plasma concentrations in the millimolar range (≈1-5 mM), far exceeding those achievable with oral dosing (<100 µM), thereby reaching thresholds required for pharmacodynamic effects on oxidative stress, immune signaling, and endothelial function. This exposure-dependent transition distinguishes vitamin C as a pharmacological agent rather than a nutritional supplement in critically ill populations. Therapeutic response may be influenced by timing relative to disease progression, with earlier administration representing a biologically plausible strategy that warrants prospective evaluation rather than a clinically established therapeutic window. Interindividual variability in transporter function, redox status, and genetic background may further contribute to heterogeneous responses. Biomarkers such as interleukin-6 (IL-6), C-reactive protein (CRP), D-dimer, and markers of endothelial injury provide a framework for patient stratification and monitoring of pharmacodynamic effects. Integrated with pharmacokinetic principles, these markers support a shift toward biomarker-guided, precision-based therapeutic strategies. This review synthesizes current clinical and mechanistic evidence through an exposure-response conceptual framework, framing HDIVC as a context-dependent pharmacological intervention and advancing a shift toward biomarker-guided, precision-based therapeutic strategies in critical illness.