Vitamin C—Beyond Deficiency: Mechanisms, Clinical Applications, Formulation and Dosing Considerations, and Safety Across Stress-Responsive Conditions

Vitamin C (L-ascorbic acid) is an essential micronutrient involved in collagen biosynthesis, redox regulation, immune function, endothelial biology, carnitine synthesis, neurotransmitter metabolism, and non-heme iron absorption. Dietary reference values are designed primarily to prevent deficiency in general populations, but vulnerability to low-vitamin C status may increase during trauma, surgery, chronic inflammation, malignancy, metabolic disease, smoking, poor intake, environmental exposure, and tissue repair. This narrative review synthesizes mechanistic, pharmacokinetic, clinical, and safety evidence on vitamin C as a stress-responsive micronutrient. Evidence is reviewed across tissue repair and wound healing, orthopedic recovery and selected complex regional pain syndrome risk contexts, fatigue, neuropsychiatric vulnerability, cancer-supportive care, vascular homeostasis, dermatologic biology, and preliminary microbiota-gut-brain axis hypotheses. The strength of evidence differs substantially across domains: biochemical functions and deficiency correction are well established, whereas benefits of supraphysiologic oral supplementation in vitamin C-replete patients remain uncertain. Oral nutritional supplementation is distinguished from intravenous pharmacologic ascorbate, with attention to route, formulation, dose division, gastrointestinal tolerance-limited adjustment, and safety monitoring. Because evidence for high-dose oral supplementation remains limited and condition-specific, such use should be individualized, time-limited, and clinician-monitored rather than presented as a population-level recommendation or evidence-defined therapeutic target. Taken together, the clinical value of vitamin C depends on baseline status, patient vulnerability, route, formulation, dosing interval, clinical endpoint, and safety review.