The primary objective of the study aims to compare the effect of high-dose intravenous vitamin C vs. placebo on a composite of death or persistent organ dysfunction - defined as continued dependency on mechanical ventilation, new renal replacement therapy, or vasopressors - assessed at 28 days on intensive care unit (ICU) patients. As secondary objectives, the study aims: * To compare the effect of high-dose intravenous vitamin C vs. placebo on: 1. 6-month mortality; 2. 6-month HRQoL; 3. organ function (days 1, 2, 3, 4, 7, 10, 14, and 28 if in ICU); 4. global tissue dysoxia (at baseline); 5. oxygenation Index (FiO2 x Mean Airway Pressure/PaO2) (days 1, 2, 3, 4, 7, 10, 14, and 28 if in ICU, and if still intubated); 6. occurrence of stage 3 acute kidney injury as defined by KDIGO (Kidney Disease: Improving Global Outcomes) criteria20; 7. acute hemolysis as defined by: * clinician judgment of hemolysis, as recorded in the chart, or * hemoglobin drop of at least 25 g/L within 24 hours of a dose of investigational product PLUS 2 of the following: * reticulocyte count \>2 times upper limit of normal at clinical site lab; * haptoglobin \< lower limit of normal at clinical site lab; * indirect (unconjugated) bilirubin \>2 times upper limit of normal at clinical site lab; * lactate dehydrogenase (LDH) \>2 times upper limit of normal at clinical site lab. Severe hemolysis: \- hemoglobin \< 75 g/L AND at least 2 of the above criteria AND requires 2 units of packed red blood cells; 8. hypoglycemia as defined as core lab-validated glucose levels of less than \< 3.8 mmol/L. * To assess baseline vitamin C levels in study participants (before the first dose of investigational product).