In this multicenter, randomized, single-blind, placebo-controlled clinical trial. Patients will be randomly assigned to receive Vitamin C or placebo for 4 days or until ICU discharge (whatever come first). The primary outcome is 28-day all-cause mortality.

Investigational drug: Vitamin C for injection Study title: Clinical efficacy of megadose vitamin C in sepsis (CEMVIS): A Multicenter, Randomized, Single-blind, Placebo-controlled Clinical Trial Principal Investigator: Zhanguo Liu, professor, Department of Critical Care Medicine, Zhujiang Hospital, Southern Medical University Study subjects: Adult septic/septic shock patients with procalcitonin(PCT)≥2ng/ml at recruitment. Study phase: Investigator Initiated Trial(IIT) Study objectives: The objective of the study is to determine whether megadose vitamin c, compared to placebo, improve the prognosis of sepsis, including the reduction in mortality, the protection of organ function and reduction of inflammatory response, and to determine the safety of megadose vitamin c in patients with sepsis. Study design: A Multicenter, Randomized, Single-blind, Placebo-controlled Clinical Trial Method： Megadose vitamin C group: routine treatment follow the recommendation of the guidelines for sepsis in 2021+ 12 g vitamin C (48 ml) injection every 12 h for 4 days or until ICU discharge (death or transfer from ICU to general ward or discharge), whatever come first. Placebo control group: routine treatment follow the recommendation of the guidelines for sepsis in 2021 + 48ml 5% glucose injection every 12 h for 4 days or until ICU discharge (death or transfer from ICU to general ward or discharge), whatever come first. Course: 4 days Sample size: 234 The number of study center: 4 Study center: 1. Department of Critical Care Medicine of Zhujiang Hospital,Guangzhou, Guangdong, China 2. Department of Critical Care Medicine of The First People's Hospital of Foshan, Foshan, Guangdong, China 3. Department of Critical Care Medicine of Dongguan People's Hospital, Dongguan, Guangdong,China 4. Department of Critical Care Medicine of Yunfu People' s Hospital, Yunfu, Guangdong, China 5. Department of Critical Care Medicine of Zhongshan People's Hospital, Zhongshan, Guangdong, China Primary endpoint: 28-day all-cause mortality. Secondary endpoints: 1. The state of liver function: the serum level of transaminase(AST、ALT)、total bilirubin at 96 h after randomization 2. The state of lung function: oxygenation index(PaO2/FiO2) at 96h after randomization 3. The state of kidney function: serum level of Creatinine (Cr)、blood urea nitrogen(BUN)、Cystatin(Cys) at 96 h after randomization 4. The state of inflammatory response: the serum level of interleukin-6(IL-6) and C-reactive protein(CRP) at 96 h after randomization. 5. The state of infection: the serum level of procalcitonin(PCT) and white blood cell （WBC） at 96 h after randomization. 6. The state of circulation system: the serum level of lactate at 96 h after randomization 7. Organ dysfunction assessed by Sequential Organ Failure Assessment (SOFA) score at 96 h after randomization 8. The duration of successful cessation of supportive therapies for organ dysfunction including vasoactive agents, mechanical ventilation. 9. The duration of continuous renal replacement therapy(CRRT) 10. The length of stay in ICU Safety endpoints： 1. adverse events 2. Serious adverse events