Vitamin

Vitamin C

L-Ascorbic acid (Ascorbate)

Evidence TierAWADA NOT PROHIBITED

tuneTypical Dose

500-2,000 mg/day

watchEffect Window

Acute plasma effects within hours. Cold-duration signal across the illness window.

check_circleCompliance

WADA NOT PROHIBITED

Overview

Clinical Summary

Vitamin C is an antioxidant and collagen synthesis cofactor. It is used to prevent scurvy, support wound healing, and improve non-heme iron absorption, especially when dietary intake is low.

Vitamin C correction prevents scurvy and supports collagen formation and wound healing. Newer meta-analytic evidence suggests vitamin C also modestly reduces common-cold severity, with stronger effects on the more severe phase of illness than on mild symptoms. Improved non-heme iron absorption remains well established. High doses can cause gastrointestinal upset and may increase kidney stone risk in susceptible individuals.

Vitamin C functions as a water-soluble antioxidant and an enzymatic cofactor for collagen formation and immune-cell function.

Outcomes

What This Is Expected To Influence

Primary Outcomes

  • Modestly shortens common-cold duration in routine-use populations.

Secondary Outcomes

  • Can reduce cold incidence in sustained high-physical-stress cohorts.
  • Supports collagen synthesis and non-heme iron absorption.

Safety

Contraindications and Interactions

Contraindications

  • Recurrent calcium-oxalate kidney stones
  • Severe renal impairment
  • Known hypersensitivity

Side effects

  • Diarrhea (more likely at higher doses)
  • Nausea
  • Abdominal cramping or pain
  • Rash

Interactions

  • Amphetamines (Probable/Moderate) - Vitamin C can decrease the absorption of amphetamines and potentially reduce their effectiveness.
  • Warfarin (Possible/Moderate) - While some cases report vitamin C reducing warfarin's effectiveness, other trials suggest that this interaction is not clinically significant.
  • Levothyroxine sodium (Possible/Moderate) - Vitamin C may increase the absorption of levothyroxine, which could necessitate a dose reduction to maintain an appropriate therapeutic effect.
  • Iron (Possible/Minor) - Vitamin C may enhance the absorption of non-heme iron.
  • Acetaminophen (Possible/Minor) - Vitamin C can increase the half-life and decrease the excretion of acetaminophen.
  • Aluminum hydroxide (Possible/Minor) - Vitamin C may increase the absorption of aluminum.
  • Indinavir (Possible/Minor) - Vitamin C may decrease the blood levels of indinavir.
  • Propranolol (Possible/Minor) - Vitamin C may reduce blood levels of propranolol.
  • Possible interference with some glucose/lab assay interpretation.

Avoid if

  • Active kidney-stone recurrence
  • Severe uncontrolled renal disease
  • People using amphetamines
  • People using warfarin
  • People using levothyroxine sodium

Evidence

Study-level References

vitamin-c-SRC-001Systematic review and meta-analysis
Sourceopen_in_new

Hemila H, Chalker E. "Vitamin C for preventing and treating the common cold." Cochrane Database Syst Rev. 2013.

Population: General population and cold-treatment cohorts

Key findings: Routine supplementation does not prevent colds in the general population but modestly reduces duration and severity.

Paper content

Routine supplementation does not prevent colds in the general population but modestly reduces duration and severity.

vitamin-c-SRC-002Narrative review with pooled trial context
Sourceopen_in_new

Hemila H. "Vitamin C and infections." Nutrients. 2017.

Population: Physically stressed adults (athletes, military)

Key findings: Stronger preventive signal appears in short-term, high-physical-stress settings.

Paper content

Stronger preventive signal appears in short-term, high-physical-stress settings.

vitamin-c-SRC-003Meta-analysis of randomized double-blind placebo-controlled trials
Sourceopen_in_new

Hemila H, Chalker E. Vitamin C reduces the severity of common colds: a meta-analysis. BMC Public Health. 2023;23(1):2468. doi:10.1186/s12889-023-17229-8. PMID:38082300.

Population: Generally healthy participants from 10 placebo-controlled vitamin C common-cold trials.

Dose protocol: Oral vitamin C at 1 g/day or higher across placebo-controlled common-cold trials

Key findings: Reduced common-cold severity by about 15 percent and preferentially shortened the more severe phase of illness.

Notes: Useful for repairing broad immune-boosting wording into a more specific symptom-burden claim.

Paper content

This 2023 meta-analysis sharpens how vitamin C should be described in common-cold guidance. The supplement did not transform cold prevention in the general population, but it did reduce overall severity and had a stronger effect on the more severe phase of colds than on mild symptoms. That supports more specific wording around symptom burden rather than broad immune-boosting claims.

vitamin-c-SRC-004Systematic review.
Sourceopen_in_new

Alangari A, Arif J, Al Qureshah F, et al. Clinical benefits and risks of high-dose intravenous vitamin C: a systematic review. J Med Life. 2026. doi:10.25122/jml-2025-0176. PMID:41815850.

Population: Patients across multiple clinical settings including sepsis, oncology, and symptom management, drawn from studies published 2010 to 2025.

Dose protocol: High-dose intravenous vitamin C across sepsis, oncology, and symptom management settings (2010 to 2025)

Key findings: Evidence does not support routine IVC in sepsis. Early-phase oncology trials show safety and QoL gains. Major safety risks include oxalate nephropathy and hemolysis in G6PD deficiency.

Notes: Positions high-dose IVC as exploratory rather than standard of care. Relevant for safety boundary-setting.

Paper content

This systematic review synthesized evidence from 2010 to 2025 on high-dose intravenous vitamin C (IVC). IVC achieves plasma concentrations far exceeding what oral supplementation can deliver. The review found plausible mechanistic support for antioxidant and immune benefits, and some phase trials in oncology showed safety and quality-of-life improvements with a promising survival signal in pancreatic cancer when combined with chemotherapy. However, evidence does not support routine IVC use in sepsis. Major safety risks include oxalate-related kidney damage and hemolysis in patients with glucose-6-phosphate dehydrogenase deficiency. The review positions high-dose IVC as supportive and exploratory rather than standard of care for any indication.

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