Is high dose vitamin C (ascorbic acid) effective for treating viral illnesses or flu?

High-Dose Vitamin C for Viral Illness or Flu

High-dose vitamin C is not recommended for treatment or prevention of viral illnesses including influenza in healthy individuals, but regular supplementation may be worthwhile for common cold patients to test on an individual basis given its consistent effect on symptom duration and severity, low cost, and safety profile. 1

Evidence-Based Recommendations by Clinical Scenario

For Prevention in Healthy Individuals

• Do not use high-dose vitamin C as monotherapy for infection prevention in healthy people - there is no evidence supporting this practice (Grade B recommendation, 96% consensus). 2
• Doses exceeding 10 times the dietary reference intake should not be used without proven severe deficiency. 2
• The recommended daily intake is 75-90 mg/day for healthy adults, which can be achieved through diet. 3

For Treatment of Common Cold/Viral Upper Respiratory Infections

• Regular vitamin C supplementation may be worthwhile for common cold patients to test individually given consistent effects on duration and severity of symptoms in supplementation studies, combined with low cost and safety. 1
• Vitamin C does not reduce the average incidence of colds in the general population, but it halved the number of colds in physically active people. 4
• When taken regularly, vitamin C has shortened the duration of colds, indicating a biological effect. 1
• The therapeutic benefit remains unclear, though two controlled trials found statistically significant dose-response effects with 6-8 g/day for symptom duration. 4

For Influenza Treatment

• Early treatment within 48 hours of symptom onset should use established antivirals (oseltamivir, zanamivir for influenza A and B; amantadine or rimantadine for influenza A only). 1
• Vitamin C is not recommended as primary therapy for influenza - no high-quality evidence supports its use for this specific indication. 1

For Malnourished or At-Risk Populations

• Malnourished patients should ensure sufficient supplementation with vitamins including vitamin C to maximize anti-infection nutritional defense. 1
• High-risk populations with chronic oxidative stress may require 200-500 mg/day. 3
• Physically active individuals may benefit more from supplementation, as effects are more prominent when baseline levels are low. 5

Critical Safety Considerations

Absolute contraindications to high-dose vitamin C include: 2

• Hemochromatosis or iron overload conditions
• G6PD deficiency (risk of hemolysis)
• Active oxalate kidney stones
• Severe renal dysfunction

Dosing Guidance When Used

For Common Cold (if choosing to trial)

• Therapeutic doses studied: 6-8 g/day showed dose-response benefits for symptom duration. 4
• Lower doses of 3-4 g/day may explain negative findings in some studies. 4
• Must be initiated within 24 hours of symptom onset for potential benefit. 1

For Documented Deficiency

• Standard repletion: 100 mg three times daily (300-500 mg/day total) for at least one month. 3
• Alternative: 250 mg twice daily for 3 weeks for severe cases. 3

Common Pitfalls to Avoid

• Do not delay or substitute proven antiviral therapy (for influenza) or standard supportive care with vitamin C supplementation. 1
• Do not assume high-dose supplementation prevents viral infections in well-nourished individuals - this is not supported by evidence. 5
• Oral absorption is limited at higher doses due to saturation of intestinal transporters, making very high oral doses inefficient. 3
• Do not measure plasma vitamin C levels during acute illness - inflammation (CRP >10 mg/L) makes interpretation unreliable. 3, 6

What Actually Works for Viral Illness Prevention

More effective evidence-based interventions include: 1

• Zinc lozenges (≥75 mg/day within 24 hours of symptom onset) significantly reduce common cold duration. 1
• Regular moderate-intensity exercise may prevent common colds. 1
• Influenza vaccination for influenza prevention (not common cold). 1
• Probiotics may reduce acute upper respiratory infection incidence, though evidence quality is low. 1

The Bottom Line

The European guidelines provide the most balanced perspective: given vitamin C's consistent effect on cold duration and severity in regular supplementation studies, combined with low cost and excellent safety profile, it may be reasonable for common cold patients to test whether therapeutic vitamin C benefits them individually. 1 However, this is distinctly different from recommending high-dose vitamin C as standard therapy for all viral illnesses or influenza, which lacks supporting evidence and should not replace proven treatments. 1, 2