What is the recommended dose and frequency of vitamin C for treating viral illnesses?

Vitamin C Dosing for Viral Illnesses

There is no established evidence-based dose or frequency of vitamin C specifically recommended for treating viral illnesses, and current high-quality guidelines recommend against its routine use for this indication. 1

Guideline-Based Position

The most recent and authoritative guidance comes from the Surviving Sepsis Campaign pediatric guidelines (2020), which recommend against the use of vitamin C in sepsis or sepsis-associated organ dysfunction (weak recommendation, very low quality of evidence). 1 While this addresses sepsis specifically rather than all viral illnesses, it represents the highest-quality guideline evidence available for vitamin C in acute infectious disease management.

No major medical society or guideline body recommends vitamin C as a treatment for viral respiratory infections, including influenza or COVID-19. 1

If Vitamin C Deficiency is Documented

The only scenario where vitamin C dosing is clearly indicated during viral illness is when documented vitamin C deficiency (scurvy) is present:

• Initial treatment: 100 mg orally three times daily (300-500 mg/day total) for at least one month 2
• Alternative regimen: 250 mg orally twice daily for 3 weeks 2
• Treatment should begin immediately when clinical symptoms suggest scurvy (bleeding gums, petechiae, poor wound healing) without waiting for laboratory confirmation 2

Research Context (Not Guideline-Supported)

While research studies have explored various dosing regimens, these remain investigational and are not endorsed by clinical guidelines:

Common Cold Prevention

• Regular supplementation of 1 g or more daily decreased cold severity by 15% in meta-analyses, but did not prevent colds in the general population 3
• In physically active individuals, vitamin C halved the incidence of colds 4, 3

Therapeutic Dosing (Investigational Only)

• Two trials found that 6-8 g/day was twice as effective at reducing cold duration compared to 3-4 g/day, though results remain inconsistent 3
• The most commonly studied sepsis protocol used 1.5 g IV every 6 hours for 4 days (6 g/day total), which showed no significant benefit in the rigorous C-EASIE 2025 trial 1

Critical Safety Considerations

Contraindications to high-dose vitamin C include: 1

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

Oral absorption is saturable at higher doses due to intestinal transporter limitations, making IV administration necessary if pursuing high-dose therapy in critical illness. 2

Common Pitfalls to Avoid

• Do not delay or substitute standard antiviral therapy (such as oseltamivir for influenza) with vitamin C 5, 1
• Do not measure plasma vitamin C levels during acute viral illness - inflammation (CRP >10 mg/L) makes interpretation unreliable 2
• Do not assume that "natural" or "safe" means effective - the evidence does not support routine vitamin C supplementation for treating viral illnesses 1
• Avoid using low doses (3-4 g/day) if attempting therapeutic intervention, as research suggests higher doses (6-8 g/day) may be necessary for any potential effect, though this remains unproven 3

Practical Algorithm

For patients with viral illness:

1. Is there clinical suspicion of scurvy? (bleeding gums, petechiae, poor wound healing)

   • Yes → Treat with 100 mg orally three times daily for 1 month 2
   • No → Proceed to step 2

2. Is the patient critically ill with sepsis or severe viral pneumonia?

   • Yes → Follow standard sepsis guidelines; vitamin C is NOT recommended 1
   • No → Proceed to step 3

3. Does the patient want to try vitamin C for symptom reduction?

   • For prevention in physically active individuals: 1 g daily may reduce cold incidence 3
   • For common cold treatment: 6-8 g/day in divided doses may reduce duration, though evidence is weak 3
   • Ensure no contraindications exist before recommending 1

The bottom line: Vitamin C has no established role in treating viral illnesses beyond correcting documented deficiency states. Standard antiviral therapy and supportive care remain the evidence-based approach. 5, 1