What is the recommended daily dose of vitamin C (ascorbic acid) for a generally healthy adult or child with symptoms of a cold or flu?

Vitamin C Dosage for Cold and Flu Symptoms

For generally healthy adults and children experiencing cold or flu symptoms, there is no strong evidence to support routine vitamin C supplementation for treatment, as it does not prevent colds and provides only modest symptom reduction (approximately 8-9% decrease in symptom days). 1

Evidence-Based Recommendations

For Prevention (Not Recommended for General Population)

• Daily supplementation with 1 gram or more of vitamin C does not prevent common colds in the general population 2, 1
• The exception is individuals under heavy physical stress, where 1+ gram daily reduced cold incidence by 50% 2

For Treatment (When Symptoms Already Present)

Adults

• If choosing to supplement therapeutically, 6-8 grams per day divided into multiple doses appears more effective than lower doses (3-4 grams/day) for reducing cold duration 2
• Regular supplementation of 1 gram or more daily decreased cold severity by 15% in trials, but this represents only a modest reduction of approximately half a symptom day per episode 1
• The standard daily intake for healthy adults is 75-90 mg/day for women and men respectively, which should be maintained through diet 3

Children

• Children may derive greater benefit than adults from vitamin C supplementation during colds 4
• In trials using 2 grams/day in children, cold duration decreased by a median of 26%, compared to only 6% reduction in adults taking 1 gram/day 4
• For children on parenteral nutrition: 15-25 mg/kg/day for infants, 80 mg/day for older children 5

Clinical Algorithm for Decision-Making

Step 1: Assess if supplementation is warranted

• Is the patient under heavy physical stress or extreme conditions? → Consider 1+ gram daily for prevention 2
• Does the patient have chronic oxidative stress (diabetes, smoking, heart failure, severe COPD)? → Consider 200-500 mg/day 5
• Is this a generally healthy person with a common cold? → Supplementation provides minimal benefit 1

Step 2: If proceeding with therapeutic supplementation

• Adults: 6-8 grams/day divided into multiple doses (e.g., 1-2 grams every 3-4 hours) 2
• Children: 2 grams/day divided into doses 4
• Duration: Continue for the duration of symptoms (typically 5-7 days)

Step 3: Monitor for contraindications

• Screen for hemochromatosis, G6PD deficiency, active oxalate kidney stones, or severe renal dysfunction before high-dose supplementation 3

Important Caveats and Pitfalls

Absorption Limitations

• Oral vitamin C absorption is limited at higher doses due to saturation of intestinal transporters 3
• Dividing doses throughout the day improves absorption and tolerability 2

Realistic Expectations

• The benefit is modest at best: even with optimal dosing, expect only 8-9% reduction in symptom days (less than half a day per cold episode) 1
• Therapeutic trials starting after symptom onset have shown inconsistent results 2, 1
• Constitutional symptoms (chills, malaise) may improve more than respiratory symptoms 6

When NOT to Supplement

• Do not delay or replace standard medical care for influenza with vitamin C supplementation 5
• Plasma vitamin C levels are unreliable during acute illness (CRP >10 mg/L makes interpretation difficult) 5
• Routine supplementation for cold prevention in healthy individuals is not cost-effective given the lack of benefit 1

Safety Considerations

• High-dose vitamin C (>1 gram/day) is generally safe but may cause gastrointestinal upset 2
• Avoid in patients with renal dysfunction, hemochromatosis, G6PD deficiency, or history of oxalate stones 3

Bottom Line for Clinical Practice

The evidence does not support routine vitamin C supplementation for treating common colds in otherwise healthy individuals, as the benefit is minimal (less than half a day reduction per cold episode). 1 If patients insist on trying vitamin C therapeutically, 6-8 grams/day divided into multiple doses appears more effective than lower doses, but they should understand the benefit is modest and inconsistent. 2 For children under heavy physical stress or extreme conditions, there may be greater benefit, but for typical cold symptoms in the general population, focus on symptomatic care and appropriate medical evaluation rather than vitamin C supplementation. 4, 2