What is the recommended dosage of vitamin C (ascorbic acid) to take at the onset of a cold?

Vitamin C Dosage for Common Cold Treatment

For the treatment of common cold symptoms, taking vitamin C at a dose of 75-200 mg/day at the onset of symptoms may be worthwhile to test on an individual basis, though evidence for significant therapeutic benefit is limited. 1, 2

Recommended Dosage

• Vitamin C supplementation may be considered when cold symptoms first appear, though the evidence for therapeutic benefit (starting after symptoms begin) is less consistent than for regular preventive supplementation 2
• The European Position Paper on Rhinosinusitis and Nasal Polyps (EPOS 2020) suggests that due to vitamin C's consistent effect on cold duration and severity in supplementation studies, plus its low cost and safety profile, it may be worthwhile for common cold patients to test whether therapeutic vitamin C benefits them 1
• For general supplementation during illness, doses of 200-500 mg/day are commonly recommended for conditions with increased oxidative stress 1

Evidence on Effectiveness

• Vitamin C has shown a modest but consistent effect on reducing the duration and severity of cold symptoms in regular supplementation studies, with an average reduction of 8-9% in symptom days 3, 2
• Children may experience greater benefits than adults, with some studies showing up to 14-18% reduction in cold duration in children taking 1-2 g/day 2
• Higher doses (≥2 g/day) may provide greater benefits than lower doses (1 g/day), though this finding is not consistently supported across all studies 4

Important Considerations

• Vitamin C is generally safe even at higher doses, making it a low-risk option to try 1
• The therapeutic effect (starting after cold symptoms begin) has been less studied than preventive supplementation, and results have been variable 2
• One randomized controlled trial found that doses of vitamin C in excess of 1 g daily taken shortly after onset of a cold did not significantly reduce duration or severity of symptoms compared to placebo 5
• Zinc lozenges (75 mg/day) taken within 24 hours of symptom onset may be more effective than vitamin C for reducing cold duration 1

Alternative and Complementary Approaches

• Zinc acetate or zinc gluconate lozenges at ≥75 mg/day taken within 24 hours of symptom onset significantly reduce common cold duration 1
• Nasal saline irrigation may provide some symptomatic relief, particularly in children 1
• Some herbal medicines like BNO1016 (Sinupret), Cineole, and Andrographis paniculata extract have shown positive effects on common cold symptoms 1
• Antibiotics should not be used for common cold as they are ineffective and may lead to adverse effects and antimicrobial resistance 1

Common Pitfalls

• Taking extremely high doses of vitamin C (>1 g/day) at cold onset has not consistently shown greater benefit than more moderate doses 5
• Expecting vitamin C to prevent colds is not supported by evidence for the general population (though it may help in people undergoing intense physical stress) 2
• Delaying other appropriate treatments while waiting for vitamin C to work could prolong symptoms 1
• Echinacea products have not been consistently shown to provide significant benefits for treating colds 1

While the evidence for vitamin C's therapeutic effect on common colds is modest, its safety profile makes it a reasonable option to try when cold symptoms first appear. The most consistent evidence supports doses in the range of 75-200 mg/day, though some may benefit from higher doses.