Does vitamin C (ascorbic acid) supplementation have any therapeutic effect on reducing symptom severity in an adult patient with no underlying medical conditions after they have already become sick?

Vitamin C After You're Already Sick

Yes, vitamin C can modestly reduce the duration and severity of cold symptoms when taken after you're already sick, though the benefit is relatively small and requires high doses started within 24 hours of symptom onset. 1

Evidence for Therapeutic Use

The European Position Paper on Rhinosinusitis (2020) concludes that because there is a consistent effect of vitamin C on the duration and severity of colds in supplementation studies, and given the low cost and safety, it may be worthwhile for common cold patients to test on an individual basis whether therapeutic vitamin C is beneficial for them. 1

Magnitude of Benefit

• Vitamin C reduces cold duration by approximately 8% in adults and 14% in children when taken regularly, with therapeutic doses potentially offering similar benefits. 2

• A 2023 meta-analysis found that vitamin C significantly decreased the severity of common cold symptoms by 15%, with the greatest effect observed on severe symptoms rather than mild symptoms. 3

• The effect appears more pronounced for severe symptoms - vitamin C may have greater impact on reducing days of severe illness and absenteeism compared to overall cold duration. 3

Dosing for Treatment

For therapeutic use after symptom onset, doses of 6-8 g/day appear more effective than lower doses of 3-4 g/day, though this must be initiated within 24 hours of symptom onset for potential benefit. 4, 5

• The American College of Rhinology notes that therapeutic doses of 6-8 g/day may be beneficial when started early. 4

• Two therapeutic trials found that 6-8 g/day was twice as effective at reducing cold duration as 3-4 g/day. 5

• Oral absorption is limited at higher doses due to saturation of intestinal transporters, making very high oral doses less efficient. 4

Important Caveats and Limitations

Mixed Evidence on Therapeutic Trials

The evidence for vitamin C started after symptom onset is less consistent than for regular supplementation:

• The 2013 Cochrane review found no consistent effect of vitamin C on duration or severity of colds in therapeutic trials (7 comparisons, 3,249 episodes). 2

• However, more recent analyses suggest benefit when higher doses are used and treatment is initiated promptly. 3, 5

Safety Considerations

Absolute contraindications to high-dose vitamin C include: 4

• Hemochromatosis or iron overload conditions

• G6PD deficiency

• Active oxalate kidney stones

• Severe renal dysfunction

• The tolerable upper level is 2 g/day based on gastrointestinal upset; doses above this may cause severe diarrhea. 6

Clinical Bottom Line

For an otherwise healthy adult with a new cold, taking 6-8 grams of vitamin C daily (divided into multiple doses) starting within 24 hours of symptom onset is reasonable given the low cost, good safety profile, and potential for modest symptom reduction. 1, 4

The benefit is modest - reducing cold duration by less than a day on average and decreasing severity by about 15% - but this may be clinically meaningful for individual patients, particularly those with more severe symptoms. 3, 2 The European guidelines appropriately frame this as something patients can "test on an individual basis" rather than a universal recommendation. 1

This is NOT a substitute for proven antiviral therapy if influenza or other treatable viral illness is suspected. 4