Key messages

• Vitamin C has little or no impact on the duration of the cold and on days at home or out of work.
• Despite the lack of relevant adverse effects, investing in this treatment is not justified.

What is the evidence.
See evidence matrix  in Epistemonikos later

We found four systematic reviews [2],[3],[4],[5] that included eight primary studies [6],[7],[8],[9],[10],[11],[12],[13] of which, seven were randomized trials [6],[7],[8],[9],[10],[11],[12]. This table and the summary in general are based on the latter, since the observational study did not increase the certainty of the existing evidence, or provide additional relevant information.

What types of patients were included*

Four trials included patients of both sexes [7],[8],[11],[12], one only included women [9] and two did not specify it [6],[10].

Six trials included adults[6],[7],[8],[9],[11],[12] and one did not specify it [10].

The trials were conducted in Canada [6],[7], the United States [5],[9], the United Kingdom [8],[10] and Australia [11].

What types of interventions were included*

All the trials evaluated vitamin C administered in different doses, starting the first day of cold. 

All the trials compared against placebo, and two of them also evaluated the effectiveness of vitamin C by comparing different doses: 4 or 8 grams [7] and 1 or 3 grams [12].

The dose and duration of the treatment varied in all the studies: 3 grams per day for 5 days [6], 4 or 8 grams per day for one day [7], 1 gram per day for 5 days [8] , 4 grams per day for 2.5 days [9],[11], 3 grams per day for 2 days [10] and 1 or 3 grams per day for 3 days [12].

What types of outcomes
were measured

The measurement method for the duration of the cold was very heterogeneous among the included trials, so, in order to standardize the results, the systematic review that included more trials [2] decided to report the duration in the form of percentages (being placebo equivalent to a 100%). To facilitate interpretation, these percentages were converted into absolute numbers according to the average number of days of symptoms in the trials.

The severity was also measured heterogeneously in the different trials. The reviews pooled the results as days at home or out of work, or with a symptom score.

To whom this evidence does and does not apply

• The results of this summary are widely applicable to people with the common cold, regardless of gender or race. Even though the participants in the trials did not include children, pregnant women, groups with comorbidities or people subjected to high physical stress or extreme cold, there are no compelling clinical reasons for not applying the evidence to these groups.
• All of the included trials were conducted in Anglo-Saxon countries with temperate climates, however, there are no clinical or pathophysiological reasons for not applying the results to different geographic locations or climates. Therefore, in the absence of direct evidence in these contexts, it is reasonable to extrapolate the results of this summary to any person with the common cold.

• The outcomes selected are those considered critical for decision-making based on the opinion of the authors of this summary, and are in agreement with those presented by the main systematic reviews identified.

• None of the systematic reviews conducted a meta-analysis of the adverse effects. One of the systematic reviews [2] delved into the topic reporting that seven studies (not specified) recorded data from a total of 2,490 patients who had used ≥ 1 g/day of vitamin C compared to 2066 who took placebo. In total, 5.8% of patients who received vitamin C had adverse effects (not detailed) that could be attributed to the medication, compared to 6.0% of those who took placebo. No serious adverse effects were reported.
• Even though it is an intervention with minimal adverse effects, it has no benefit, so the balance between benefits and risks is not favorable.

• Notwithstanding vitamin C is easy to acquire and relatively inexpensive, it has no benefits, so the balance between benefits and costs is not favorable.

• Faced with the evidence presented in this summary, most patients and doctors should lean against using this intervention.
• However, as the belief in the efficacy of vitamin C is deeply rooted in the general population, additional efforts are probably needed to get the message across.

Differences between this summary and other sources

• The conclusions presented are consistent with the main systematic review [2] and are also in agreement with the guideline ‘The Common Cold’ [1].

• The probability that future studies change the conclusion of this summary is very low, since the certainty of the evidence is high.
• We did not identify ongoing trials addressing this question in the International Clinical Trials Registry Platform of the World Health Organization or systematic reviews in progress in the PROSPERO registry.