Is vitamin C effective in treating the common cold?

This article was published in Medical Observer.

The article is also available on the Medical Observer website (may need registration).

Clinical scenario

Jenny, a stay-at-home mum, presented with an URTI.  After a clinical assessment, I advise her that she had the common cold.  She said, “Yeah, I thought so.  I just wanted to be sure.  So I should just rest and take some vitamin C right?”

Vitamin C has been widely used for the treatment of colds since the 1970s, when Linus Pauling controversially promoted the idea in his book “Vitamin C and the Common Cold”.

Clinical question

Does taking oral vitamin C reduce the duration or severity of an episode of the common cold?

What does the research evidence say?

A search for “vitamin C and cold” in the Cochrane Library finds a Cochrane systematic review published in 2010 examining the evidence for both vitamin C as treatment for colds, and vitamin C as prophylaxis. ¹  Using PubMed, there are no new randomised controlled trials (RCTs) using vitamin C for colds since 2010.  We will examine the Cochrane review by Hemilä et al. (2010) in detail.

Critical appraisal

Like the December 2011 issue, this will be a critical appraisal of a systematic review rather than a therapeutic trial.  I will use the appraisal sheet for systematic reviews available from the Centre for Evidence Based Medicine. ²

What PICO question does the systematic review ask?

In children and adults of either gender, and any age (Participants); what is the effect of oral vitamin C of at least 0.2 g/daily, for a single day or for a period (Intervention); compared to placebo (Comparator); on the incidence, duration, and severity of colds (Outcome).

Is it clearly stated?

Yes.

Is it unlikely that important studies were missed?

Yes.  This was an update of a previous Cochrane systematic review.  The authors searched three large electronic databases in the update, screened the reference lists in two other systematic reviews in the literature, and reviewed the references of all identified studies.  One of the authors had a decades-long research history in the area and reviewed her personal reference list of papers in the grey literature.

Were the criteria used to select articles for inclusion appropriate?

Yes.  The authors included any trial testing vitamin C of at least 0.2 g daily, for the prevention or treatment of colds, where it was placebo controlled.  They did not restrict to RCTs which are generally best for testing therapeutic interventions – however, the majority of the included trials were double-blinded and randomised.

Were the included studies sufficiently valid for the question asked?

Yes – probable.  The authors collected data and formally assessed the methodological quality of the identified studies.  Although most of the trials were described as randomised, the actual method of randomisation was unclear in many.

Were the results similar between studies?

Unclear.  There is significant heterogeneity when the results from the prophylaxis trials are looked together.  However, there appeared to have been two subgroups of studies – general community and heavy acute physical stress trials (e.g., alpine skiers).  There is moderate heterogeneity between therapeutic studies.

What were the results?

Looking at the community therapeutic studies, there are no consistent and clinically meaningful differences between the vitamin C and placebo groups.  This includes studies where the participants took doses greater than 1 g of vitamin C per day.

Conclusion

There is reasonable evidence that oral vitamin C is not an effective treatment for the common cold in the general community.  Moreover, there is good evidence that vitamin C prophylaxis does not reduce the incidence of colds, but does appear to have a modest effect on the duration and severity of an episode.

So how does this apply to Jenny?

There is no compelling reason to recommend the regular use of vitamin C for the treatment or prevention of colds, but, it is inexpensive and mostly harmless.  The common cold is part of the human condition, and supportive care and common sense is the mainstay of therapy.

Stats Facts

Meta-analysis

A meta-analysis combines the data from separate studies (that have similar hypotheses and methods) using statistical methods.  This can improve the precision of the results, and the strength of conclusions. ³  They are often an important component of systematic reviews.

References

1. Hemilä H, Chalker E, Douglas B. Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews 2007, Issue 3. Art. No.: CD000980.
2. Systematic Review: Are the results of the review valid? Centre for Evidence Based Medicine, University of Oxford. http://www.cebm.net/index.aspx?o=1157 Retrieved: 11 March 2012
3. Combining studies; What is a meta-analysis? The Cochrane Collaboration, 2002. http://www.cochrane-net.org/openlearning/html/mod12-2.htm Retrieved: 11 March 2012