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This question already has an answer here:

The study Preventing the common cold with a garlic supplement: a double-blind, placebo-controlled survey shows a reduction of days challenged virally by about 70% just by taking 180mg allicin every day.

This would be really huge. The economic impact of the common cold per year seems to be in the billions (Productivity losses related to the common cold.).

But the author of the allicin study (Peter Josling) appears to be involved in selling garlic supplements, so I'm not sure if one can trust his study fully.

The only related study I found is: Supplementation with aged garlic extract improves both NK and γδ-T cell function and reduces the severity of cold and flu symptoms: a randomized, double-blind, placebo-controlled nutrition intervention.

Are there any further studies verifying or refuting the existence of this effect?

marked as duplicate by Sean Duggan, pericles316, Larian LeQuella Mar 23 '16 at 1:19

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Summary: There is not yet enough evidence to form an opinion.

The 2014 Cochrane review, Garlic for the common cold by Elizabeth Lissiman, Alice L Bhasale and Marc Cohen, examined the study Preventing the common cold with a garlic supplement: a double-blind, placebo-controlled survey shows, and concluded it wasn't too bad:

The included trial was of reasonable quality

but still not perfect:

bias assessment

Figure 1. 'Risk of bias' graph: review authors' judgements about each risk of bias item presented as percentages across all included studies.

Cochrane reviews normally systematic examine many different papers. However, this review was unable to find any other high quality papers on the subject, and concluded:

Implications for practice

There is no conclusive evidence to recommend garlic supplements as a preventative or treatment option for the common cold. A single, small trial was found suggesting garlic might reduce the incidence of the common cold if taken continuously as a daily prophylactic (preventive treatment) but the results require validation. There is currently no evidence to help decide whether treating common colds with garlic will reduce symptom severity or days of illness. Anecdotally, adverse events reported include odour and minor skin or respiratory irritation. The frequency of adverse effects could not be determined from the evidence available.

We need more evidence though to seriously recommend such supplements. I hope there will be more studies in the future.

  • Can you summarize the title and results of the study you're quoting? – Sean Duggan Mar 21 '16 at 16:58
  • Is the summary of the study in me question too terse? – Tobias Hermann Mar 21 '16 at 18:51
  • :) You have links to two studies up there, simply saying "this guy says this guy is OK". All it takes is one or both links failing and your statement is absolutely devoid of information. Name the review, and describe why you feel it proves that Josling's study is "OK". – Sean Duggan Mar 21 '16 at 19:06
  • Um, are all the risks of bias 100%? Or is that just a list with a huge color-coded tag section and a spurious precentage axis? – StarWeaver Mar 21 '16 at 21:44
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    I made a not insubstantial edit, because the reference found was excellent - a Cochrane review is pretty much the best we can hope to find on this, and it was worth quoting the conclusion of the review, not just their opinion on one paper. – Oddthinking Mar 21 '16 at 23:58

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