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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?

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Taken from skeptics.stackexchange.com:

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.

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