Garlic pills are not the best way to fight colds, as a new review suggests inconclusive evidence of the benefit of this treatment. Only one garlic study had strong enough data to be included in the review, but that study did find a large effect.
It included 146 patients randomly assigned to take garlic pills or a placebo for 12 weeks.
The researchers observed that the number of days they were sick, if they caught a cold, decreased from five to less than two, and there was also a dramatic reduction in the number of colds.
"The one relevant trial that we found did report a significant benefit: of those people taking garlic supplements, only 24 reported coming down with a cold, compared to 65 of the people taking the placebo tablet," said lead review author Elizabeth Lissiman, a medical student at the University of Western Australia.
She added: "Unfortunately, that trial was small and reported an unusually high number of people getting colds within the study period, so it cannot be considered conclusive."
However, the participants suffered only mild side effects: the expected bad breath, body odour and in some cases, a skin rash.
Explaining how garlic might work in colds, Lissiman said: "Some laboratory investigations have suggested that some components of garlic have antimicrobial properties. Theoretically, these compounds in garlic could kill the viruses that cause the common cold."
Christopher Gardner, an associate professor of medicine at Stanford University, who has studied the use of garlic to lower cholesterol, said that he was skeptical of the results on colds.
He said that the findings from the included study "could be a fluke or an outlier."
He also notes that reviews cannot answer questions about collections of data if they only include one study.
Gardner says that it is very difficult to study garlic, as there are more than 100 different types of garlic and each type contains many different compounds.
"It's incredibly complicated. There are 14 sulfur-containing compounds and two non-sulfur compounds," said Gardner.
He added: "It's not as simple as just freeze-dry the powder and stick it in a pill. There are issues there; you might ruin some of molecules in real garlic. The biochemistry of garlic is really quite complex and it's not even clear what the active agent might be."
The review has appeared in the latest issue of The Cochrane Library, which is a publication of The Cochrane Collaboration, an international organization that evaluates medical research.