Summary
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What does garlic do?
Garlic can modestly reduce cholesterol levels (by reducing triglycerides) and blood pressure. It may also slightly reduce the risk of atherosclerosis, and may help prevent colds, but garlic does not help treat an existing cold. Preliminary evidence suggests garlic may have some protective role with regard to gastric cancer in limited situations (See What It Is). -
What are the best forms of garlic?
Clinical studies have typically used garlic extract (providing allicin or potentially providing it from an allicin precursor, alliin) or aged garlic (providing an allicin-derivative known as S-allylcysteine or SAC). Some studies have used fresh garlic or dried garlic as garlic powder. Be aware that black garlic (not tested in this Review) has little allicin potential and has not been found to lower cholesterol, although it may have some benefit for chronic heart failure (see Forms of Garlic). -
What did ConsumerLab find in garlic supplements?
One widely sold garlic supplement appeared to contain little or no garlic, based on finding no detectable amounts of alliin, allicin, or SAC. In contrast, all others had significant amounts, ranging from roughly 5,000 to 40,000 mcg of total allicin and from 27 to 12,900 mcg of SAC per daily serving. Interestingly, some of the products that were not labeled as aged garlic had higher amounts of SAC than the aged garlic supplements. None of the supplements exceeded acceptable limits of contamination with lead, cadmium, or arsenic, and products claiming to be enteric-coated were found to live up to this claim. 
Which are the best garlic supplements?
ConsumerLab selected three garlic supplements as Top Picks based on their total allicin and/or SAC levels, overall quality, and value.-
Does garlic have side effects?
Other than bad breath (which is not a problem with aged garlic and some odor-controlled garlic), the most common side effect of garlic supplements is gastrointestinal upset. These and other side effects, as well as potential drug interactions, are discussed in the Concerns and Cautions section.
