See what clinical studies, cold & flu studies say about Garlic
All of the beneficial components of garlic can be found in fresh garlic, which makes aged garlic supplements and fresh garlic the two best ways to supplement garlic. Garlic should be crushed, sliced, or chewed (prior to cooking) in order to ensure maximum allicin
Source – https://examine.com/supplements/garlic/
Cold & Flu Studies
The rate (frequency of occurring) of the common cold has twice been found to be reduced by 60-70% in persons who take garlic supplementation daily; this is associated with both allicin and the aged garlic extract, and requires higher doses (2.5g aged extract or 180mg allicin).
Supplementation of garlic powder (180mg allicin) over the course of twelve weeks noted that the supplementation group recovered from sickness faster (1.52 days of sickness relative to 5.02 in placebo) and experienced sickness at a lower rate (111 cumulative sick days, relative to 366 in placebo), suggesting immunity from the common cold by reducing both length and occurrence by 70%.
Source – https://examine.com/supplements/garlic/#hem-rate-of-sickness