Garlic has it benefits but there is some debate on its effectiveness. Garlic can work as a food attractant, an immune system booster and has antibacterial, antifungal, antiviral and antiprotozoan properties to it. Allicin is the component in garlic that does most of the magic. Problem is that it is highly unstable. If forms when garlic is crushed and the enzyme Allinase comes in contact with the Allin and then forms Allicin. Being unstable, it quickly breaks down into other sulfur containing compounds. Most of the products you that advertise having garlic in them or garlic extract may or may not have Allicin in them. If they do, it will be limited. Research shows varying successes and failures using garlic. This is primarily due to the instability of Allicin. You can buy stabilized Allicin (Allimax Liquid) but it is expensive, at about $20.00 an ounce.
Adding a drop of garlic to the food won't hurt anything, but when I use it, I get the stabilized stuff. Dans' Feed Immune Boosting formula has it and Beta Glucan in it. My personal preference is to use live food that has been gut loaded but if live food is hard to get and expensive, then adding the garlic may be good.
Dan