Not to sure about pills. I use pure garlic cloves, I have tried the Jedds garlic juice the birds liked it also. but it has a strong oder to it, to strong. the smell of garlic doesn't bother me however after using the juice! the smell would not go away after a few days in the loft it didn't smell to fresh! so I went back to the using garlic cloves! to me its easier to give it to flock threw the water instead of giving each bird a tablet or pill. unless you only have a few birds.
There are several articles written by Dr. Gordon Chalmers on the web re: pigeon feeding & etc.
One in particular deals with garlic use for pigeons. One of the points made is that any type of processing of the garlic is bound to affect its quality, and that fresh garlic is best to use.
I crush a couple of cloves and put them is a water container. Let it sit over night and put it in the birds waterer the next day. The chemicals that are in the fresh garlic are a stimulant to the birds immune system.
I use about 4 half tooths of fresh cloves per 1 gallon of water peel the cloves then with a knife or fork poke lots of holes in the clove to where it's still intact and not falling apart. then placed them in the water I leave them in the water for about 2 days then replace with pure water. I do it once a week good for the birds
I have used fresh garlic and garlic juice form Jedds. Both are fine when I use the fresh garlic I use a garlic press and squeeze 1 or 2 cloves to 1 gallon of distilled water and add the pulp also. When I use the juice I use 2 tsp per gallon. I also add apple cider vinigar 1 or 2 tsp to the mix .GEORGE