I am not sure why this is called "medicated" as citric acid (also the main ingredient in tonic water) is not an antiparasitic or antibiotic but might be used to preserve the freshness of the pellets or the real "medicine" that was added.
It's used as a preservative, as far as I know. It's also used in chicken feed to push the uptake of calcium, which improves egg shell quality and is said to improve development of broilers.
look at the label a little more, medicated -"usually" means a coccidiostat has been added [and the amount they add is useless]
citric acid is listed as GRAS [generally regarded as safe] and would / should not be listed as a "medication"