Yes tylan is over the counter, yes it is injected in livestock this is commercial farming what commercial farming as it's best , tylan is use to treat and also prevent RI that are due to overcrowding and poor conditions in which livestock is being kept, not to mention the effects of those antibiotics as well as the fact that those animals are not meant to live long anyway, (but that’s another debate and as nothing to do with pets)

Recommending using any antibiotics whether prescribed or not is not the way to go, pumping an animal with antibiotics just because they are cheap, over the counter or both is not gonna help if this is not the right antibiotic which that can only be assessed by a culture.

Antibiotics are not a one fit all, resistance and complications can also arise.