Good morning,
We don't give anything that has to go in a vein. We call the vet for that. Also, bear in mind that some drugs can cause an extreme reaction in some horses so you don't want to be giving that unless you know the antidote and have it on hand.
When I give a shot, I always make sure I know these things. I think it was antibiotics, but a girl that was in our Roundup club, kill a horse by putting it in the veing when it shouldn't have been in the vein. Dropped the horse before she even could pull it out. Was a nice horse too:(
Becky