Geeze, how you guys getting this drug, your Vet? It's abig time, big money horse tranq. I use rompin for the price or Acepromizine for the light weight stuff.
I would use antibiotics, just because it sound like you made more than one stab at the vein. If you used the same needle it could have pick up dirt or what ever going through the skin. and may have been introduced into the blood stream. Better safe than sorry. I like SMZ tablets as an antibiotic for horses, they are fairly cheap and easy to give. I put the tablets, seems like about 10 to 15 a dose depending on the horse wieght, in a 20 to 50ml syringe add water to the sryinge and it becomes a paste in no time have to shake it a few times. Then you give it orally, just like a wormer. I have tried mixing in grain but most of my horses won't eat it very well.
Alan