An 18 is much larger than a 20, but necessary for some things
such as Penicillin. That being said, if you are giving PNC, take it out of
the refrigerator, draw it up in the syringe, and warm it in your hands for a few
minutes. It will hurt the horse less and be easier to inject. (I
assume your are giving a med to a horse?)
If you have a choice of giving the med with a 20 or 18, definitely go with
the 20. Trust me on this one.
