When making choices wrt antibiotic selection, one needs to consider risk vs. benefit.
In most cases, there isn't a significant advantage to procaine penicillin G over either oral trimethoprim/sulfa or other injectable antibiotics ( ex. ceftiofur) that are less likely to result in an adverse reaction than is PPG, which is why its use has diminished greatly.
All my research says no more than 10mL injected per site...MAX. Dosing instructions are 1mL per 100lbs. of horse for Penicillin procaine. That would be 12 mL for a 1200 lb. horse per day, split am. and pm. is 6 mL per injection site per day. 50 mL seems a bit radical to me but I'm not a vet. Proper injection is critical on any drug, no matter what. If a horse has lost a lot of blood then its blood pressure is low and veins will be flatter. Lots of vets have gone thru the neck vein into the carotid artery and killed horses with a dose of dormosedan or any sedative "cocktail". Accidents happen. With IM injection of antibiotics, you don't want blood in the aspiration...in sedatives you have to have the correct blood if you are shooting IV.
Originally Posted by twadwis
1) this is a very old thread that has been answered by several
Originally Posted by nvhorseman
and 2) one of the answers was provided by a vet.