Just like people, horses can develop allergic reactions to penicillin . This requires previous exposure to the drug, and then is manifested by hives and head swelling, and occasionally, massive constriction of the airways and sudden death. Penicillin is very commonly administered to horses in a formulation known as penicillin procaine G . It helps to give the formulation a long-acting effect, so that it can be administered only twice a day. When correctly injected into the horse's muscle, it causes no problem.
Most horses begin to tremble violently and throw themselves over backwards.
There is no antidote when this occurs and an affected horse will recover in just a few minutes as long as it doesn't damage its skull or spine. Obviously, this reaction can be very dangerous for bystanders. This is why when injecting penicillin into a horse's neck or hindquarters, you should always place the needle first, and watch for blood to fill the hub of the needle. If you see blood, remove the needle and place a clean needle in another site. When you attach the syringe full of penicillin, pull back and again watch for blood to appear in the syringe.