The use of intramuscular injections of Procaine penicillin G (bensylpenicillin Procaine) in the horse is now and then associated with acute adverse reactions (penicillin-chock). The etiology is not yet clearly understood, but the theories are several. One possibility is that it can be caused by Procaine toxicity. Procaine penicillin G is a salt which is quickly dissolved in plasma. Procaine is then metabolized by plasmaesterases to non-toxic metabolites, p-aminobenzoic acid (PABA) and diethylaminoethanol.

Penicillin G (benzylpenicillin) is the most commonly used antimicrobial substance used in equine medicine today. Two forms of penicillin G are available for equine use; one form is the poorly soluble penicillin G Procaine and the other is the very soluble sodium salt. Although penicillin has a wide safety margin and is considered to possess a good tolerance, reactions due to side effects are sometimes encountered, which commonly is named ?penicillin-shock?. The etiology behind penicillin shock in horses is not completely understood, but the most common reason is believed to be Procaine toxicity.

In veterinary medicine, penicillin is the most used antibiotic in Sweden. Two dosage forms of penicillin are available in Sweden for use in horses, sodium benzylpenicillin (Na-pc) and Procaine benzylpenicillin (proc-pc). Proc-pc is the most used dosage form and is used for intramuscular administration which allows horse owners to treat an animal at home under veterinary instruction. Na-pc is only approved for intravenous use in horses. Penicillin is normally well tolerated by horses but a serious, sometimes life-threatening, adverse reaction called penicillin shock may occur.