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.