If you or your vet has recently given your horse a medication or vaccine, it is important to monitor the horse for an adverse reaction afterwards. While adverse reactions are rare, they do occur.

Common reactions include a procaine penicillin reaction (when accidentally injected into the vein, a serious and often violent response quickly ensues), intracarotid injection (misdirected IV injection into the carotid artery, causing an almost immediate seizure-like response with the horse falling and flailing on the ground), allergic or anaphylactic reactions (resulting in swellings, bumps, hives, itchiness, and/or difficulty breathing), and local inflammatory reactions (swelling, redness) that can occur soon after injection or hours later.