Horse Side Vet Guide ®

Equine Health Resource

Reaction after Penicillin IM Injection

Code Red - Call Your Vet Immediately, Even Outside Business Hours

Code Red - Call Your Vet Immediately, Even Outside Business Hours

    You gave your horse a shot of intramuscular procaine penicillin G. Moments later, the horse began acting abnormally.

    Procaine penicillin is commonly given to horses intramuscularly “IM” with no adverse effects. If even a small amount is accidentally administered into a vessel, however, a violent reaction may occur.

    A horse having a “procaine reaction” usually appears hyper-alert or spooked. This progresses into a seizure-like fit, with the horse scrambling and sometimes falling down. Horses may spin around or lurch backwards with their eyes wide, and nostrils flared, sneezing, or blowing.

    This reaction lasts 2-5 minutes, but it seems like a lot longer. Horses generally quiet down after 5-10 minutes. By the time the vet arrives, the reaction is usually over. This reaction is not allergic in nature, but is caused by the procaine additive. It is more likely to occur after multiple injections in the same site because of inflammation and increased blood supply to the area.


    If you notice this happening, leave the horse loose in a stall. Get out of the stall. When it happens in our presence (yes, it can rarely happen to a vet too), we may use sedatives to try to slow the horse down until the reaction passes.

    If there is not a closed, small, safe space in which to release the horse, a confident handler may be able to manage a horse through the fit by circling the horse around them. When in doubt get out of the stall and call your vet.


    Your vet will assess the horse for injuries sustained during the reaction. If the horse is still having a reaction they may sedate the horse or give anti-seizure drugs. They may discuss alternatives to the use of penicillin in the future, depending upon the condition being treated.

    What Not To Do

    Do not try to restrain or handle your horse if you are not confident in doing so, you could easily be injured. It is best not to release the horse in a large area because in some cases, the horse may gallop and injure itself severely. Do not tie the horse fast.

    Author: Doug Thal DVM Dipl. ABVP


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