What you see. The starting point for addressing any equine health related issue is your observation.


Strange Behavior after Medication Given


You have given your horse a shot or oral dose of medication (an anti-inflammatory, antibiotic or vaccine), and you now notice that they are acting strangely.

Maybe the horse is suddenly excited, anxious, or agitated. Maybe they are tossing their head, sweating, showing signs of abdominal pain (colic), or showing other odd behavior.

Problems are more common following injection than oral medication. In rare cases, horses can have seizures or excitable behavior following injection. Adverse reactions to medications can occur very quickly after administration or can take place hours later, and they can take many forms.

- Local pain and swelling can develop after an injection.

- Anaphylaxis is a body-wide allergic reaction to a medication. It usually takes minutes to hours to develop. It might appear as depression, loss of appetite, hives, rapid breathing, diarrhea or colic.

- Procaine (penicillin) reaction is a common reaction to the carrier in injectable penicillin. This is seizure like, excitable behavior within minutes of an injection of Procaine Penicillin G.

- Intra-carotid injection (inadvertent injection into the carotid artery) usually occurs following attempted intravenous injection. This is immediate and results in seizure like activity, often violent falling and flailing. It usually lasts 1-2 minutes but full recovery can take an hour.

  • Code Red

    Call Your Vet Immediately, Even Outside Business Hours
    • If the behavior is persistent and the horse seems to be distressed.
    • If the results of the Whole Horse Exam (WHE) in the resting horse indicate fever (Temp >101F/38.3C) or heart rate greater than 48 BPM.
  • Code Orange

    Call Your Vet at Their First Available Office Hours
    • If this seems mild or occasional and the horse seems normal otherwise.

your role


What To Do

If it is safe to do so, perform the Whole Horse Exam. Given these possibilities, it is wise for you to call your vet immediately. Tell your vet about the type of medication given, the method of delivery, the amount given, and location of where it was given. Provide your vet with detailed information about why you gave your horse the medicine in the first place and describe the abnormal observations you are making. If it is possible, take a video or photos.

What Not To Do

Do not delay. If you gave your horse medication in hopes of not having to call your vet, and you are now faced with a compounded problem, it is time to call your vet.

your vet's role

Your vet may or may not need to see your horse, but it is best to talk to them as soon as possible so you can make that important decision together.
Questions Your Vet Might Ask:
  • What medication did you give, how much, and by what route?
  • How long ago did you give the medication?
  • Describe what the horse is doing now?
  • What is the horse's age, sex, breed and history?
  • What condition were you treating?
  • What are the results of the Whole Horse Exam (WHE)?

Treatments Your Vet May Recommend

A way to resolve the condition or diagnosis. Resolving the underlying cause or treating the signs of disease (symptomatic treatment)

Very Common
more treatments

Author: Doug Thal DVM Dipl. ABVP