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.