Dexamethasone is a prescription medication that is to be used only under the supervision of a veterinarian. It is a potent steroid (anti-inflammatory) that can be given intravenously, intramuscularly and orally. Your vet may dispense this medication to you to have on hand in case of hives or a seeming allergic reaction.
Dexamethasone is used inappropriately in the horse show world for its supposed sedative properties and anti-inflammatory, pain relieving effects. Like most medications, this one can be detected even at low levels by drug testing.
Dexamethasone can be given IV, IM and the solution can even be given orally (it is absorbed quite well from the GI tract).
In an emergency case of hives or wheals in which you cannot contact your vet, you could give 1cc per 100 lbs (of the 2mg/ml strength) by either IM or oral route. Continue trying to contact your vet even after administration to let them know what has taken place.