Final answer:
Epinephrine is the first-choice drug for treating anaphylactic reactions due to its rapid action in raising blood pressure, improving breathing, modulating heart rate, and reducing swelling.
Step-by-step explanation:
The drug of choice for an anaphylactic reaction is epinephrine. Anaphylaxis is a severe and potentially life-threatening allergic reaction that requires immediate medical treatment. The primary action of epinephrine is to raise blood pressure, relax bronchial smooth muscle to improve breathing, modulate heart rate, and reduce swelling to prevent airway blockage. While antihistamines and oral corticosteroids have roles in the follow-up treatment to prevent potential late-phase reactions, they are not considered first-line treatments for anaphylactic shock. Epinephrine is administered as an emergency measure to counteract the dangerous effects of the reaction, and it is available in the form of autoinjectors like EpiPen® for immediate self-administration in the event of exposure to an allergen.