Final answer:
The first and most critical action in treating a patient with impending anaphylaxis due to a drug allergy is to administer epinephrine immediately. Epinephrine is the life-saving drug that acts quickly to counteract the severe allergic reactions associated with anaphylaxis.
Step-by-step explanation:
The first action in emergency treatment for a patient with impending anaphylaxis due to hypersensitivity to a drug is administering epinephrine. Anaphylaxis is a rapid, life-threatening systemic allergic reaction that can include symptoms such as a drop in blood pressure, difficulty breathing, and swelling of the tongue and throat. Epinephrine, the drug used to counteract the effects of anaphylactic shock, rapidly reverses these reactions. It constricts blood vessels to increase blood pressure, relaxes the lungs' smooth muscles to improve breathing, and modulates heart rate to prevent suffocation. Epinephrine autoinjectors, for example, EpiPens®, provide a means for patients to self-administer the medication in emergency situations, buying critical time to seek medical care.