Anaphylaxis is a severe, life-threatening allergic reaction that affects a number of different body systems. Anaphylaxis is most often caused by food allergy especially peanuts and tree nuts.
A true drug allergy is caused by the immune system identifying the medication as foreign and responding as such. Most reactions occur to medications to which the person has already been exposed and within hours to weeks of exposure. The most severe form of immediate reaction is anaphylaxis which may present with hives, facial or throat swelling, wheezing, vomiting and shock.