But sometimes, exposure to an allergen can cause a life-threatening allergic reaction known as anaphylaxis. This severe reaction happens when an over-release of chemicals puts the person into shock. Allergies to food, insect stings, medications and latex are most frequently associated with anaphylaxis. A second anaphylactic reaction, known as a biphasic reaction, can occur as long as 12 hours after the initial reaction. Call and get to the nearest emergency facility at the first sign of anaphylaxis, even if you have already administered epinephrine, the drug used to treat severe allergic reactions.