Anaphylactic (Anaphylactoid) Reaction

A group of symptoms which represent (or resemble) a sometimes overwhelming and dangerous allergic reaction due to extreme hypersensitivity to a drug. Anaphylactic reactions, whether mild, moderate, or severe, often involve several body systems. Mild symptoms consist of itching, hives, nasal congestion, nausea, abdominal cramping and/or diarrhea. Sometimes these precede more severe symptoms such as choking, shortness of breath, and sudden loss of consciousness (usually referred to as anaphylactic shock).

Characteristic features of anaphylactic reaction must be kept in mind. It can result from a very small dose of drug; it can be rapidly progressive and can lead to fatal collapse in a short time if not reversed by appropriate treatment. A developing anaphylactic reaction is a true medical emergency. Any adverse effect that appears withing 20 minutes after taking a drug should be considered the early manifestation of a possible anaphylactic reaction. Obtain medical attention immediately.

--From The Essential Guide to Prescription Drugs