A drug-allergy is an adverse reaction resulting from an immunologic response following a previous sensitizing exposure to the drug. The reaction can range from a rash or hives to anaphylactic shock. Tolerance to a drug means that the patient experiences a decreasing physiologic response to repeated administration of the drug in the same dosage. Idiosyncrasy is an individual’s unique hypersensitivity to a drug, food, or other substance; it appears to be genetically determined. Synergism, is a drug interaction in which the sum of the drug’s combined effects is greater than that of their separate effects.