However, the responses I've heard to this question indicate to me that there is a crucial misunderstanding about what an allergy is. On several occasions I've heard people say they are allergic to a drug, and when asked what reaction occurred, they respond with nausea or something of the sort. Of course, nausea can be a symptom of an allergic reaction, particularly in anaphylaxis. Usually though, nausea is a side effect of a medication, which is separate from an allergy. Now, time for some science and jargon!
An allergic reaction (Type I Hypersensitivity for those of you so inclined) occurs when IgE, a type of immunoglobulin (antibody), interacts with an allergen. IgE bound to the allergen can then bind to receptors on mast cells, basophils, and eosinophils to stimulate an allergic reaction. Mast cells in particular release large quantities of histamine (along with cytokines and other chemicals), which causes vasodilation as part of the inflammatory response. When vasodilation of the blood vessels in the skin occurs, the affected area will turn redder in color (also know as rubor). Other inflammatory agents like leukotrienes, cytokines, and prostaglandins contribute to the inflammatory response, causing further vasodilation, pain, and broncoconstriction. Usually the allergic reaction is limited to the immediate area where the allergen made contact. Anaphylaxis is a severe, systemic allergic response, involving broncoconstriction and global vasodilation, which leads to difficulty breathing and shock. Anaphylaxis is a life-threatening emergency, but is treatable with prompt intervention. The allergic response to a drug may occur in several different ways (Type I, II, or III), and the signs and symptoms will vary depending on the mechanism and severity. Rash and itchiness are the most common presentations of a drug allergy.
The reason allergic reactions should not be confused with side effects is because of the outcome. An allergic reaction to a drug can be very dangerous, so it is important to avoid it by using alternative medications. Side effects are often unpleasant, but rarely immediately life-threatening. For instance, the flushing that occurs when taking niacin is not an immune hypersensitivity reaction, even though it appears as one. Opioids like hydrocodone or oxycodone may cause nausea, but not due to an allergic response. Additionally, almost all medications have side effects, so it is harder to pick and choose in that case. This is why healthcare workers ask for the reaction associated with an allergy; they are trying to distinguish a true allergy from an unpleasant side effect.
I have two points, one for healthcare professionals and one for the general public. First, if you are asking a patient about any drug allergies, be sure to ask for the type of reaction that occurs. It is reasonable that the average individual may not know the difference between a side affect and allergy, but the RN/MD/DO/PA/NP needs to know whether there are any contraindications to particular medications. Second, if you are telling a healthcare worker about an adverse drug reaction, be sure to mention what happens when you take it. Additionally, if you are unsure about whether or not you are allergic to a medication, be sure to ask (and write down the response for that matter).
Sources
1. Drug allergies. PubMed Health. 2010. http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001822/. Accessed June 21, 2012.
2. Levinson W. Chapter 65. Hypersensitivity (Allergy). In: Levinson W, ed. Review of Medical Microbiology and Immunology. 11th ed. New York: McGraw-Hill; 2010. http://www.accessmedicine.com/content.aspx?aID=6459867. Accessed June 21, 2012.
3. Papaliodis D, Boucher W, Kempuraj D, Michaelian M, Wolfberg A, House M, Theoharide TC. Niacin-induced "flush" involves release of prostaglandin D2 from mast cells and serotonin from platelets: Evidence from human cells in vitro and an animal model. J Pharmacol Exp Ther. 2008;327:665-672.
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