What is an Adverse Reaction?

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  • Written By: G. Wiesen
  • Edited By: Heather Bailey
  • Last Modified Date: 15 October 2019
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An adverse reaction — sometimes also called an adverse drug reaction, adverse drug event, adverse event, or adverse effect — is a reaction between a person and a drug or other type of medication that is typically unintended and negative. Unlike a side effect which can be unintended but potentially positive, an adverse reaction is almost always negative and can have a number of different causes and levels of severity. In many areas, thorough testing of a new drug or pharmaceutical product is required by government agencies to ensure the safety of the product and to attempt to discover any adverse effects before a product is made widely available to the public.

There are several different major categories in which an adverse reaction can fall, including dose-dependent and predictable reactions, unpredictable or idiosyncratic reactions, and allergic reactions. Dose-dependent reactions are typically predictable and these types of reactions can usually be recreated in an experimental or laboratory environment with reliable results. As research and understanding of pharmacology has improved over the years, these types of reactions have become more common since fewer reactions are left unexplained. Understanding of these types of reactions is especially important since it can better ensure fewer accidental cases of an adverse reaction to a drug.


Idiosyncratic reactions, however, are not dose-dependent and are typically unpredictable. This type of adverse reaction may not be able to be easily replicated in experiments or studies. These are often allergic reactions in which the particular process by which a body reacts negatively to the drug is not fully understood, or where no clear commonality between subjects with such an adverse reaction can be established. Idiosyncratic reactions can be the most problematic for a pharmaceutical company since they can be the most difficult to fully understand and may lead to inadvertent illness or fatality.

The level of severity of an adverse reaction can change greatly and depends upon the drug and how it interacts with the body. A number of different scales are used by different organizations, with different language and specifics regarding each level of severity. Typically, the word “serious” is used to describe a potentially fatal or life-threatening reaction that can lead to hospitalization or disability, though “lethal” is sometimes used instead. “Severe” is often used as a modified term to indicate a sliding scale such as “low severity” or “high severity” depending on how unpleasant or painful a reaction may be.

In the United States, the Food and Drug Administration typically oversees the release of drugs and medication to ensure the safety of such products. The European Medicines Agency represents the European Union in investigating any potential reactions involving medicine. On a global scale, the World Health Organization monitors various pharmaceutical products for potential adverse effects.


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