What is an Adverse Event?

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  • Written By: Elle Jay
  • Edited By: Daniel Lindley
  • Last Modified Date: 13 November 2019
  • Copyright Protected:
    Conjecture Corporation
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The term adverse event describes an unexpected, usually negative physical reaction in direct response to a drug or other medical product. Also called a serious adverse event, adverse effect, or adverse drug reaction, these occurrences are usually discovered during a clinical trial and can vary in severity. The medical and pharmaceutical communities work to prevent adverse drug reactions through research, testing, and information sharing. Negative drug reactions are monitored by medical regulatory agencies throughout the world.

Adverse events are sometimes compared to side effects, but serious adverse events may be life threatening and frequently require immediate medical attention. Health problems stemming from an adverse event can vary from mild to serious to fatal, and reactions are rated on a sliding scale. Episodes classified as low severity include mild effects that do not need emergency care, such as nausea or headache. Medium and high severity cases can lead to hospitalization, permanent damage, long-term disability, birth defects, or death.

Multiple factors can lead to an adverse event, and the causes are categorized to make research easier. Dose-dependent events describe a reaction associated with the typical dosage for a particular drug or with a change in a patient’s dose. Interactions with other medications or herbal supplements can also cause drug-related adverse effects.


Researchers may classify an adverse event as predictable or unpredictable. Predictable events are generally not surprising and can be easily recreated in a lab setting to further test the outcome. Unpredictable or idiosyncratic events are far from typical and are more difficult to mimic in the lab. It can be difficult to determine the cause of this type of reaction.

Pharmacovigilance is a science created for and dedicated to finding out what causes an adverse event and preventing it from happening on a broad public scale. During a clinical trial, researchers, doctors, and patients provide valuable information that helps decipher the cause of an adverse effect. Good clinical practice means sharing data to develop solutions and prevent future negative reactions, including short-term and long-term side effects.

Medical regulatory agencies monitor clinical testing and closely track data regarding adverse events, especially for pharmaceuticals. Organizations such as the United Stated Food and Drug Administration and the European Medicines Agency require pharmaceutical companies to conduct in-depth trials before approving a new drug to prevent widespread adverse reactions. These official groups will also ban a drug, medical product, or procedure from the market if it is determined to be dangerous and likely to have an adverse effect on the public.



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