An adverse drug reaction (ADR) is a harmful, unintended result caused by taking medication.

Adverse drug reactions can also be categorized using time-relatedness, dose-relatedness, and susceptibility, which collectively are called the DoTS classification.

[12] The terms "severe" and "serious", when applied to adverse events, are technically very different.

One example of this is multiple medications administered concurrently that prolong the QT interval, such as antiarrhythmics like sotalol and some macrolide antibiotics, such as systemic azithromycin.

[29] Some of the medications that can contribute to serotonin toxicity include MAO inhibitors, SSRIs, and tricyclic antidepressants.

[3] Assigning causality to a specific agent often proves difficult, unless the event is found during a clinical study or large databases are used.

[35] In the United States, the Food and Drug Administration (FDA) is responsible for monitoring post-marketing studies.

[36] Healthcare professionals, consumers, and the pharmaceutical industry can all submit information to this system.

[40] A study by the Agency for Healthcare Research and Quality (AHRQ) found that in 2011, sedatives and hypnotics were a leading source for adverse drug events seen in the hospital setting.

[41] A second study by AHRQ found that in 2011, the most common specifically identified causes of adverse drug events that originated during hospital stays in the U.S. were steroids, antibiotics, opiates/narcotics, and anticoagulants.

[42] Medication related harm (MRH) is common after hospital discharge in older adults, but methodological inconsistencies between studies and a paucity of data on risk factors limits clear understanding of the epidemiology.

Nearly 8 in 1,000 adults aged 65 years or older experienced one of the four most common ADEs (steroids, antibiotics, opiates/narcotics, and anticoagulants) during hospitalization.

[44] In 2012, McKinsey & Company concluded that the cost of the 50-100 million preventable error-related adverse drug events would be between US$18–115 billion.

[45] An article published in The Journal of the American Medical Association (JAMA) in 2016 reported adverse drug event statistics from emergency departments around the United States in 2013-2014.

[46] This article reported that 57.1% of these adverse drug events presented to the ED were in females.

[46] As well, out of all of the adverse drug events presented to the emergency department documented in this article, 17.6% were from anticoagulants, 16.1% were from antibiotics, and 13.3% from diabetic agents.