Overview of Adverse Drug Reactions

(Drug Side Effects; Drug Adverse Effects)

ByDaphne E. Smith Marsh, PharmD, BC-ADM, CDCES, University of Illinois at Chicago College of Pharmacy
Reviewed/Revised Jan 2025
VIEW PROFESSIONAL VERSION

Adverse drug reactions (adverse effects) are any unwanted effects of a drug or medication.

Medications are prescribed to provide the therapeutic effect (the intended treatment of a particular disorder), but most can produce other effects as well. The other effects may be regarded as unwanted, whether they are intrinsically harmful or not. For example, certain antihistamines cause drowsiness as well as control the symptoms of allergies. When an over-the-counter sleep aid containing an antihistamine is taken, drowsiness is considered a therapeutic effect. But when an antihistamine is taken to control allergy symptoms during the daytime, drowsiness is considered an annoying, unwanted effect.

Most people, including health care professionals, refer to unwanted effects as side effects; another term used is adverse drug effect. However, the term adverse drug reaction is technically more appropriate for drug effects that are unwanted, unpleasant, noxious, or potentially harmful.

Prevalence of adverse drug reactions

Not surprisingly, adverse drug reactions are common. Most adverse drug reactions are relatively mild, and many disappear when the medication is stopped or the dose is changed. Some gradually subside as the body adjusts to the medication.

Other adverse drug reactions are more serious and last longer. In the United States, a national system for reporting adverse effects of medications, the National Electronic Injury Surveillance System–Cooperative Adverse Drug Event Surveillance Project (NEISS–CADES), estimated that there were 6 emergency department visits for medication harms per 1000 persons per year from 2017 to 2019. Approximately 39% of these visits resulted in hospitalization. Approximately 3 to 6% of hospital admissions in the United States and 2.5 to 10.6% of admissions in Europe are due to adverse drug reactions. Additionally, an estimated 134 million adverse drug events occur annually in low- and middle-income countries due to unsafe care in hospitals, resulting in 2.6 million deaths.

Adverse drug reactions occur during 10 to 20% of hospital admissions, and about 10 to 20% of these reactions are severe. These statistics do not include the number of adverse drug reactions that occur in people who live in nursing homes and other care facilities.

Although the exact number of adverse drug reactions is uncertain, they clearly represent a significant public health problem that is often preventable.

According to the World Health Organization, fatal adverse drug reactions occur mainly in people older than 75 years of age. Prescription errors and inability or failure to take correctly prescribed medications as indicated both contribute to the incidence of ADRs.

Did You Know...

  • In the United States, 3 to 6% of all hospitalizations are for treatment of adverse drug reactions.

Common adverse drug reactions

Digestive disturbances—loss of appetite, nausea, bloating, constipation, and diarrhea—are particularly common adverse drug reactions, because most drugs and medications are taken by mouth and pass through the digestive tract. However, almost any organ system can be affected. In older adults (see Aging and Medications), the brain is commonly affected, often resulting in drowsiness and confusion.

Identification and reporting of adverse drug reactions

Many adverse drug reactions are identified when a drug is being tested before it is submitted to the U.S. Food and Drug Administration (FDA) for approval. Other adverse drug reactions, typically those that are uncommon, are not detected until the medication has been on the market long enough to be used by a large number of people. Thus, health care providers are required to report suspected adverse drug reactions to the FDA.

Table
Table

More Information

The following English-language resources may be useful. Please note that The Manual is not responsible for the content of these resources.

  1. America's Poison Centers: Access to information on a variety of poisons, an emergency helpline (1-800-222-1222), and tips on prevention.

  2. FDA Adverse Event Reporting System (FAERS): Access to Questions and Answers on FDA's Adverse Event Reporting System (FAERS).

quizzes_lightbulb_red
Test your KnowledgeTake a Quiz!
Download the free Merck Manual App iOS ANDROID
Download the free Merck Manual App iOS ANDROID
Download the free Merck Manual App iOS ANDROID