MSD Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Overview of Adverse Drug Reactions

(Drug Side Effects; Drug Adverse Effects)


Daphne E. Smith Marsh

, PharmD, BC-ADM, CDCES, University of Illinois at Chicago College of Pharmacy

Full review/revision Apr 2021 | Modified Sep 2022
Topic Resources

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

In the early 1900s, German scientist Paul Ehrlich described an ideal drug as a "magic bullet." Such a drug would be aimed precisely at a disease site and would not harm healthy tissues. Although many new drugs are aimed more accurately than their predecessors, none of them, as of yet, hit the target exclusively.

Most drugs produce several effects, but usually only one effect—the therapeutic effect—is wanted for the treatment of a disorder. 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 practitioners, 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 drug is stopped or the dose is changed. Some gradually subside as the body adjusts to the drug. Other adverse drug reactions are more serious and last longer. About 3 to 7% of all hospital admissions in the United States are for treatment of adverse drug reactions. 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 not certain, they clearly represent a significant public health problem that is often preventable.

Common adverse drug reactions

Digestive disturbances—loss of appetite, nausea, a bloating sensation, constipation, and diarrhea—are particularly common adverse drug reactions, because most drugs are taken by mouth and pass through the digestive tract. However, almost any organ system can be affected. In older people (see Aging and Drugs Aging and Medications Medications, the most common medical intervention, are an important part of medical care for older people. Without medications, many older people would function less well or die at an earlier... read more 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 Food and Drug Administration (FDA) for approval. Other adverse drug reactions, typically those that are uncommon, are not detected until the drug 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.


More Information

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

quiz link

Test your knowledge

Take a Quiz!