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Adverse Drug Reactions

(Adverse Drug Effects)


Daphne E. Smith Marsh

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

Last full review/revision Apr 2021| Content last modified Apr 2021

Adverse drug reaction (ADR, or adverse drug effect) is a broad term referring to unwanted, uncomfortable, or dangerous effects that a drug may have.

Adverse drug reactions can be considered a form of toxicity; however, toxicity is most commonly applied to effects of overingestion (accidental or intentional) or to elevated blood levels or enhanced drug effects that occur during appropriate use (eg, when drug metabolism is temporarily inhibited by a disorder or another drug). For information on toxicity of specific drugs see the table Symptoms and Treatment of Specific Poisons. Side effect is an imprecise term often used to refer to a drug’s unintended effects that occur within the therapeutic range.

Because all drugs have the potential for adverse drug reactions, risk-benefit analysis (analyzing the likelihood of benefit vs risk of ADRs) is necessary whenever a drug is prescribed.

In the US, 3 to 7% of all hospitalizations are due to adverse drug reactions. ADRs occur during 10 to 20% of hospitalizations; about 10 to 20% of these ADRs are severe. These statistics do not include the number of ADRs that occur in ambulatory and nursing home patients. Although the exact number of ADRs is not certain, ADRs represent a significant public health problem that is, for the most part, preventable (1, 2).

Incidence and severity of adverse drug reactions vary by patient characteristics (eg, age, sex, ethnicity, coexisting disorders, genetic or geographic factors) and by drug factors (eg, type of drug, administration route, treatment duration, dosage, bioavailability). Incidence is higher with advanced age and polypharmacy. ADRs are more severe among older patients (see Drug-Related Problems in Older Adults), although age per se may not be the primary cause. The contribution of prescribing and adherence errors to the incidence of ADRs is unclear.

Pearls & Pitfalls

  • Adverse drug reactions occur in 10 to 20% of hospitalizations.
  • About 10 to 20% of these reactions are severe.

General references

  • 1. Weiss AJ, Freeman WJ, Heslin KC, et al: Adverse drug events in U.S. hospitals, 2010 versus 2014. Agency for Healthcare Research and Quality. Statistical Brief #234. January 2018. Accessed 3/31/21.
  • 2. PSNet (Patient Safety Network), Agency for Healthcare Research and Quality: Medication Errors and Adverse Drug Events. Accessed 3/31/21.

Etiology of Adverse Drug Reactions

Most adverse drug reactions are dose-related; others are allergic or idiosyncratic. Dose-related ADRs are usually predictable; ADRs unrelated to dose are usually unpredictable.

Dose-related ADRs are particularly a concern when drugs have a narrow therapeutic index (eg, hemorrhage with oral anticoagulants). ADRs may result from decreased drug clearance in patients with impaired renal or hepatic function or from drug-drug interactions.

Allergic ADRs are not dose-related and require prior exposure. Allergies develop when a drug acts as an antigen or allergen. After a patient is sensitized, subsequent exposure to the drug produces one of several different types of allergic reaction. Clinical history and appropriate skin tests can sometimes help predict allergic ADRs.

Idiosyncratic ADRs are unexpected ADRs that are not dose-related or allergic. They occur in a small percentage of patients given a drug. Idiosyncrasy is an imprecise term that has been defined as a genetically determined abnormal response to a drug, but not all idiosyncratic reactions have a pharmacogenetic cause. The term may become obsolete as specific mechanisms of ADRs become known.

Symptoms and Signs of Adverse Drug Reactions

Adverse drug reactions are usually classified as mild, moderate, severe, or lethal (see table Classification of Adverse Drug Reactions [ADRs]). Severe or lethal ADRs may be specifically mentioned in black box warnings in the physician prescribing information provided by the manufacturer.

Symptoms and signs may manifest soon after the first dose or only after chronic use. They may obviously result from drug use or be too subtle to identify as drug-related. In older adults, subtle ADRs can cause functional deterioration, changes in mental status, failure to thrive, loss of appetite, confusion, and depression.

Classification of Adverse Drug Reactions (ADRs)





No antidote or treatment is required; hospitalization is not prolonged.

ACE inhibitor: Cough

Antidepressants: Dry mouth

Antihistamines (some): Drowsiness


A change in treatment (eg, modified dosage, addition of a drug), but not necessarily discontinuation of the drug, is required; hospitalization may be prolonged, or specific treatment may be required.

Hormonal contraceptives: Venous thrombosis

NSAIDs: Hypertension and edema

Opioids: Constipation


An ADR is potentially life threatening and requires discontinuation of the drug and specific treatment of the ADR.

ACE inhibitors: Angioedema

Macrolide antibiotics: Abnormal heart rhythm


An ADR directly or indirectly contributes to a patient’s death.

Acetaminophen overdosage: Liver failure

Anticoagulants: Hemorrhage

Allergic ADRs typically occur soon after a drug is taken but generally do not occur after the first dose; typically, they occur when the drug is given after an initial exposure. Symptoms include itching, rash, fixed-drug eruption, upper or lower airway edema with difficulty breathing, and hypotension.

Idiosyncratic ADRs can produce almost any symptom or sign and usually cannot be predicted.

Diagnosis of Adverse Drug Reactions

  • Consideration of rechallenge
  • Reporting of suspected ADRs to MedWatch

Symptoms that occur soon after a drug is taken are often easily connected with use of a drug. However, diagnosing symptoms due to chronic drug use requires a significant level of suspicion and is often complicated. Stopping a drug is sometimes necessary but is difficult if the drug is essential and does not have an acceptable substitute. When proof of the relationship between drug and symptoms is important, rechallenge should be considered, except in the case of serious allergic reactions.

Physicians should report most suspected adverse drug reactions to MedWatch (the Food and Drug Administration's [FDA’s] ADR monitoring program), which is an early alert system. Only through such reporting can unexpected ADRs be identified and investigated. MedWatch also monitors changes in the nature and frequency of ADRs. Online reporting of ADRs is encouraged. Forms for and information about reporting ADRs are available in the Physicians’ Desk Reference and the FDA News Daily Drug Bulletin, as well as at (MedWatch: The FDA Safety Information and Adverse Event Reporting Program); forms may also be obtained by calling 800-FDA-1088. Nurses, pharmacists, and other health care practitioners should also report ADRs. The FDA's Adverse Event Reporting System (FAERS) is a search tool that improves access to data about adverse drug reactions (1).

The incidence of severe or fatal adverse drug reactions is very low (typically <1 in 1000) and may not be apparent during clinical trials, which are typically not powered to detect low-incidence ADRs. Thus, these ADRs may not be detected until after a drug is released to the general public and is in widespread use. Clinicians should not assume that because a drug is on the market that all ADRs are known. Postmarketing surveillance is extremely important for tracking low-incidence ADRs.

Diagnosis reference

  • 1. FDA Adverse Event Reporting System (FAERS): Questions and Answers on FDA's Adverse Event Reporting System (FAERS). Accessed 3/31/21.

Treatment of Adverse Drug Reactions

  • Modification of dosage
  • Discontinuation of drug if necessary
  • Switching to a different drug

For dose-related adverse drug reactions, modifying the dose or eliminating or reducing precipitating factors may suffice. Increasing the rate of drug elimination is rarely necessary. For allergic and idiosyncratic ADRs, the drug usually should be discontinued and not tried again. Switching to a different drug class is often required for allergic ADRs and sometimes required for dose-related ADRs. For example, opioid-induced constipation may be improved with the use of an opioid receptor antagonist such as lubiprostone.

Prevention of Adverse Drug Reactions

Prevention of adverse drug reactions (ADRs) requires familiarity with the drug and potential reactions to it. Computer-based analysis should be used to check for potential drug interactions; analysis should be repeated whenever drugs are changed or added. Drugs and initial dosage must be carefully selected for older adults (see Reasons for Drug-Related Problems). If patients develop nonspecific symptoms, ADRs should always be considered before beginning symptomatic treatment. Various genes have been identified as having an association with ADRs. For example, multiple liver enzymes that affect liver metabolism of cytochrome P450 have been characterized, and many are affected by single nucleotide polymorphisms, leading to clinically meaningful effects on a wide range of commonly prescribed drugs. Therefore, pharmacogenomics may help to predict, reduce, and minimize ADRs (1, 2). However, only a limited number of such tests are used in routine clinical practice (eg, genotype-guided warfarin therapy [3]).

Prevention references

  • 1. Zhou Z-W, Chen X-W, Sneed KB, et al: Clinical association between pharmacogenomics and adverse drug reactions. Drugs 75:589-631, 2015. doi: 10.1007/s40265-015-0375-0
  • 2. Gerogianni K, Tsezou A, Dimas K: Drug-induced skin adverse reactions: The role of pharmacogenomics in their prevention. Mol Diagn Ther 22(3): 297-314, 2018. doi: 10.1007/s40291-018-0330-3
  • 3. Bardolia C, Matos A, Michaud V, et al: Utilizing pharmacogenomics to educe adverse drug events. Am J Biomed Sci & Res 11(3). doi: 10.34297/AJBSR.2020.11.00163

Drugs Mentioned In This Article

Drug Name Select Trade
Acetaminophen TYLENOL
lubiprostone AMITIZA
warfarin COUMADIN

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