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, including medications, see the table Symptoms and Treatment of Specific Poisons.
Side effect is an imprecise term often used to refer to the unintended effects of a medication that occur within the therapeutic range.
Because all drugs have the potential for adverse drug reactions, conducting a risk-benefit analysis (the likelihood of benefit vs risk of ADRs) is necessary whenever a medication is prescribed.
The National Electronic Injury Surveillance System–Cooperative Adverse Drug Event Surveillance Project (NEISS–CADES) estimated that there were 6 emergency department (ED) visits for medication harms per 1000 persons per year in the period from 2017 to 2019 based on a sample of almost 100,000 cases (1). About 39% of these visits resulted in hospitalization. In previous estimates in the United States, 3 to 7% of all hospitalizations were due to adverse drug reactions. ADRs occurred during 10 to 20% of hospitalizations; about 10 to 20% of those ADRs were severe. These statistics do not include the number of ADRs that occur in other 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.
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. According to the National Electronic Injury Surveillance system, in older adults, therapeutic use of anticoagulants and diabetes medications led to ED visits most often. Nontherapeutic use of sedative and hypnotic drugs such as benzodiazepines and analgesics also contributed to drug-related harm. For children under 5 years of age, antibiotic use was a frequent cause of ED visits due to medication-related harm.
ADRs are more severe among older patients (see Drug-Related Problems in Older Adults), although age per se may not be the primary cause. Fatal adverse drug reactions occur mainly in patients older than 75 years of age, according to the World Health Organization’s pharmacovigilance database.The contribution of prescription errors and poor adherence to the incidence of ADRs is unclear.