Adverse Events: ADRs, MedWatch, FAERS and VAERS

Types of Drug-Induced Harm: ADEs, ADRs, and Hypersensitivity

In pharmacy practice, patient safety requires understanding the differences between various negative drug responses. An adverse drug event is defined broadly as any injury or harm resulting from a medical intervention related to a drug. This broad category includes harm resulting from medication errors and overdoses. By contrast, an adverse drug reaction is a harmful and unintended response to a medication that occurs specifically at normal therapeutic doses. Because of these specific definitions, all adverse drug reactions are classified as adverse drug events, but not all adverse drug events are adverse drug reactions. Harm from an accidental overdose is an adverse drug event (ADE) outside the usual therapeutic-dose definition of an adverse drug reaction (ADR); harm after a wrong-drug dispensing error is also an ADE and may include an ADR if the drug causes a harmful, unintended response at a normal dose.

Other drug-induced effects are distinguished by their predictability and mechanisms. A side effect is an unintended effect of a drug occurring at a therapeutic dose; unlike many severe reactions, a side effect is often predictable from the known actions of the drug and may be dose-related. An idiosyncratic drug reaction is an unusual, unexpected response that may reflect an individual's unique genetic susceptibility rather than a typical dose-related effect. Drug toxicity refers to a harmful effect of a medication that is often caused by an excessive dose or drug accumulation within the body, although toxicity can also occur at normal therapeutic concentrations. Additionally, pharmacy personnel must recognize teratogenicity, which is the potential of a medication taken during pregnancy to cause developmental abnormalities in an embryo or a fetus.

Immune system involvement represents another critical distinction in patient safety. An allergic reaction is an immune-mediated hypersensitivity response to a medication. Common clinical signs of allergic drug reactions include urticaria, pruritus, angioedema, wheezing, and hypotension. Anaphylaxis is an acute, potentially life-threatening systemic hypersensitivity reaction requiring immediate emergency treatment; other drug hypersensitivity reactions can also be severe or fatal. Whenever a patient presents with sudden systemic signs such as wheezing or severe swelling, it must be recognized as an emergency.