Adverse effect
Harmful outcomes from medical treatments or interventions.
An adverse effect is an unwanted and harmful outcome caused by a medication or other medical intervention like surgery. When it occurs as a secondary result alongside the intended therapeutic effect, it may be called a side effect. This is different from a medical error, which happens when the wrong dose or procedure is used. Adverse effects are sometimes described as iatrogenic, meaning they originate from a physician or treatment. They can worsen a disease or procedure’s outlook, create complications, or cause a patient to stop following their treatment plan. Globally, adverse effects from medical care led to 142,000 deaths in 2013, compared to 94,000 in 1990.
The harmful result may show up as illness, death, changes in body weight or enzyme levels, loss of function, or a pathological change seen at the microscopic, macroscopic, or physiological level. It can also be reported by the patient as symptoms. These effects may be reversible or irreversible, and they can alter a person’s sensitivity to other chemicals, foods, or procedures—such as through drug interactions.
In drug-related contexts, an adverse event is defined as any unwanted medical occurrence in a patient or clinical trial subject given a pharmaceutical product, even if it is not clearly caused by the treatment. Clinical trials distinguish between an adverse event and a serious adverse event. A serious adverse event generally includes events that cause death, permanent damage, birth defects, or require hospitalization. The term "life-threatening" in this context means the patient was actually at risk of death at the time of the event, not that it might have caused death if it were worse. Trial results are often included in medication labeling for patients and prescribers.
Many countries legally require adverse effects to be reported, studied in clinical trials, and included in patient information for drugs and medical devices. Investigators in human trials must report these events in study reports, though research shows they are often poorly reported in public documents. Because of missing data and unclear methods for combining it, systematic reviews and meta-analyses of treatments may unknowingly overemphasize benefits. Scholars have called for more complete reporting of harm from clinical trials.
In the United Kingdom, the Yellow Card Scheme, run by the Medicines and Healthcare products R
- field
- Pharmacology and medicine
- known_for
- Undesired harmful effects from medical interventions
- global_deaths_2013
- 142000
- global_deaths_1990
- 94000
- classification
- Adverse event vs. serious adverse event
- reporting_systems
- Yellow Card Scheme (UK), VAERS (US), ADRAC (Australia), CARM (New Zealand), HPFB (Canada)
Lore & Background
Adverse effects are defined as undesired harmful results from medications or other interventions like surgery. They may be called side effects when secondary to the main effect, and are distinct from medical errors, which stem from incorrect dosage or procedure. The term 'iatrogenic' is used because the effect is generated by a physician or treatment. Some adverse effects occur only when starting, increasing, or discontinuing a treatment. Using a contraindicated drug or intervention may increase risk. The harmful outcome can be indicated by morbidity, mortality, weight change, enzyme levels, loss of function, or pathological changes, and may be reversible or irreversible.
Reader's Guide
Adverse effects are a central concern in pharmacology and clinical practice, as they can undermine treatment benefits and cause significant harm. The article notes that adverse effects of medical treatment caused 142,000 deaths globally in 2013, up from 94,000 in 1990, highlighting their growing impact. Reporting systems exist in many countries—such as the UK's Yellow Card Scheme, the US's MedWatch, Australia's ADRAC, New Zealand's CARM, and Canada's HPFB—to collect data on these events. However, reporting is often incomplete; investigators in clinical trials are obligated to report adverse events, but research suggests they are inadequately reported in publicly available documents. This lack of data can lead systematic reviews to overemphasize benefit. The distinction between an adverse event and a serious adverse event (e.g., causing death, permanent damage, or requiring hospitalization) is critical in clinical trials. Understanding adverse effects helps clinicians weigh risks and benefits, and informs patient consent and drug labeling.
Did You Know?
- Adverse effects are sometimes called iatrogenic because they are generated by a physician or treatment.
- The term 'life-threatening' in a serious adverse event refers to an event where the patient was at risk of death at the time, not a hypothetical worse scenario.
- In the United States, MedWatch is the main reporting center for adverse effects, operated by the Food and Drug Administration.
- Adverse effects may cause a reversible or irreversible change, including an increase or decrease in susceptibility to other chemicals, foods, or procedures.
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