In the medical sphere, an adverse effect or reaction is a harmful outcome that results from medication or another form of medical intervention. There exists some confusion within the medical community as to the proper use of the term. Healthcare professionals sometimes use adverse drug reaction, adverse effect, and adverse event interchangeably. However, each of these terms refers to similar, but different results. Additionally, some physicians may describe an adverse effect as a side effect. Following strict definitions, this is inaccurate.
An adverse effect differs from an adverse drug reaction because adverse effects can result from any form of medical intervention, whereas an adverse drug reaction occurs exclusively because of a drug. Adverse events are very similar to adverse effects. The latter directly cause harmful outcomes while adverse events may not have a causal relationship to treatment -- the harmful outcome merely appeared during treatment.
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