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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Adverse Effects

Some medical texts describe adverse effects as “iatrogenic” because they originate from the physician or the treatment. This can occur for a variety of reasons. In some cases, an adverse effect may only occur when a physician starts, increases, or ends a treatment. The risk for adverse effects rises if a physician uses a treatment option that is contraindicated, which could present a reason for the physician to withhold the treatment. Adverse effects may manifest as morbidity, mortality, or changes in body weight, function levels, or enzyme levels.

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Adverse Drug Reactions

When the adverse effect is the result of a medication or drug, it is an adverse drug reaction (ADR). There are six types of ADR with appropriate mnemonics: dose-related, non-dose-related, dose-related and time-related, time-related, withdrawal, and failure of therapy. The mnemonics are, in respective order, augmented, bizarre, chronic, delayed, end of use, and failure, or types A, B, C, D, E, and F. When attempting to discover the causality of a reaction, physicians may refer to the timing, pattern of illness, and test results to ensure the medication was the cause.

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Adverse Events

Many healthcare professionals use the terms adverse events and adverse effects interchangeably. However, there is a notable difference between the two terms, particularly in a clinical trial. Adverse events refer to an unexpected and unfavorable outcome that did not necessarily result from the treatment option -- in other words, it is a wider categorization that can include adverse effects. In clinical trials, researchers must report all adverse events to the study sponsor and possibly an ethics committee. Additionally, researchers distinguish between an adverse event and a serious adverse event. Any event that results in death, irreversible damage, hospitalization, or birth defects is a serious adverse event.

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Side Effect vs. Adverse Effect

Some medical experts describe adverse effects as a type of side effect. However, this is not entirely accurate. Side effects are entirely dose-independent, while adverse effects have a variety of classifications, including dose-related. Side effects may resolve on their own after treatment ends, while some adverse effects are permanent. In some cases, a physician may intentionally use a drug for its possible side effects to treat an unrelated condition. Medical experts never desire an adverse effect.

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Safety, Tolerability, Serious, and Severe

Four terms describe the qualities of a treatment option and its relation to adverse effects. The safety of a treatment is its potential to cause adverse effects. Tolerability describes the possible unpleasant side effects of drugs, such as tiredness, dry mouth, or headaches. If an adverse effect is serious, it has major medical consequences. Typically, this refers to death, disability, or long-term hospitalization. A severe quality refers to the intensity of an adverse effect. A non-serious effect such as a headache may have a severe intensity but will not result in permanent damage.

What are Adverse Effects?



For safety reasons, many countries have laws and regulations that require physicians to report adverse effects. In the United States, several reporting systems allow healthcare professionals to report their findings. The most prevalent is the Food and Drug Administration’s MedWatch. Though regulations require physicians to report all adverse effects, studies show that experts tend to underreport because medical professionals use judgment to determine causality. For example, a broken leg is likely not a result of a medication prescription from several years prior, and an expert would not report it. In addition, some effects are complex and determining the cause may be incredibly difficult.

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Diagnosis and Determining Causality

Physicians can easily link adverse effects to drugs they prescribed recently because the symptoms typically appear soon after. Diagnosing symptoms that develop during long-term medication use is significantly more difficult. In some events, the medical professional will stop the use of a drug to determine if it is responsible. Adverse effects that result from treatment options such as surgery typically manifest soon after the procedure. A patient could develop an infection or show signs of inflammation, scarring, or loss of function. This clear causality allows professionals to link the events easily.

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Treatments for Adverse Effects

There are several methods of treatment for adverse drug reactions. A physician may modify the dosage or discontinue a drug to see if the reaction clears up. They may also transfer the patient to a similar medication. Treating other adverse effects can be complex. For example, if a person develops untreatable gangrene in a limb following surgery, surgeons must make a difficult choice. They can amputate the limb and potentially save the patient’s life, or they can take a risk and use antibiotics to try and treat the condition without an operation. In some instances, the adverse effect may be irreversible and untreatable.

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Over the years, there have been many controversies around physicians and researchers underreporting or misreporting adverse effects. A major, recent controversy developed when one researcher claimed vaccines lead to autism. However, the ingredient he claimed as the cause has not been present in vaccines in around a decade and autism rates have not fallen. Recent studies show no link between vaccines and autism. Another such controversy occurred due to misinformation about silicone breast implants. Many people believe that the implants can cause immune system damage, though studies cannot conclusively confirm this.

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