An adverse drug reaction (ADR) is a harmful, unintended result caused by taking medication. ADRs may occur following a single dose or prolonged administration of a drug or result from the combination of two or more drugs. The meaning of this term differs from the term "side effect" because side effects can be beneficial as well as detrimental. The study of ADRs is the concern of the field known as pharmacovigilance. An adverse drug event (ADE) refers to any unexpected and inappropriate occurrence at the time a drug is used, whether or not associated with the administration of the drug. An ADR is a special type of ADE in which a causative relationship can be shown. ADRs are only one type of medication-related harm, as harm can also be caused by omitting to take indicated medications. Classification
ADRs may be classified by e.g. cause and severity. Cause
Type A: Augmented pharmacologic effects - dose dependent and predictableType A reactions, which constitute approximately 80% of adverse drug reactions, are usually a consequence of the drugs primary pharmacological effect (e.g. bleeding when using the anticoagulant warfarin) or a low therapeutic index of the drug (e.g. nausea from digoxin), and they are therefore predictable. They are dose-related and usually mild, although they may be serious or even fatal (e.g. intracranial bleeding from warfarin). Such reactions are usually due to inappropriate dosage, especially when drug elimination is impaired.
The term side effects is often applied to minor type A reactions.Type B: IdiosyncraticTypes A and B were proposed in the 1970s, and the other types were proposed subsequently when the first two proved insufficient to classify ADRs. Seriousness
The U.S Food and Drug Administration defines a serious adverse event as one when the patient outcome is one of the following:
Death
Life-threatening
Hospitalization (initial or prolonged)
Disability - significant, persistent, or permanent change, impairment, damage or disruption in the patients body function/structure, physical activities or quality of life. Congenital abnormality
Requires intervention to prevent permanent impairment or damageSeverity is a point on an arbitrary scale of intensity of the adverse event in question. The terms "severe" and "serious", when applied to adverse events, are technically very different. They are easily confused but can not be used interchangeably, requiring care in usage. A headache is severe if it causes intense pain. There are scales like "visual analog scale" that help clinicians assess the severity. On the other hand, a headache is not usually serious (but may be in case of subarachnoid haemorrhage, subdural bleed, even a migraine may temporally fit criteria), unless it also satisfies the criteria for seriousness listed above. Location
Adverse effects may be local, i.e. limited to a certain location, or systemic, where medication has caused adverse effects throughout the systemic circulation. For instance, some ocular antihypertensives cause systemic effects, although they are administered locally as eye drops, since a fraction escapes to the systemic circulation.