According to WHO, an adverse event is “any untoward medical occurrence associated with the use of a drug in humans whether or not considered drug related”.

An adverse event is also referred to as an adverse experience and can be any unfavorable and unintended sign, symptom, or disease temporarily associated with the use of a drug.

The term “adverse event” should be distinguished from the term “side effect”. The former may occur at any dose, while the latter means “unintended effect occurring at normal dose”.

Serious adverse event is an unwanted reaction that results in death or a significant disability and requires inpatient hospitalization or prolongation of existing one.

Other adverse event is an adverse event that is not serious, it is not life-threatening and it does not cause any significant incapacities.