"A response which is noxious and unintended and which occurs at doses normally used in humans for the prophylaxis, diagnosis or therapy of disease, or for the modification of physiological function." (WHO, 1972)

Karch and Lasagna defined ADR as “Any response to a drug that is noxious and unintended, and that occurs at doses used in humans for prophylaxis, diagnosis, or therapy, excluding failure to accomplish the intended purpose.”

"Any untoward medical occurrence that may present during treatment with a pharmaceutical product but which does not necessarily have a causal relationship with this treatment."(WHO)

Side effect:
Any unintended effect of a pharmaceutical product occurring at normal dosage which is related to the
pharmacological properties of the drug.

Unexpected adverse reaction:
An adverse reaction, the nature or severity of which is not consistent with domestic labelling or market authorization, or expected from characteristics of the drug.

Signal:
Reported information on a possible causal relationship between an adverse event and a drug, the relationship being unknown or incompletely documented previously. Usually more than a single report is required to generate a signal, depending upon the seriousness of the event and the quality of the information. The publication of a signal usually implies the need for some kind of review or action.

Serious unexpected adverse drug reaction(SUADR):
A serious adverse drug reaction that is not identified in nature, severity or frequency by the risk information provided in the clinical investigator's brochure (CIB) or on the drug label.