Sometimes people seek a way to determined whether a specific clinical event is likely to reflect an underlying adverse drug reaction. This can be important for a variety of reasons, primarily to prevent inadvertent re-exposure of a patient to a drug treatment that has the potential to cause significant harm. Although there is no substitute for a careful clinical history and targeted investigations to elucidate a potential adverse drug reaction, at time a structured approach can be useful. Published nearly 40 years ago, the Naranjo Algorythm is a structured instrument that when applied carefully generates a numerical score that is used to assign a probability rating for the likelihood that a specific clinical event reflects an underlying adverse drug reaction. The outcome is that the probability rating can be assigned as doubtful, possible, probably, or in some very rare cases, certain. Information about the Naranjo Algorythm can be accessed  at