An adverse reaction is a harmful or unpleasant effect from a medicine used at a normal dose. Regulators distinguish it from an overdose effect and from a treatment failure. Official labeling contains an Adverse Reactions section listing effects reported during studies and after marketing.
A listed adverse reaction is not a prediction. It means the effect has been reported by someone, somewhere, in association with the medicine. Some listed effects are common and mild, others are rare and serious, and the label does not always separate the two clearly.