An adverse event is a negative reaction caused by taking a drug, more commonly known as a side effect. This can include any unexpected or unwanted symptoms experienced by the patient, including temporary reactions and those previously not associated with that drug.

A serious adverse event is a reaction that causes life-threatening harm to the individual, resulting in hospitalisation, significant disability or incapacitation, congenital abnormality, or at worst, death. Anything that requires intervention to prevent these is also classified as serious. In any case, adverse event reporting is compulsory and critical to the success of PV.