“On-target,” “off-target,” and cytotoxic ADRs, with the exception of overdose, are related to the use of therapeutic dosages of drugs and are predicated on the same pharmacokinetic and pharmacodynamic pathways as the therapeutic effects of drugs. Prerequisites to considering ADRs in the differential diagnosis of a patient's signs and symptoms include awareness that an ever increasing number of patients are taking more and more medications (polypharmacy) and familiarity with relevant literature about ADRs.