adverse drug reaction, n a detrimental outcome from a drug. Two types of ADRs exist: Type 1 results from dosage mismatch and Type 2 from rare conditions often as a consequence of a small dose.See also risk or sensitive type.dverse drug reaction, n a detrimental outcome from a drug. Two types of ADRs exist: Type 1 results from dosage mismatch and Type 2 from rare conditions often as a consequence of a small dose.