Adverse Effect or Reaction:

An abnormal, unexpected, infrequent and usually unpredictable injurious response to a drug. Used in this restrictive sense, the term adverse reaction does NOT include effects of a drug which are normally a part of its pharmacological action, even though such effects may be undesirable and unintended. Adverse reactions are of three basic types: those due to drug allergy, those caused by individual idiosyncrasy and those representing toxic effects of drugs on tissue structure and function.

--From The Essential Guide to Perscription Drugs