Adverse Drug Reactions

Types:

• Toxic at normal dose:
  Intolerance (including unwanted pharmacological effects) - a drug produces side effects at the usual dosage that would normally be only expected to occur in overdosage  (may be related to the presence of enzyme defects in some patients)
  Idiosyncrasy - the adverse reaction is unusual and pharmacologically unexpected (may be related to the presence of enzyme defects in some patients)

• Toxic at overdosage:
  Deliberate or accidental

• Drug / drug Interactions:
  e.g. one drug impairs the excretion or breakdown of another causing the unexpected and unwanted effect of overdosage.  For example patients on an anticoagulant who start taking an additional drug and the anticoagulant effect is increased causing the risk of bleeding 

• Drug / food Interactions:
  e.g. a particular kind of antidepressant (MAO Inhibitor) inhibits an enzyme (MAO) and if food containing certain compounds (amines) are eaten with it a dangerous false allergy reaction is induced

• Disease / drug Interactions:
  e.g. ampicillin (antibiotic) if given to patients with glandular fever will commonly cause a rash
  e.g. certain drugs given to patients with certain infections will occasionally cause a serious kind of rash called Stevens-Johnson Syndrome

• Allergy:  
  The reaction is caused by an immune system response (= a true drug allergy).  True drug allergy is important even though it probably only accounts for around 10% of all adverse drug reactions.