What is the difference between a side effect and an adverse drug reaction?

A side effect is any unintended pharmacological effect of a drug at normal therapeutic doses, which may be beneficial, neutral, or harmful. For example, sedation from diphenhydramine is a side effect that is harmful when treating allergies but beneficial when used as a sleep aid. An adverse drug reaction, or ADR, is specifically a harmful and unintended response to a medication. ADRs range from mild, like nausea, to life-threatening, like anaphylaxis or Stevens-Johnson syndrome. Type A reactions are dose-dependent and predictable from pharmacology.