A rash appears three days after starting a new antibiotic. That is an adverse reaction, a harmful response to a medication taken at a normal dose for a legitimate purpose. The definition matters because it separates a genuine drug effect from an accidental overdose, a prescribing error, or a coincidental illness.

Reactions come in several flavors. Some are predictable and dose-related, like the drowsiness that follows a sedative. Others are unpredictable and immune-mediated, like anaphylaxis to penicillin. Still others are idiosyncratic, meaning they occur in a small number of patients for reasons that are not well understood. The World Health Organization divides them into six types, labeled A through F, a scheme clinicians use to reason about cause and prevention.

Common categories include:

• Type A: augmented, dose-dependent, predictable from the drug's known action
• Type B: bizarre, unpredictable, often immune-related
• Type C: chronic, related to long-term use
• Type D: delayed, appearing years after exposure

Reporting a suspected reaction to a national authority helps regulators spot trends. Most reactions resolve when the drug is stopped, though some leave lasting damage.