An adverse drug reaction (ADR) occurs when a prescription or nonprescription medicine is administered in the proper dose and method, according to standard practice, but the patient nevertheless, suffers an unexpected and unintended serious or significant reaction, injury, toxicity, or sensitivity.

The history of ADRs is over 2000 years old, and dates back to 400 BC when Hippocrates counseled physicians to "first, do no harm."

The way your body reacts to a medication may be different than the way someone else’s does. For example, your age, gender, or medical conditions may influence your ADR risk. Similarly, any other prescription, non-prescription or herbal medicines you take might increase your risk for an ADR. Even the way your body processes medications may influence your chances for an ADR. What’s most important to know is that there are usually multiple factors that contribute to your likelihood of experiencing an undesirable and potentially dangerous reaction to a medicine.