This document discusses adverse drug reactions and drug interactions. It begins by defining adverse drug reactions as unwanted effects caused by normal drug doses. Reactions are classified as Type A, which are common and dose-related, or Type B, which are unpredictable and idiosyncratic. Drug interactions can occur through pharmaceutical, pharmacodynamic, or pharmacokinetic mechanisms. While some interactions are useful to increase effects or minimize side effects, others can be harmful and even severe, causing issues like hypertensive crisis or hemorrhage. Identifying the culprit drug can be difficult, requiring a careful history, provocation testing, or stopping all medications one by one. Caution is important as multiple drug use commonly occurs.