Which term describes an agent that counteracts an unwanted drug effect?

• A. Reversal
• B. Antidote
• C. Countermeasure
• D. Antagonist

Answer: (B)

An antidote is the agent used to counteract a harmful or unwanted drug effect. It works by neutralizing the toxic action, blocking the offending mechanism, or reversing the physiological consequences so normal function is restored. For example, naloxone acts as an antidote to opioid overdose by binding to opioid receptors and reversing respiratory depression and sedation. Protamine reverses heparin’s anticoagulant effect, and vitamin K can reverse warfarin’s action. The term is preferred here because it specifically describes a substance that neutralizes or reverses the adverse effects of a drug or toxin, whereas other terms are more general or describe the mechanism rather than a dedicated counteracting agent.