What is the term for a drug that can reverse the effect of another drug?

• A. Agonist
• B. Synergist
• C. Barbiturate
• D. Antagonist

Answer: (D)

In pharmacology, the term for a drug that reverses the effect of another drug is an antagonist. An antagonist binds to the same receptor as the other drug but does not activate it in the same way, so it blocks or reduces the other drug’s response. This can happen by competing directly for the receptor (competitive antagonism) or by binding in a way that cannot be overcome by increasing the other drug (non-competitive antagonism). A classic real-world example is naloxone, which reverses opioid effects by occupying opioid receptors. The other terms don’t fit because an agonist activates receptors to produce a response, a synergist enhances the combined effect of two drugs, and a barbiturate is a drug class, not the mechanism of reversing another drug’s action.