In pharmacology, antagonism occurs when?

• A. The effect of one drug is canceled out or reduced by another drug
• B. There is no interaction
• C. Two drugs produce a new effect
• D. The combined effect is greater than each alone

Answer: (A)

Antagonism is when one drug cancels out or reduces the effect of another. This usually happens when a drug blocks the receptor the other drug would act on or otherwise opposes its action, so the net effect is diminished or prevented.

That’s why the statement that the effect of one drug is canceled out or reduced by another best describes antagonism. It’s different from no interaction (which would mean the drugs don’t affect each other), from two drugs producing a new effect (that’s a synergistic or additive interaction), or from the combined effect being greater than each alone (that’s synergy or potentiation). A classic example is a blocking agent like naloxone restoring normal function by preventing opioid receptors from being activated.