An antagonistic drug interaction is best described as what?

• A. Increases the effect of the other
• B. Decreases the effect of the other
• C. No effect
• D. Only additive effects

Answer: (B)

An antagonistic drug interaction occurs when the presence of one drug reduces the effect of another. This means the opposing action dampens the overall response, so the combined effect is smaller than what you’d expect from the drugs acting individually. Mechanisms can be pharmacodynamic (one drug blocks or counters the effect at its target, like a receptor antagonist preventing an agonist from stimulating the receptor) or pharmacokinetic (one drug changes the levels of another by altering absorption, metabolism, or distribution). A classic example is naloxone reversing opioid effects by blocking opioid receptors.

This isn’t about increasing the effect—that would be potentiation or synergy. It isn’t about no effect; there is a real interaction that alters outcomes. And it isn’t about only additive effects, where the total equals the sum of each drug’s effect without interference.