In pharmacology, antagonism is defined as?

• A. The effect of one drug is amplified by another
• B. The effect of one drug is canceled out or reduced by another drug
• C. Two drugs produce a new effect
• D. Drugs have no interaction

Answer: (B)

Antagonism means one drug blocks or reduces the effect of another. This is a type of interaction where the presence of the second drug diminishes, cancels, or prevents the first drug from producing its full effect. This can happen in a couple of ways: a receptor antagonist can bind to the same receptor as an agonist and block its action, or a different mechanism can counteract the first drug’s effect so the net response is smaller. For example, an antagonist that blocks a receptor will prevent the usual response to an agonist, or a drug that counters a physiologic change can lessen the overall effect. In short, the defining idea is that the combined action is decreased or nullified compared with using the drugs separately.

The other ideas describe different interactions: one describes potentiation, where one drug enhances the effect of another; another describes a combined effect that is new or greater than either drug alone; and the last describes no interaction at all.