What Role Antagonists Play in the Study of Drugs?
An antagonist is a substance that blocks a particular pathway (a receptor). You can therefore test whether a particular drug acts through the opioid system by using an opioid antagonist. If your substance still works despite the presence of the antagonist (which should block the opoiod receptors), you can conclude that it is not working thorough the opioid system.

Conversely, if your drug is blocked by an opioid antagonist, that is good evidence that your drug works through the opioid system.