What distinguishes an agonist from an opioid antagonist?

An agonist is a substance that binds to a receptor and stimulates a biological response, effectively activating the receptor's function. In the case of opioids, agonists mimetically engage the opioid receptors in the brain and throughout the body, producing effects such as pain relief, euphoria, and sedation. This stimulation enhances the activity of the receptor, resulting in the expected physiological responses.

On the other hand, an opioid antagonist binds to the same receptors but does not activate them. Instead, it blocks the action of agonists, preventing other substances (like natural opioids or opioid medications) from having their effect. This blocking action is critical in situations such as opioid overdose, where an antagonist can reverse the depressant effects of opioids on the central nervous system.

The other options do not accurately reflect the definitions or actions of agonists and antagonists. The first option incorrectly states that an agonist blocks action, which is the role of an antagonist. The statement that indicates both have the same effect undermines the distinct roles of these substances, while the last option regarding drug absorption doesn't relate to the definitions of agonists or antagonists in pharmacology.