An antagonist is a drug designed to directly oppose the actions of an agonist. Again, using the lock and key analogy, an antagonist is like a key that fits nicely into the lock but doesn’t have the right shape to turn the lock.
When this key (antagonist) is inserted in the lock, the proper key (agonist) can’t go into the same lock. So the actions of the agonist are blocked by the presence of the antagonist in the receptor molecule.
Again, let’s think of morphine as an agonist for the opioid receptor.
If someone is experiencing a potentially lethal morphine overdose, the opioid receptor antagonist naloxone can reverse the effects. This is because naloxone (marketed as Narcan) quickly occupies all the opioid receptors in the body and prevents morphine from binding to and activating them. Morphine bounces in and out of the receptor in seconds. When it’s not bound to the receptor, the antagonist can get in and block it. Because the receptor can’t be activated once an antagonist is occupying the receptor, there is no reaction. The effects of Narcan can be dramatic. Even if the overdose victim is unconscious or near death, they can become fully conscious and alert within seconds of injection.
The effects of Narcan can be dramatic. Even if the overdose victim is unconscious or near death, they can become fully conscious and alert within seconds of injection.