opioid antagonist

An opioid antagonist is a drug that does the opposite of an opioid: it blocks the opioid receptor without switching it on. Naloxone, the best-known one, is the antidote pulled out in an overdose. Picture a key that slides into the lock and fills it but will not turn — and in doing so it shoulders out any opioid that was turning that lock, instantly reversing the effect.

These drugs are competitive antagonists with high affinity for the mu opioid receptor. They bind tightly but have no intrinsic activity, so they trigger no signal themselves; instead they occupy the receptor and displace the opioid drug from it. Given to someone whose breathing has stopped from an opioid overdose, naloxone can restore normal breathing within a minute or two, which is why it is now distributed widely as a nasal spray to bystanders and first responders.

The same blockade has two important catches. First, naloxone is short-acting — it can wear off before a long-acting opioid like methadone has cleared, so the overdose can return and repeat doses or monitoring are needed. Second, in someone who is physically dependent, abruptly stripping every receptor of its opioid throws them into sudden, severe withdrawal. A longer-acting relative, naltrexone, is used differently: not for rescue but to maintain a persistent blockade that helps people in recovery resist relapse.