opi·oid
ˈō-pē-ˌȯid 
plural opioids
: a natural, semisynthetic, or synthetic substance that typically binds to the same cell receptors as opium and produces similar narcotic effects (such as sedation, pain relief, slowed breathing, and euphoria):
a
or less commonly opioid peptide
: any of various of endogenous polypeptides (such as an endorphin or enkephalin) produced by neurons of the peripheral and central nervous system
The people who rated pain the lowest were those whose brains began producing natural painkillers called opioids the fastest …—
John O'Neil
John O'Neil
b
: any of various opiates (such as morphine), semisynthetic opiate derivatives (such as heroin, hydrocodone, or oxycodone), or synthetic preparations (such as fentanyl or methadone) that may be used illicitly for their narcotic properties and are associated with physiological tolerance (see tolerance sense 4a(1)), physical and psychological dependence, or addiction upon repeated or prolonged use
… fentanyl, a synthetic opioid at least 75 times more potent than morphine.—Ryan Trimble and Eric S. Peterson
Ryan Trimble and Eric S. PetersonNaltrexone is an opioid antagonist used for the treatment of heroin addiction.—Scientific American
Scientific AmericanThe prototypical opioids are morphine and codeine (which is milder than morphine).—Harvard Health Letter
Harvard Health LetterWhen Walker County was identified as the epicenter of Alabama's opioid crisis, it was no surprise to residents who watched as drugs brought death and devastation down upon their families, neighbors and communities.—Ashley Remkus
Ashley RemkusNote: The word opioid was originally used only for morphine-like substances not derived from opium, but it has now become widely accepted as a broader term encompassing any substance—natural or synthetic, opium-derived or not—that binds to opiate cell receptors and induces sedation, analgesia, and euphoria.
: possessing narcotic properties characteristic of opiates : of, relating to, involving, or being an opioid
opioid drugs
opioid addiction
endogenous opioid endorphins
opioid cell receptors
Examples of opioid in a Sentence
Recent Examples on the Web
Avalon Hester, PEOPLE, 18 Sep. 2025
This is the final part of a three-part series about the rise of the gas-station opioid 7-OH and its ties to the Kansas City company CBD American Shaman.
—Kansas City Star, 18 Sep. 2025
John Ruwitch, NPR, 19 Sep. 2025
Getty Images That upward trajectory stands in contrast to the United States, where life expectancy virtually flatlined during the 2010s and even dipped slightly on occasions during the 2010s—a trend blamed in large part on a surge in deaths from overdoses from the synthetic opioid fentanyl.
—Micah McCartney, MSNBC Newsweek, 18 Sep. 2025
See All Example Sentences for opioid
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Noun
The actress, who has spoken openly about her addictions to alcohol and opioids, gave Klitschko full custody of Kaya in 2018.
—Avalon Hester, PEOPLE, 18 Sep. 2025
This is the final part of a three-part series about the rise of the gas-station opioid 7-OH and its ties to the Kansas City company CBD American Shaman.
—Kansas City Star, 18 Sep. 2025
Adjective
The fate of TikTok became increasingly wrapped up in sweeping trade talks between Washington and Beijing that include strategically thorny topics like tariffs, microchips, rare earth minerals and China's export of chemicals used to make the synthetic opioid fentanyl.
—John Ruwitch, NPR, 19 Sep. 2025
Getty Images That upward trajectory stands in contrast to the United States, where life expectancy virtually flatlined during the 2010s and even dipped slightly on occasions during the 2010s—a trend blamed in large part on a surge in deaths from overdoses from the synthetic opioid fentanyl.
—Micah McCartney, MSNBC Newsweek, 18 Sep. 2025
See All Example Sentences for opioid
Cite this Entry
“Opioid.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/opioid. Accessed 20 Sep. 2025.
Copy CitationKids Definition
opioid
noun
opi·oid
ˈō-pē-ˌȯid
: any substance that binds to the same cell receptors as opium, produces similar effects (as sleepiness and pain relief), includes drugs used medically especially as painkillers, is often used illicitly for its narcotic properties, and often causes physical and psychological dependence
Medical Definition
opioid
1 of 2 noun
: a natural, semisynthetic, or synthetic substance that typically binds to the same cell receptors as opium and produces similar narcotic effects (such as sedation, pain relief, slowed breathing, and euphoria):
a
also opioid peptide
: any of various of endogenous polypeptides (such as an endorphin or enkephalin) produced by neurons of the peripheral and central nervous system
b
: any of various opiates (such as morphine), semisynthetic opiate derivatives (such as heroin, hydrocodone, or oxycodone), or synthetic preparations (such as fentanyl or methadone) that may be used illicitly for their narcotic properties and are associated with physiological tolerance (see tolerance sense 1), physical and psychological dependence, or addiction upon repeated or prolonged use
… fentanyl, a synthetic opioid at least 75 times more potent than morphine.—Ryan Trimble and Eric S. Peterson
Ryan Trimble and Eric S. PetersonThe prototypical opioids are morphine and codeine (which is milder than morphine).—Harvard Health Letter
Harvard Health LetterNote: The word opioid was originally used only for morphine-like substances not derived from opium, but it has now become widely accepted as a broader term encompassing any substance—natural or synthetic, opium-derived or not—that binds to opiate cell receptors and induces sedation, analgesia, and euphoria.
opioid
2 of 2 adjective
opi·oid
: possessing narcotic properties characteristic of opiates : of, relating to, involving, or being an opioid
opioid drugs
opioid addiction
endogenous opioid endorphins
opioid cell receptors
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