Opioids are a class of drugs that derive from, or mimic, natural substances found in the opium poppy plant. Opioids work on opioid receptors in the brain and other organs to produce a variety of morphine-like effects, including pain relief.There are a number of broad classes of opioids:[267]

Natural opiates: alkaloids contained in the resin of the opium poppy, primarily morphine, codeine, and thebaine, but not papaverine and noscapine which have a different mechanism of action
Esters of morphine opiates: slightly chemically altered but more natural than the semi-synthetics, as most are morphine prodrugs, diacetylmorphine (morphine diacetate; heroin), nicomorphine (morphine dinicotinate), dipropanoylmorphine (morphine dipropionate), acetylpropionylmorphine, dibenzoylmorphine, diacetyldihydromorphine;[268][269]
Semi-synthetic opioids: created from either the natural opiates or morphine esters, such as hydromorphone, hydrocodone, oxycodone, oxymorphone, ethylmorphine, buprenorphine and desomorphine;
Fully synthetic opioids: such as fentanyl, pethidine, levorphanol, methadone, tramadol, tapentadol, and dextropropoxyphene;
Endogenous opioid peptides, produced naturally in the body, such as endorphins, enkephalins, dynorphins, and endomorphins.
Endogenous opioids, non-peptide: Morphine, and some other opioids, which are produced in small amounts in the body, are included in this category.
Natural opioids, non-animal, non-opiate: the leaves from Mitragyna speciosa (kratom) contain a few naturally occurring opioids, active via Mu- and Delta receptors. Salvinorin A, found naturally in the Salvia divinorum plant, is a kappa-opioid receptor agonist.
Tramadol and tapentadol, which act as monoamine uptake inhibitors also act as mild and potent agonists (respectively) of the μ-opioid receptor. Both drugs produce analgesia even when naloxone, an opioid antagonist, is administered.Opium alkaloids and derivatives
Opium alkaloids
Phenanthrenes naturally occurring in (opium):

Codeine
Morphine
Thebaine
Oripavine[276]
Preparations of mixed opium alkaloids, including papaveretum, are still occasionally used.

Esters of morphine
Diacetylmorphine (morphine diacetate; heroin)
Nicomorphine (morphine dinicotinate)
Dipropanoylmorphine (morphine dipropionate)
Diacetyldihydromorphine
Acetylpropionylmorphine
Desomorphine
Methyldesorphine
Dibenzoylmorphine
Ethers of morphine
Dihydrocodeine
Ethylmorphine
Heterocodeine
Semi-synthetic alkaloid derivatives
Buprenorphine
Etorphine
Hydrocodone
Hydromorphone
Oxycodone (sold as OxyContin)
Oxymorphone
Synthetic opioids
Anilidopiperidines
Fentanyl (see also list of fentanyl analogues)
Alphamethylfentanyl
Alfentanil
Sufentanil
Remifentanil
Carfentanyl
Ohmefentanyl
Ohmecarfentanil
Benzimidazoles
Main article: List of benzimidazole opioids
Metodesnitazene (Metazene)
Etodesnitazene (Etazene)
Etonitazepyne
Etonitazepipne
Nitazene opioids, which are a subgroup of benzimidazoles opioids
Metonitazene
Etonitazene
Isotonitazene
Clonitazene
Phenylpiperidines
Pethidine (meperidine)
Ketobemidone
MPPP
Allylprodine
Prodine
PEPAP
Promedol
Diphenylpropylamine derivatives
Propoxyphene
Dextropropoxyphene
Dextromoramide
Bezitramide
Piritramide
Methadone
Dipipanone
Levomethadyl acetate (LAAM)
Difenoxin
Diphenoxylate
Loperamide (does cross the blood–brain barrier but is quickly pumped into the non-central nervous system by P-Glycoprotein. Mild opiate withdrawal in animal models exhibits this action after sustained and prolonged use including rhesus monkeys, mice, and rats.)
Benzomorphan derivatives
Dezocine—agonist/antagonist
Pentazocine—agonist/antagonist
Phenazocine
Oripavine derivatives
Buprenorphine—partial agonist
Dihydroetorphine
Etorphine
Morphinan derivatives
Butorphanol—agonist/antagonist
Nalbuphine—agonist/antagonist
Levorphanol
Levomethorphan
Racemethorphan
Others
Lefetamine
Meptazinol
Mitragynine
Tilidine
Tramadol
Tapentadol
Eluxadoline
Bucinnazine
7-Hydroxymitragynine
Allosteric modulators
Plain allosteric modulators do not belong to the opioids, instead they are classified as opioidergics.

Opioid antagonists
Nalmefene
Naloxone
Naltrexone
Methylnaltrexone (Methylnaltrexone is only peripherally active as it does not cross the blood–brain barrier in sufficient quantities to be centrally active. As such, it can be considered the antithesis of loperamide.)
Naloxegol (Naloxegol is only peripherally active as it does not cross the blood–brain barrier in sufficient quantities to be centrally active. As such, it can be considered the antitheses of loperamide.)