There are a number of broad classes of<unk> s: * 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; The following could be considered natural opiates: The leaves from Mitragyna speciosa (also known as kratom) contain a few naturally-occurring<unk> s, active via Mu- and Delta receptors. Salvinorin A, found naturally in the Salvia divinorum plant, is a kappa-opioid receptor agonist. * 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), desomorphine, acetylpropionylmorphine, dibenzoylmorphine, diacetyldihydromorphine; * Semi-synthetic<unk> s: created from either the natural opiates or morphine esters, such as hydromorphone, hydrocodone, oxycodone, oxymorphone, ethylmorphine and buprenorphine; * Fully synthetic<unk> s: such as fentanyl, pethidine, levorphanol, methadone, tramadol, tapentadol, and dextropropoxyphene; * Endogenous<unk>  peptides, produced naturally in the body, such as endorphins, enkephalins, dynorphins, and endomorphins. Morphine, and some other<unk> s, which are produced in small amounts in the body, are included in this category. 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<unk>  antagonist, is administered. Some minor opium alkaloids and various substances with<unk>  action are also found elsewhere, including molecules present in kratom, Corydalis, and Salvia divinorum plants and some species of poppy aside from Papaver somniferum. There are also strains which produce copious amounts of thebaine, an important raw material for making many semi-synthetic and synthetic<unk> s. Of all of the more than 120 poppy species, only two produce morphine. Amongst analgesics there are a small number of agents which act on the central nervous system but not on the<unk>  receptor system and therefore have none of the other (narcotic) qualities of<unk> s although they may produce euphoria by relieving pain—a euphoria that, because of the way it is produced, does not form the basis of habituation, physical dependence, or addiction. Foremost amongst these are nefopam, orphenadrine, and perhaps phenyltoloxamine or some other antihistamines. Tricyclic antidepressants have painkilling effect as well, but they're thought to do so by indirectly activating the endogenous<unk>  system. Paracetamol is predominantly a centrally acting analgesic (non-narcotic) which mediates its effect by action on descending serotoninergic (5-hydroxy triptaminergic) pathways, to increase 5-HT release (which inhibits release of pain mediators). It also decreases cyclo-oxygenase activity. It has recently been discovered that most or all of the therapeutic efficacy of paracetamol is due to a metabolite, AM404, which enhances the release of serotonin and inhibits the uptake of anandamide. Other analgesics work peripherally (i.e., not on the brain or spinal cord). Research is starting to show that morphine and related drugs may indeed have peripheral effects as well, such as morphine gel working on burns. Recent investigations discovered<unk>  receptors on peripheral sensory neurons. A significant fraction (up to 60%) of<unk>  analgesia can be mediated by such peripheral<unk>  receptors, particularly in inflammatory conditions such as arthritis, traumatic or surgical pain. Inflammatory pain is also blunted by endogenous<unk>  peptides activating peripheral<unk>  receptors. It was discovered in 1953, that humans and some animals naturally produce minute amounts of morphine, codeine, and possibly some of their simpler derivatives like heroin and dihydromorphine, in addition to endogenous<unk>  peptides. Some bacteria are capable of producing some semi-synthetic<unk> s such as hydromorphone and hydrocodone when living in a solution containing morphine or codeine respectively. Many of the alkaloids and other derivatives of the opium poppy are not<unk> s or narcotics; the best example is the smooth-muscle relaxant papaverine. Noscapine is a marginal case as it does have CNS effects but not necessarily similar to morphine, and it is probably in a category all its own. Dextromethorphan (the stereoisomer of levomethorphan, a semi-synthetic<unk>  agonist) and its metabolite dextrorphan have no<unk>  analgesic effect at all despite their structural similarity to other<unk> s; instead they are potent NMDA antagonists and sigma 1 and 2-receptor agonists and are used in many over-the-counter cough suppressants. Salvinorin A is a unique selective, powerful ĸ-opioid receptor agonist. It is not properly considered an<unk>  nevertheless, because: # chemically, it is not an alkaloid; and # it has no typical<unk>  properties: absolutely no anxiolytic or cough-suppressant effects. It is instead a powerful hallucinogen.<unk>  peptides Skeletal molecular images Adrenorphin Chemical structure of Adrenorphin Amidorphin Chemical structure of Amidorphin. Casomorphin Chemical structure of Bovine β-casomorphin. DADLE Chemical structure of DADLE. DAMGO Chemical structure of DAMGO. Dermorphin Chemical structure of Dermorphin. Endomorphin Chemical structure of Endomorphin 1. Morphiceptin Chemical structure of Morphiceptin. Nociceptin Chemical structure of Nociceptin. Octreotide Chemical structure of Octreotide. Opiorphin Chemical structure of Opiorphin. TRIMU 5 Chemical structure of TRIMU 5. Endogenous<unk> s<unk> -peptides that are produced in the body include: * Endorphins * Enkephalins * Dynorphins * Endomorphins β-endorphin is expressed in Pro-opiomelanocortin (POMC) cells in the arcuate nucleus, in the brainstem and in immune cells, and acts through μ-opioid receptors. β-endorphin has many effects, including on sexual behavior and appetite. β-endorphin is also secreted into the circulation from pituitary corticotropes and melanotropes. α-neo-endorphin is also expressed in POMC cells in the arcuate nucleus. met-enkephalin is widely distributed in the CNS and in immune cells; met-enkephalin is a product of the proenkephalin gene, and acts through μ and δ-opioid receptors. leu-enkephalin, also a product of the proenkephalin gene, acts through δ-opioid receptors. Dynorphin acts through κ-opioid receptors, and is widely distributed in the CNS, including in the spinal cord and hypothalamus, including in particular the arcuate nucleus and in both oxytocin and vasopressin neurons in the supraoptic nucleus. Endomorphin acts through μ-opioid receptors, and is more potent than other endogenous<unk> s at these receptors. Opium alkaloids and derivatives Opium alkaloids Phenanthrenes naturally occurring in (opium): * Codeine * Morphine * Thebaine * Oripavine 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<unk> s Anilidopiperidines * Fentanyl * Alphamethylfentanyl * Alfentanil * Sufentanil * Remifentanil * Carfentanyl * Ohmefentanyl 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 * AP-237 * 7-Hydroxymitragynine Allosteric modulators Plain allosteric modulators do not belong to the<unk> s, instead they are classified as<unk> ergics.<unk>  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.) Tables of<unk> s Table of morphinan<unk> s Table of morphinan<unk> s: click to Chemical structure of Morphine. Morphine Chemical structure of 2,4-Dinitrophenylmorphine. 2,4-Dinitrophenylmorphine Chemical structure of 6-Methylenedihydrodesoxymorphine. 6-MDDM Chemical structure of Chlornaltrexamine. Chlornaltrexamine Chemical structure of Desomorphine. Desomorphine Chemical structure of Dihydromorphine 2D structure. Dihydromorphine Chemical structure of Hydromorphinol. Hydromorphinol Chemical structure of Methyldesorphine. Methyldesorphine Chemical structure of N-phenethylnormorphine. N-Phenethylnormorphine Chemical structure of RAM-378 RAM-378 3,6-diesters of morphine Chemical structure of acetylpropionylmorphine. Acetylpropionylmorphine Chemical structure of dihydroheroin. Dihydroheroin Chemical structure of dibenzoylmorphine. Dibenzoylmorphine Chemical structure of dipropanoylmorphine. Dipropanoylmorphine Chemical structure of diacetylmorphine. Heroin Chemical structure of Nicomorphine. Nicomorphine Codeine-dionine family Chemical structure of Codeine. Codeine Chemical structure of 6-MAC. 6-MAC Chemical structure of Benzylmorphine. Benzylmorphine Chemical structure of Codeine methylbromide. Codeine methylbromide Chemical structure of Dihydroheterocodeine. Dihydroheterocodeine Chemical structure of ethylmorphine. Ethylmorphine Chemical structure of heterocodeine. Heterocodeine Chemical structure of pholcodine. Pholcodine Chemical structure of myrophine. Myrophine Chemical structure of Methyldihydromorphine. Methyldihydromorphine Morphinones and morphols Chemical structure of 14-Cinnamoyloxycodeinone. 14-Cinnamoyloxycodeinone Chemical structure of 14-ethoxymetopon. 14-Ethoxymetopon Chemical structure of 14-methoxymetopon. 14-Methoxymetopon Chemical structure of 14-phenylpropoxymetopon. PPOM Chemical structure of 7-Spiroindanyloxymorphone. 7-Spiroindanyloxymorphone Chemical structure of Acetylmorphone. Acetylmorphone Chemical structure of Codeinone. Codeinone Chemical structure of Conorphone. Conorphone Chemical structure of Codoxime. Codoxime Chemical structure of Thebacon. Thebacon Chemical structure of Hydrocodone. Hydrocodone Chemical structure of Hydromorphone. Hydromorphone Chemical structure of Metopon.
Opioid
Heroin
Synthetic cannabinoids
Nicotine
Phencyclidine
Cocaine 
Cocaine
Benzphetamine