Yohimbine, also known asquebrachine, is anindolealkaloid derived from the bark of the African treePausinystalia johimbe (yohimbe) and from the bark of the unrelated South American treeAspidosperma quebracho-blanco.[3][4] It is a veterinary drug used to reverse sedation in dogs and deer.
Substances that have purported to beextracts from the yohimbe tree have been marketed asdietary supplements for various purposes, but they contain highly variable amounts of yohimbine, if any; no publishedscientific evidence supports their efficacy for treating sexual dysfunction or any disease.[3][4][5]
There is no evidence from randomized, placebo-controlled trials that yohimbine is effective as an aphrodisiac, for treating sexual dysfunction, or for any health condition.[3][4]
Yohimbine has been used to treatfemale sexual dysfunction, but there are few reported clinical trials and these do not show it to be better than placebo.[6][7][8][9]
Yohimbine has been largely superseded by thePDE5 inhibitor drugs such assildenafil (Viagra). Prescriptions for it are now rare, and most United States pharmaceutical manufacturers have discontinued production of prescription capsules and tablets.[10]: 357–8
In the United States, "yohimbe" preparations are sold as a dietary supplements for enhancinglibido, for weight loss and as aids for bodybuilding; but "There is virtually no published research on yohimbe which supports these or any other claims".[8]: 861 Often, these products explicitly claim to contain yohimbine.[10]
One study found that samples of brands sold in American brick-and-mortar stores contained highly variable amounts of yohimbine, and sometimes none at all.[10]: 368 Labelling claims were often misleading.[10]: 368 Similar results have been reported by other laboratories for products sold in the US, in other countries and on the internet.[11][12][13][14][15] One study found that many brands of "yohimbe" might not derive from theP. johimbe tree in the first place.[16] According to yet another source, the yohimbe sold in markets in West Africa, where the tree grows, is frequently adulterated with other species of the genusPausinystalia; these contain little yohimbine.[17] The amounts of alkaloid found even in genuineP. johimbe bark vary considerably, depending on the source of the bark (roots, stem, branches, height, etc.).[18]
Some brands sold over-the-counter were found to contain more yohimbine per serving than a standard pharmaceutical dose.[10]: 368 It is illegal to introduce or deliver "drugs" into interstate commerce without the permission of the USFood and Drug Administration (FDA). The FDA has asserted that some yohimbine-containing products are "drugs" because they are so promoted as to show "they are intended for use in the cure, mitigation, treatment or prevention of disease": 21 U.S.C. § 321(g)(1)(B).[19] However the legal position is not entirely straightforward,[20] and as of 1 February 2019 there does not appear to be any record of a successful prosecution.
Because of the lack of reliable scientific data on yohimbe, the European Food Safety Authority Panel on Food Additives determined that it was not possible to conclude on its safety or to establish a health-based guidance value.[5]: 38 They wrote:
Overall the missing information include[s] quantitative data on the composition and specifications of yohimbe bark and its preparations used in food and food supplements covering other alkaloids besides yohimbine, data on the bioavailability of active ingredients from the yohimbe bark extract and data on the toxicity of well specified individual preparations of yohimbe bark and the major yohimbe bark alkaloids, especially regarding subchronic toxicity, genotoxicity and reproductive toxicity.
It has been studied as a potential treatment forerectile dysfunction, but there is insufficient evidence to rate its effectiveness.[21][22][23] It is illegal in the United States to market an over-the-counter product containing yohimbine as a treatment for erectile dysfunction without getting FDA approval to do so.[24] Nevertheless, the quantity of yohimbine in dietary supplements, often advertised as promoting sexual function, has been found to overlap with prescription doses of yohimbine.[25]
Yohimbine selectively blocks the pre-synapticα2-adrenergic receptors. Blockade of post-synaptic α2-adrenergic receptors causes only minorcorpus cavernosumsmooth muscle relaxation, due to the fact that the majority ofadrenoceptors in the corpus cavernosum are of the α1 type. Blockade of pre-synaptic α2-adrenergic receptors facilitates the release of several neurotransmitters in the central and peripheralnervous system—thus in the corpus cavernosum—such asnitric oxide andnorepinephrine. Whereas nitric oxide released in the corpus cavernosum is the major vasodilator contributing to the erectile process, norepinephrine is the major vasoconstrictor through stimulation ofα1-adrenergic receptors on the corpus cavernosum smooth muscle. Under physiologic conditions, however, nitric oxide attenuates norepinephrine vasoconstriction.[39]
Yohimbine binds to the a2 adrenergic receptor at a ratio of 40:1 and is the only a2 adrenergic receptor antagonist with noimidazoline receptor activity.[40]
Yohimbine should not be confused withyohimbe[41] but often is.[42] Yohimbe is the common English name for the tree speciesP. johimbe (also calledCorynanthe johimbe) and, by extension, the name of a medicinal preparation made from the bark of that tree, sold as anaphrodisiac.[43] In contrast, yohimbine is a pure alkaloid that can be isolated from yohimbe bark.
Yohimbine is just one of at least 55 indole alkaloids that have been isolated from the bark;[44] and, while it has been described as the most active of these,[10] it constitutes only 15% of the total alkaloid content.[8] Others includerauwolscine,corynanthine andajmalicine;[8] the bark also contains non-alkaloids about which virtually nothing is known.[8]
Yohimbe, thus a complex mixture, has been studied far less thoroughly than yohimbine, the pure compound.[8] Pharmaceutical grade yohimbine is usually presented as thehydrochloride,[5][8] which is more soluble.
The traditional source of yohimbine is the bark of the African treeP. johimbe. It has other uses, but the tree is sought out primarily for its bark; in practice, harvesting the bark kills the tree. Tree density is relatively low (average ≈ 4 harvestable trees/hectare). The high demand for medicines based on the bark has led to the tree's over-exploitation. The bark is traded in local markets and, because it is scarce, it is often adulterated with that of other species which contain little yohimbine.[17] The species is becoming endangered.[45]
Around the year 2000,Cameroon was shippingP. johimbe to Europe at the rate of about 100 tonnes annually. Most bark is collected illegally by local people who are paid 150CFA francs per kilo (about US$0.10 per pound) for delivery of pre-dried bark at the roadside. In practice they confuse and mix it withP. macroceras ("false yohimbe"), a species that contains little yohimbine.[46]
Aspidosperma quebracho-blanco is an unrelated tree whose common name isquebracho blanco.[47] It is found in large areas of central South America, particularly theGran Chaco, where it is often the dominant species in thecanopy.[48] It is one of the most widely distributed Argentine arboreal species.[49][50] Traditionally it was logged for fuel, timber and railway sleepers.[50] While in recent times cattle ranching and soya cultivation have led to considerable habitat loss,[51] and while there is stillillegal logging, no shortage of the bark is reported. The tree has not been described as endangered: a few members of the genusAspidosperma are on theIUCN Red List. but thequebracho blanco species is not one of them.[52]
In its bark an alkaloid is found which was given the name quebrachine. In 1914, two scientific papers claimed quebrachine was chemically identical to yohimbine.[53] This was disputed,[54] and the matter long remained in doubt.[55] However, in 1972, Effler and Effler using modern analytical techniques, includingmass spectrometry,UV absorption,IR absorption, andNMR, established that quebrachine and yohimbine are one and the same thing. They wrote:
While it was almost unthinkable in 1914 ... that the same alkaloid was formed in [completely] different plants, recent studies have shown that this is certainly the case for indole alkaloids.[56]
Strictly speaking, wroteGeorge Barger, yohimbine should have been given the scientific namequebrachine, seeing that it was first isolated from the quebracho tree and first named in the scientific literature. However the later work onP. yohimbe was better known, so the newer name stuck.[53]
Yohimbe (Pausinystalia johimbe) is a tree that grows in western and centralAfrica;[67] yohimbine was named as originally extracted from the bark of yohimbe in 1896 by Adolph Spiegel[68] (but see§ Aspidosperma quebracho-blanco below). Yohimbe is used in folk medicine as anaphrodisiac. In 1900, it attracted scientific interest in Germany, where an initial report claimed that yohimbe exerted a strong aphrodisiacal effect in animals and humans.[8] Attention soon shifted from the plant to its active constituents, particularly yohimbine.[8] In 1943 the correct constitution of yohimbine was proposed by Witkop.[69] Fifteen years later, a team led byEugene van Tamelen used a 23-step synthesis to become the first persons to achieve the synthesis of yohimbine.[70][71][72]
There was a case in theWorld Anti-Doping Agency practice in 2007, when an athlete, who reportedly consumed Yohimbine prior to a given athletic event, was later tested positive for19-norandrosterone, which is a prohibited substance.[73] However, WADA did not yet list Yohimbine (which can come into a body via anenergy drink,[74] also in a form ofpre-workout supplement orfat burner[75]) as aprohibited substance, nor did it confirm that its use can increase the endogenous level of anabolic steroids, in particular of 19-norandrostenedione and testosterone.
Yohimbine has been used since the 1970s to reverse the effects of xylazine.[40] A 2011 preliminary study found thatintravenous yohimbine has slow elimination and a large distribution in horses.[40][79] Yohimbine is not commonly used in small animal medicine anymore but is still commonly used in large animal medicine to reverseα2 adrenergic receptor agonists such asxylazine.[40]
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^Zanolari B, Ndjoko K, Ioset JR, Marston A, Hostettmann K (2003). "Qualitative and quantitative determination of yohimbine in authentic yohimbe bark and in commercial aphrodisiacs by HPLC-UV-API/ MS methods".Phytochemical Analysis.14 (4):193–201.Bibcode:2003PChAn..14..193Z.doi:10.1002/pca.699.PMID12892413.Twenty commercial aphrodisiac preparations were analysed and the amount of yohimbine measured and expressed as the maximal dose per day suggested on product labels ranged from 1.32 to 23.16 mg.
^Raman V, Avula B, Galal AM, Wang YH, Khan IA (January 2013). "Microscopic and UPLC-UV-MS analyses of authentic and commercial yohimbe (Pausinystalia johimbe) bark samples".Journal of Natural Medicines.67 (1):42–50.doi:10.1007/s11418-012-0642-2.PMID22402817.S2CID11977944.Of 12 commercial samples tested, yohimbine was not detected in one; its presence in other samples was found to be in the range 0.1–0.91%.
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