Keywords

FormalPara Vernaculars:

Urd.: Aslussoos ; Hin. : Jethi-madh , Muletthi , Mithi-lakri ; San. : Madhuka , Yashtimadhu ; Ben. : Jaishto-modhu ; Mal.: Irattimadhuram ; Mar. : Jeshti-madh ; Tam.: Adhimadhuram , Athimathuram , Athimathurappal (extract ), Atimad-uram (root); Tel.: Athimathuram , Yashti-madhukam ; Ara.: Oudussoos (root), Irk-es-sus , Rubbus-soos (extract), Soos ; Chi.: Gām chóu , Gancao , Yáng gān cǎo ; Dut.: Zoethout ; Eng.: Licorice , Liquorice , Sweetwood ; Fre.: Bois doux , Réglisse , Réglisse glabre ; Ger.: Gewöhnliches süßholz , Kahles süßholz , Lakritze , Lakritzpflanze , Süßholz ; Gre.: Glikoriza , Glykoriza ; Ita.: Legno dolce , Liquirizia , Liquirizia comune , Regolizia ; Jap.: Gurukiruriza gurabura ; Kor.: Mingamtscho ; Per.: Ausareha mahaka , Beekh-e-mehak ; Por.: Alcaçuz , Pau-cachucho , Raiz-doce , Regaliz ; Rus. : Koren solodki ; Sin.: Atimaduram ; Spa.: Agarradera , Alcarzuz , Alfendol , Chocolate del moro , Erregaliz , Fendoces , Palo dulce , Palo-luz , Regaliz ; Tha.: Cha em thet ; Tur.: Meyan kökü ; Vie.: Cam thảo .

FormalPara Description:

Liquorice is a perennial, temperate-zone herb or subshrub, native of India, Pakistan and southern Europe; also cultivated in England, Belgium, France, Germany, Spain, Italy, Greece, Turkey, Russia, South Africa, Egypt, Syria and Iraq. It has also been grown experimentally in the United States [6]. It grows up to 2 m high, with a long, cylindrical, branched, flexible, burrowing rootstock and horizontal, creeping, underground stems (stolons) of up to 1.8 m long, having buds which send up stems in the 2nd year.LV Leaves are alternate, pinnate, yellow-green leaflets, viscid on the underside. G. glabra var. typica is the most commonly used variety of glycyrrhiza. Its rootstalk and stolons are 6 to 18 mm thick, longitudinally wrinkled and sweeter than other types. It is one of the pharmaceutically important plants of India that is designated as on the verge of being endangered due to overexploitation and collection from the wild [115] (Figs. 1, 2 and 3).

Fig. 1
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Glycyrrhiza glabra, Inflorescence, Pharaoh han, WikimediaCommons; ShareAlike 3.0 Unported CC BY-SA 3.0, https://commons.wikimedia.org/wiki/File:Glycyrrhiza_glabra_inflorescence.jpg; https://creativecommons.org/licenses/by-sa/3.0/deed.en

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Fig. 2
figure 2

Glycyrrhiza glabra, Roots from India, Prof. Akbar, Original

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Fig. 3
figure 3

Glycyrrhiza glabra, Roots from South Africa, Prof. Akbar, Original

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FormalPara Actions and Uses:

Ancient historical manuscripts from China, India and Greece mention its use for symptoms of viral respiratory tract infections and hepatitis [48]. The plant has also been described by Theophrastus.XXI Author of Makhzan-el-Adwiya described licorice from Egypt to be the best, followed by from Iraq and Syria, and directs the root to be decorticated before use. The root is considered hot, dry and suppurative, demulcent and lenitive, relieving thirst and cough, and removing unhealthy humours from the body. It is also emmenagogue and diuretic, useful in asthma and irritable conditions of the bronchial passages. Avicenna recommended decoction in chronic fevers, tracheal pain, cold colic, and to clear voice; also dropped into eyes to strengthen sight.XL,LXIX,LXXVII Ibn Jazlah used it in leprosy, spleen ailments and scorpion sting, and in the Middle East it is used to relieve acute indigestion.LIII It concocts viscid humours in diseases of liver, bladder and lungs, and expectorates them.CV It has been used in Iranian herbal medicine for skin diseases, including dermatitis, eczema, pruritus and cysts [95], and for treatment of stomach disorders including peptic ulcers [58]. The herb extract inhibits gastric motility in vivo, which is regarded to be an important aspect for its antiulcer activity. GaborXLVIII reported paradoxical profile of licorice as possessing both anti-inflammatory and antiulcer activities; whereas most anti-inflammatory agents are ulcerogenic. It is a common ingredient of cough syrups, throat lozenges and pastilles for its flavor as well as its demulcent, mildly expectorant, and anti-inflammatory effects,XXI and is also used in irritable conditions of mucous membranes of urinary organs, gastric ulcers, Addison’s disease and inflammatory conditions [6]. Former German Commission E believed it to be effective in the treatment of atopic dermatitis [92]. Decoctions of peeled dried root were formerly used to allay coughs, catarrh, bronchitis, sore throat, laryngitis, urinary irritation and pain associated with diarrhea. Liquorice provides two derivatives which reduce or cure gastric ulcers [37].

Licorice root has been used for years to regulate gastrointestinal function in TCM [28], has been used for generations as an antidote, demulcent, and elixir in folk medicine of China [2], and is the most commonly used crude drug in Kampo Medicines, the Japanese form of modified TCM, for the treatment of peptic ulcer [46]. In old Chinese pharmacy the drug was regarded rejuvenating for those who consumed it for long periods. In Chinese medicine, it is known as Gancao and is also derived from G. uralensis Fisch., and described to regulate functions of the stomach, is “Qi” (vital energy)-tonifying, lung demulcent, expectorant, latent heat-clearing, antipyretic, detoxicant, anti-inflammatory, “spleen-invigorative,” and is a corrective adjuvant and harmonizing ingredient in many preparations. It is thus used in pharyngolaryngitis, cough, palpitation, stomachache due to asthenia, peptic ulcer, pyogenic infection and ulceration of the skin.XVIII Hsu described its uses in Chinese medicine for toxic states, excessive sputum, muscular pain due to tension, peptic ulcer, duodenal ulcer, and sore throat.LXVI Glycyrrhizin (GR) has been used for the treatment of chronic liver diseases in Japan, with distinct improvement in liver function tests [45]. Glabridin is commonly used in China for the treatment of CVS and CNS diseases [23]. Stronger Neo-Minophagen C® is a glycyrrhizin-containing preparation approved in Japan for the treatment of chronic hepatic diseases, and is marketed in Japan, China, Korea, Taiwan, and India [61, 66].

Licorice pieces are popular chew sticks in Italy, Spain, the Netherlands (where they are called Palu dushi) and West Indies. It is also employed in chewing gum, confectionery, soft drinks, liqueurs, ice creams, puddings, bakery products, soy sauce and soybean-protein meat substitutes [34], added to beer to enhance the “head”XXIV and aroma [6], and to porter and stout to provide more body and a darker hue.LV Mouthwashes, breath ‘purifiers’ and toothpastes [34], and tobacco industry also used it to enhance flavor, which was banned by the U.S. FDA [81]. In pharmaceutical industry it is customarily added to bitter laxative preparations of senna , aloe, cascara and other drugs to improve their flavor and because it sensitizes intestines and thus potentiates their action.CXVIII In Egypt, it was studied and promoted under the auspices of various governmental research and academic institutions [96].

FormalPara Phytoconstituents:

Roots contain glycyrrhizin (glycyrrhizic acid or glycyrrhizinic acid), the main water-soluble constituent that is 50× sweeter than sugar, 2-β-glucuronosyl glucuronic acid, and isoliquiritigenin-4-glucoside. Glycyrrhizin (GR) is a nonhemolytic saponin with foaming property [98], and one of the most potent hydroxyl radical scavengers [79]. Glycyrrhizin content in roots vary with season and age, rapidly increasing from October to November in 1-year-old roots, but did not show any significant increase from May to August in 3-years-old plants, whereas the isoliquiritigenin glycoside content increased up to October [52]. Glycyrrhizin content is higher in thick roots and rhizomes than in thin ones, and the highest content of GR is found in rhizomes 1.1–2 cm in diameter; there is little difference in the GR content of vertical and horizontal rhizomes [51, 78]. Hayashi et al. reported variation in GR contents from 3.3 to 6.1% of the dry weight, and that of glabridin, from 0.08 to 0.35% of dry weight in samples of rhizomes and roots collected from Uzbekistan [53]. Samples collected from various sites in Italy also showed remarkable differences in active constituents and biological activity [106]. Roots also contain phenolic compounds, formononetin, hemileiocarpin, hispaglabridin B, isoliquiritigenin, glabrene, glabridin, 4′-O-methylglabridin, paratocarpin B, phaseollinisoflavone (phytoalexin), glabrol, salicylic acid, and O-acetyl salicylic acid (0.15%) [73]; hispaglabridin B, isoliquiritigenin, and paratocarpin B are reported as the most potent antioxidant agents [29]. Kitagawa et al. isolated flavonoids, glucoliquiritin apioside, prenyllicoflavone A, shinflavone, shinpterocarpin and 1-methoxyphaseollin from samples collected from Xinjiang province of China [62]. Phenolic constituents, licopyranocoumarin, glycycoumarin and/or licocoumarone, are found in G. uralensis but not in G. glabra; whereas glabridin and glabrene are found in G. glabra but not in G. uralensis [50]. Glabridin is a species-specific flavonoid of G. glabra, and not found in other species of glycyrrhiza [53]. Other phenolic compounds, 5′-formylglabridin, (2R,3R)-3,4′,7-trihydroxy-3′-prenylflavane, (3R)-2′,3′,7-trihydroxy-4′-methoxyisoflavan, kanzonol X, kanzonol W, glabrol, echinatin, shinpterocarpin, licoflavanone A, shinflavanone, gancaonin L, and glabrone exhibit significant PPAR-γ ligand-binding activity [65]. Two flavonosides, glychionide A and B, were isolated from the roots by Li et al. [68]. Two caffeic acid derivative esters, viz. eicosanyl caffeate and docosyl caffeate possess potent elastase inhibitory activity, and moderate antioxidant activity [36]. Two saponins, glabranin-A and B, are glycosides of glycyrrhetic acid [113]. Oleanane-type triterpene saponins, licorice -saponin M3, licorice -saponin N4, and licorice -saponin O4, were isolated by Wei et al. [119]. Seven constituents, with antioxidant capacity were identified as the isoflavans (hispaglabridin A and B, glabridin, and 4′-O-methylglabridin), the two chalcones (isoprenylchalcone derivative and isoliquiritigenin), and the isoflavone, formononetin; glabridin being the most abundant and potent LDL antioxidant [114]. Coumarin derivatives present are herniarin and umbelliferone;XXIV also, the flavones liquiritin, liquiritigenin [55], isoliquiritin and isoliquiritigenin (responsible for yellow color) and the glycosides liquiritoside and isoliquiritoside, which, together are credited with licorice ’s antiulcer activity, and two flavonoids, rhamnoliquiritin and rhamnoisoliquiritin have been reported [112].

FormalPara Pharmacology:

The extract, powder, GR and glycyrrhetic acid possess glucocorticoid and mineralocorticoid-like activities. Licorice has been shown to inhibit tyrosinase activity in vitro, a rate-limiting oxidase enzyme involved in the formation of melanin [67]. Glyderinine, a derivative of GR produced pronounced anti-inflammatory effects [12], exceeding activities of hydrocortisone and amidopyrine; also showed analgesic and antipyretic effects, but unlike most NSAID, did not suppress hemopoiesis or cause gastrointestinal ulceration. Hydroalcohol extract showed antiulcer activity against various ulcerogenic stimuli, comparable to cimetidine and even better than omeprazole [58]. GutGard®, the standardized extract reduced ulcer index and increased gastric pH, due to its cytoprotective and antioxidant properties [76], also protected against indomethacin-induced gastric ulcers in rats, with reduced acid output and increased mucin secretion, increase in PGE2 release and decrease in leukotrienes [60]. Combination of cimetidine and deglycyrrhizinated liquorice provided greater protection against aspirin-induced gastric ulcers in rats than low doses of either drug alone [17]. Aqueous extract is a strong inhibitor of adhesion of H. pylori to human stomach tissue [120]. The flavonoids, glabridin and glabrene inhibit in vitro growth of sensitive, and clarithromycin- and amoxicillin-resistant strains of H. pylori [46]. Isoliquiritigenin produces dual dose-dependent effect on the intestine, inhibitory at low doses and prokinetic at the high doses; the former effect is predominantly due to calcium channels blockade, while the latter may involve muscarinic receptors activation [28]. Aqueous extract was protective against APAP-induced hepatorenal damage in mice [102]. Oral or i.v. administration of licorice extract caused choleretic effect in rats, and umbelliferon, a minor component, was identified responsible for it [90].

Both aqueous and ethanol licorice extracts are effective against S. mutans and L. acidophilus, comparable to chlorhexidine [3], and methanol extract was especially bactericidal to S. mutans within 2 min at 50 μg/ml [56] and also selectively active against P. falciparum and P. berghei [40]. Fresh aqueous extract inhibited growth of standard strain and two clinical isolates of C. albicans; however, the activity was lost after storage for 24 h, even at 4 °C [75]. Ethanol extract is remarkably active against P. acnes, with negligible induction of resistance [80]. The flavonoid rich standardized commercial extract (GutGard®) and one of its constituents, glabridin were significantly active against H. pylori, but even at higher concentration, GR showed no activity [10]. Glabridin was also active against M. tuberculosis and many Gram-positive and Gram-negative bacteria [48], and against drug resistant mutants of C. albicans [41]. Aqueous extract was active against HSV-1, [94] and GR inhibited plaque formation in three strains of Japanese encephalitis virus [13]. Hispaglabridin A and B, 4′-O-methylglabridin, glabridin, glabrol and 3-hydroxyglabrol, all exhibit significant antimicrobial activity [73]. Glycyrrhizin inhibits growth and cytopathology of several unrelated DNA and RNA viruses, without affecting cell activity and ability to replicate [87]. Animal studies showed reduction of mortality and viral activity in HSV encephalitis and influenza A virus pneumonia. In vitro studies revealed antiviral activity against HIV-1, SARS related coronavirus, RSV, arboviruses, vaccinia virus and vesicular stomatitis virus [43]. 18-β glycyrrhetinic acid was also active in a pH-dependent manner against C. albicans strains, isolated from patients with recurrent vulvovaginal candidiasis [86], and strong antiviral activity against rotavirus [63].

Pretreatment of mice with aqueous extract for 7-days significantly improved learning and memory [85], reversed scopolamine-induced amnesia [38], decreased brain AChE activity [37], and produced antidepressant-like effect [39]. Aqueous extract pretreatment of rats also significantly enhanced spatial memory retention [101] and learning [26]. Pretreatment of rats with glabridin [122], and isoliquiritigenin [125] significantly protected against cerebral ischemia-induced neurological deficit; glabridin also significantly antagonized scopolamine-induced amnesia and remarkably reduced brain AChE activity in mice [35], improved learning and memory in normal rats and reversed diabetes-induced cognition deficit in rats [49]. Ethanol extract acting on GABAA-benzodiazepine receptors produced hypnotic effect in mice [30], and ameliorated PTZ-induced convulsions in rats [31]. Licorice extract is protective against myocardial I/R injury [82], and isoproterenol-induced MI [83]. Aqueous extract exhibits modest in vitro thrombolytic activity [88], and GR was identified as a selective in vitro thrombin inhibitor [44], reduced thrombus size in venous thrombosis model, and in high doses caused significant hemorrhagic effect. Glycyrrhetinic acid also directly inhibited Factor Xa in vitro, increased plasma clotting time and prothrombin time, and caused moderate hemorrhagic effect [59].

Chinese researchers reported that intragastric administration of the herb powder to rabbits at a dose of 1 and 3 g/day was ineffective in preventing experimental atherosclerosis.XVIII However, plasma lipid profile was significantly positively affected in normal and dyslipidemic rats by root powder administration, while the HDL-C was significantly increased [116]. It was reported to be the most potent out of a number of Indian medicinal plants for its hypolipidemic/hypocholesterolemic and antioxidant effects [116]. Ethanol extract significantly reduced LDL-C and increased HDL-C of dyslipidemic Syrian golden hamsters [72]. Methanol and aqueous methanol extracts [121] and aqueous and ethanol extracts [117] demonstrated high in vitro radical-scavenging activity. Licorice extract, due to its antioxidant properties, protected rats against toxic effects of ochratoxin A, one of the most common food-contaminating mycotoxins [71]. Freeze-dried aqueous extract is a strong suppressor of adrenal-pituitary axis in rats, significantly decreases concentrations of cortisol, ACTH, aldosterone and K+, and increases concentrations of Na+, and stimulates renin production by the kidneys [4]. Ethanol extract has shown antiandrogenic property in castrated rats [123]. Fractions of ethyl acetate extract have displayed estrogenic activity; several of them showing higher responses than the natural hormone 17β-estradiol, but glabridin not exerting any estrogenic activity [105].

Aqueous extract demonstrates potent antiangiogenic and antitumor activity inhibiting in vivo and in vitro proliferation of Ehrlich ascites tumor cells [103]. Petroleum ether extract showed significant in vitro cytotoxic effect against Yoshida ascites sarcoma cells [111]. Glycyrrhizin feeding in drinking water to Sencar mice substantially protected against DMBA-induced skin tumorigenesis. Binding of topically applied [3H]B(α)P and [3H]DMBA to epidermal DNA was also significantly inhibited. Intraperitoneal pretreatment of animals with GR or addition of GR to the culture medium antagonized E. coli endotoxin (LPS) and hydrocortisone-induced myelosuppression of mouse bone marrow cells in culture [57]. Antimutagenic activity of extracts or their components has also been reported [99, 100, 124].

FormalPara Clinical Studies:

In a double-blind RCT, 30-days treatment with GutGuard® significantly decreased symptoms of Indian patients with functional dyspepsia [91], and was also significantly effective in patients with H. pylori infection [89]. Bardhan and colleagues reported no significant effect of deglycyrrhizinised liquorice on gastric ulcer in an RCT of British patients [15]; whereas, Brogden and associates had earlier reported it effective in peptic ulcers [20]. Treatment of healthy men with licorice for one-week decreased salivary testosterone values by 26% but no significant decrease in free testosterone [7], and nine healthy women treated with 3.5 g of licorice containing 7.6% of GR daily for two cycles, had their mean total serum testosterone decreased by 37% at the end of 2nd month. This property could be useful as an adjunct therapy of hirsutism and PCOS [8]. In a double-blind RCT, topical application of a gel preparation (2%, containing 19.6% GR) was effective in significantly reducing scores of erythema, edema and itching after two-weeks treatment of 30 Iranian patients with atopic dermatitis [95]. An emollient cream containing milk proteins and licorice extract used as adjuvant to topical corticosteroid treatment of palmoplantar psoriasis in Italian patients for four-weeks also produced significantly greater improvement than the cortiocosteroid therapy alone [24]. Licorice tincture stimulates immune cells in humans within 24 h of ingestion that continues for at least 7 days [21, 126].

Acharya and associates [1] reported a highly significant favorable response in 18 patients of subacute hepatic failure due to viral hepatitis, treated with a substance isolated from G. glabra. The substance, named Stronger Neo Minophagen-C (SNMC), is an interferon stimulator. Survival rate amongst the patients treated with SNMC was 72.2% compared to the survival rate of 31.1% in 98 patients who received only supportive therapy. RCTs confirmed that glycyrrhizin and its derivatives reduced hepatocellular damage in chronic hepatitis B and C, and the risk of hepatocellular carcinoma was reduced in hepatitis C virus-induced cirrhosis [43].

FormalPara Mechanism of Action:

Various mechanisms have been suggested for its antiulcer activity. cAMP and cGMP have been implicated in the regulation of gastric acid secretion. One of licorice constituents, glycyrrhetinic acid inhibits PDE activity, thus increasing levels of cAMP of gastric mucosa of the pylorus and cardia, and suppressing gastric acid secretion. It did not, however, affect adenylate cyclase [5]. Antiulcer effect of GR is due to increased local concentration of PGs that promotes mucous secretion, and the hepatoprotective effect is mediated through inhibition of PLA2, and increasing hepatocytes survival [72]. Isoliquiritigenin relaxes isolated guinea-pig trachea through various intracellular actions, including inhibition of PDEs [69]. Hydroalcohol extract also inhibited 5-LOX and COX-2 enzymes, inhibiting formation of both eicosanoids and LTs [54]. GR, due to the steroid-like structure of its aglycone, β-glycyrrhetinic acid is credited for its anti-inflammatory and antiallergic activities, which also possesses immunomodulatory properties [64]; and liquiritigenin is credited for the antiallergic activity [104]. However, the in vitro inhibitory effect on formation of both eicosanoids and LTs has been ascribed to glabridin and isoliquiritigenin and not GR [27, 107]. Antidepressant activity of aqueous extract is suggested to be mediated by increase of brain NE and DA, and MAO inhibiting effect [39]. Inhibition of carcinogen metabolism to active metabolite and DNA adduct formation could be the possible mechanism(s) of antitumor activity [2].

FormalPara Human A/Es, Allergy and Toxicity:

Revers first reported reduction in abdominal symptoms as well as radiographic evidence of healing of gastric ulcer, after administration of a paste prepared from dried watery extract of the roots [93]. However, later clinical studies observed that approximately 20% of treated patients developed facial and dependent edema, often accompanied by headache, shortness of breath, stiffness, and pain in the upper abdomen, which subsided after reduction of the dose [97]. Various reports of licorice toxicity emerged from Western countries in the 1950s, 60s and 70s about hypokalemia, hypertension and paralysis [11, 109]. Licorice was even declared to damage health [108]. Chopra et al. warned that licorice should be avoided by persons with cardiac problems, hypertension, kidney ailments, and those who are overweight or having difficult pregnancies.XXI Excessive licorice ingestion led to cardiac dysfunction and severe hypertension [16, 18, 32, 33, 47, 118];XXVIII most cases were resolved after cessation of licorice and potassium replacement without any sequelae. However, some cases of serious poisoning were reported, including death due to cardiac arrest [14]. A Californian woman developed pain in arms and legs, aggravated by muscular activity, after consuming large quantities of licorice to lose weight [110]; women also developed acute quadriparesis and paralysis due to severe hypokalemia, after consuming for long period a product containing licorice [9, 77]. Borst et al. [19] first reported that the GR moiety of liquorice was responsible for fluid retention and electrolyte imbalance. Glycyrrhizin in licorice has mineralocorticoid-like effect, and chronic intake induces a primary hyperaldosteronism-like syndrome resulting in sodium and water retention and loss of potassium, increasing extracellular fluid and plasma volume, leading to hypertension, hypokalemia, and metabolic alkalosis [25].CXI The hypermineralocorticoidism and pseudoaldosteronism are due to the inhibitory activity of GR on 11-hydroxysteroid dehydrogenase [42]. Kabeeruddin mentions it harmful for kidneys and spleen.LXXVII

FormalPara Animal Toxicity:

Oral LD50 of hydroalcohol extract in mice was reported as 2,950 mg/kg [58]. LD100 of the herb extract in mice by subcutaneous injection was calculated as 3,600 mg/kg; the animals died of respiratory paralysis. Mean lethal dose (MLD) of glycyrrhizin by this route was found to be 1,000 mg/kg.

FormalPara CYP450 and Potential for Drug-Herb Interactions:

Ethanol extract in vitro inhibits CYP3A4 [22], and CYP2D6 [84]. Inhibition of CYP450 enzymes could result in reduced metabolism of drugs metabolized by these enzymes, such as warfarin, synergistically increasing its activity and prolonging clotting times. An 80-year-old woman with atrial fibrillation, being treated with warfarin, developed black tarry stools and an elevated INR, after eating a pound of black licorice [70]. Pretreatment of rats with methanol extract for 6-days significantly increased cumulative biliary and urinary excretions of APAP-glucuronide conjugate [74].

In Chinese medicine, four drugs collectively known as “Zao Jie She Yuan” (Sargassum fusiforme, root of Euphorbia pekinensis, root of Euphorbia kansui, and flower buds of Daphne genkwa) are considered incompatible with Gancao . Administration of G. glabra with any of these drugs either enhances the combined effect (toxic) or inhibits activity of each other.XVIII

FormalPara Commentary:

Since the first scientific report in 1950s of healed gastric ulcers after treatment with licorice that could be proven by radiology, licorice has been a subject of controversy due to its glucocorticoid and mineralocorticoid-like activities as adverse effects. Nevertheless, it continues to be used in traditional medicines, and continues to be a subject of scientific curiosity, and investigated for various activities.