by Tugba Ucar

Many studies have explored its efficacy and safety of peppermint and its preparations. Peppermint oil has been used for alleviating the symptoms of the gastrointestinal tract such as abdominal pain, flatulence, spasm and respiratory tract problems such as coughs and oral mucosal problems, headache and inflammation. Peppermint is also widely used in the food and cosmetics industry due to its pleasant aroma. Besides its efficiency, it may have side effects such as heartburn, headache, allergic reactions, iron deficiency and interaction with other medications. Therefore, the aim of this review was to analyse the literature on the ethnopharmacological and clinical uses of peppermint in terms of its safety and efficacy.

INTRODUCTION

Peppermint (Mentha x piperita L.) is a hybrid of Mentha aquatica and Mentha spicata. It has been widely used for medicinal and culinary purposes for over 2500 years (Heinrich et al., 2012). It has been traditionally used for the relief of symptoms associated with gastrointestinal problems such as spasm, nausea, flatulence, abdominal pain and respiratory problems such as colds and coughs, generally in the form of tea and essential oil. It has also been applied topically for alleviating headache and pruritus of skin (Edwards et al., 2015). These traditional uses have highly affected medical and scientific research on peppermint today. Nowadays, aromatherapy, medicine, food and the cosmetics industry use peppermint oil for several purposes.

Peppermint belongs to the Lamiaceae. It is a perennial herb about 50 cm high, with a pungent essence. The leaves are dark green on the upper side and a little paler on the lower. The leaves have an uneven texture due to venation. Flowers are pinkish to mauve, containing many congested whorls with quadrangular stems (Bone & Mills, 2013; Heinrich et al., 2012).

HISTORICAL AND MODERN USES

The first record of peppermint was from ancient Egypt in approximately 2600-3200 BCE, and it was cultivated for medicinal and gastronomic purposes (Heinrich et al., 2012). It was also used by the Romans and Greeks in their cuisine, and it had an important place in their celebrations (Bone & Mills, 2013). In 1696, peppermint was considered as a specific species and accepted into the London Pharmacopoeia in 1721. It started to be widely used in Western Europe from the 1750s and cultivated at Mitcham, located on the southern border of today’s London.

A medicinal “essence of peppermint” was produced and patented by John Juniper in England in 1762. Essence of peppermint has been known worldwide and still being in use. In the United States, the cultivation started after 1816 in New York and began to widen into Ohio, northern Indiana, then to Michigan and the Pacific Northwest. It is also produced in Russia, southern India, Japan, Germany, Italy, France and China (Jones, 1981).

It was widely consumed to relieve digestive problems such as pain and spasm of stomach, flatulence, dysmenorrhoea, nausea, morning sickness and the symptoms of respiratory tract such as cold, cough, pneumonia and headaches (Bone & Mills, 2013). Besides these traditional uses, nowadays peppermint is widely used in many oral care products for the dental plaque reducing effect due to its phenolic content such as tannin and flavonoids (Pramila et al., 2012). Many food products, cough medicines, beverages, gums, chocolates, cigarettes and medicinal teas are flavoured by peppermint, and it is commonly used in aromatherapy (Bone & Mills, 2013). Furthermore, there are licensed peppermint oil products on the market for the treatment of irritable bowel syndrome (Pramila et al., 2012).

PHYTOCHEMISTRY

Mentha x piperita L. contains a considerable amount of essential oil (0.5-4%) which is mostly used in medicine, cooking, cosmetic and aromatherapy.

The main phytochemical constituents of peppermint essential oil are menthol (33-60%) and menthone (15-32%), with smaller amounts of 1,8-cineole (5-13%), methyl acetate (2-11%), isomenthone (2-8%), limonene (1-7%), β-myrcene, carvone and pulegone. The leaves of Mentha x piperita L. also consist of phenolic compounds such as rosmarinic acid, tannins and flavonoids, predominantly eriocitrin, hesperidin and luteolin (McKay & Blumberg, 2006). The tannin and flavonoid contents of peppermint are especially important to prevent plaque formation on teeth (Pramila et al., 2012). Linolenic acid, palmitic acid and linoleic acid constitute the fatty acid content of peppermint. Furthermore, there are studies on mineral content such as potassium, magnesium, calcium and vitamin content such as β-carotene, α, γ-tocopherols and ascorbic acid. These phytochemical constituents may change according to geographic area, processing, plant age and diversity (McKay & Blumberg, 2006).

SCIENTIFIC EVIDENCE

Peppermint as a spasmolytic

Many studies on the efficacy of peppermint oil have shown that it presents a spasmolytic effect by relieving smooth muscle constriction and thus exhibiting a treatment option for irritable colon or irritable bowel syndrome, alleviating the uneasiness of bile ducts and the gastrointestinal tract (Göbel et al., 1996).

Irritable bowel syndrome is a common gastrointestinal disorder affecting 5-20% of the world’s population. It is characterized by repetitive gastrointestinal spasm, abdominal discomfort and changes in appearance and regularity of stool continuing over three months, for a minimum of three days each month. The spasmolytic mechanism of action of peppermint oil is to reduce calcium influx into gastrointestinal smooth muscle, similar to a dihydropyridine calcium channel antagonism mechanism. An essential point is that to provide spasmolytic effect on intestinal system, enteric coated, delayed release of peppermint oil should be formulated (Shams et al., 2015).

Peppermint as an antimicrobial agent

A considerable number of studies have shown that peppermint oil displays reasonable antimicrobial activity against human and plant microorganisms, such as some Staphylococcus species, Xanthomonas and Pseudomonas strains due to its menthol and menthone content (McKay & Blumberg, 2006).

Menthol content has activity against gastrointestinal tract Helicobacter pylori infection. However, 1,8-cineole does not demonstrate activity against respiratory and gastrointestinal tract infection (McKay & Blumberg, 2006).

Many oral care products such as toothpastes and mouthwashes contain peppermint due to its phenolic content, especially tannins and flavonoids, which have antibacterial activity to protect mouth from pathogens and maintain oral health (Pramila et al., 2012). An in vitro study showed that peppermint oil demonstrated activity against Streptococcus pyogenes and mutans that cause tooth decay by biofilm formation, and peppermint oil had superior efficacy to chlorhexidine to prevent plaque formation on teeth (Bone & Mills, 2013).

Peppermint oil also exhibited activity on herpes simplex virus types 1 and 2 by preventing the entrance of the virus into the cell before penetration occurs, thus destroying the virus directly (Bone & Mills, 2013). Moreover, peppermint oil may also be used as protective agent against pests to preserve foodstuff; in particular it repels rats, thus avoiding exposure to rat-borne parasites and viruses (Maia & Moore, 2011).

Peppermint as an antiallergenic and respiratory agent

A study conducted to demonstrate the antiallergenic activity of flavonoid glycoside luteolin-7-O-rutinoside fraction acquired from Mentha x piperita found peppermint to be effective in relieving the nasal problems associated with antigen-induced allergic rhinitis (McKay & Blumberg, 2006). An in vivo study demonstrated the bronchial smooth muscle relaxing effect of menthol on guinea pigs, whereby menthol considerably reduced airway restriction by bronchoconstrictor capsaicin and neurokinin A. Therefore, it represented antitussive activity against capsaicin-induced cough (McKay & Blumberg, 2006).

Another study showed that people with nasal congestion felt wide and fine nasal passages after menthol inhalation. Although there was no difference in nasal airflow, this illusory feeling was considered to associated with induced TRPM8 thermoceptor (cold receptor), which can be activated by cool temperatures or the compounds such as menthol (Bone & Mills, 2013).

Peppermint as analgesic

Peppermint is useful for alleviating muscle pain, headache and abdominal pain. A study showing the analgesic activity of menthol applied topically suggested that the activity was due to the stimulation of endogenous opioid system or partially its local anaesthetic activity (Bone & Mills, 2013).

CLINICAL EVIDENCE

Irritable bowel syndrome

A considerable number of clinical trials have been conducted to evaluate the effect of peppermint oil on relieving the symptoms of irritable bowel syndrome. A comprehensive study aiming the determination of safety and efficacy of enteric-coated peppermint oil capsules on the treatment of irritable bowel syndrome was performed for two weeks compared with placebo (Khanna, MacDonald & Levesque, 2014). All relevant literature was searched, including randomized-controlled trials up to February 2013 and Cochrane risk of bias tool was applied. Nine studies including 726 patients were subsequently assessed. As a result, enteric-coated peppermint oil capsules were found to be considerably more effective than placebo. However, patients taking peppermint oil capsules were prone to mild to moderate side effects such as heartburn. Peppermint oil has been found to be an efficient and safe short-term cure for irritable bowel syndrome patients (Khanna et al., 2014; Shams et al., 2015).

Functional dyspepsia

A placebo-controlled, double blind, multicentre four-week trial was conducted by giving 90mg of enteric-coated peppermint oil capsule and 50 mg of caraway oil three times a day with the patients who have non-ulcer dyspepsia with moderate to severe pain. After the four week trial, 63.2% of the patients reported having no pain and 26.3% of those had a decline in the severity of pain (Bone & Mills, 2013). Moreover, a double blind, randomised trial with 118 patients having functional dyspepsia was conducted with the same preparation. As a result, a 90 mg of enteric-coated peppermint oil capsule was found to have proportionate efficacy compared with the drug cisapride (Bone & Mills, 2013).
Peppermint oil demonstrated activity in reducing the endoscopy-induced spasm on lower and upper gastrointestinal tract, and people treated with barium enema. The antispasmodic effect of peppermint oil compared favourably with hyoscine-N-butylbromide in a controlled and randomized trial with 100 patients undergoing upper gastrointestinal tract endoscopy (McKay & Blumberg, 2006).

Analgesic

A double-blind, placebo controlled, randomized, crossover study was designed to assess the efficacy of 10g of peppermint oil in ethanol solution (90%) compared with 500 mg acetaminophen as a reference preparation and placebo (90% ethanol). 41 patients aged 18 to 65 experiencing tension-type headache according to International Headache Society (IHS) classification were evaluated for 164 headache attacks, four attacks per each patient. Two capsules of acetaminophen (1000mg) or placebo were given orally and peppermint oil preparation or placebo was applied topically for each attack. As a result, the analgesic effect of 10% peppermint oil applied topically to forehead was superior to the placebo solution. 10% peppermint oil solution showed a considerable decline in the tension-type headache intensity after 15 minutes, sustained over one hour. It did not, however, show any indicative difference compared with 1000 mg acetaminophen treatment. Furthermore, patients did not report any side effects in this study. It seems that peppermint oil is an economical and satisfactory alternative to usual analgesic medication in the treatment of tension-type headache (Göbel et al., 1996).

Respiratory conditions

In a placebo-controlled, randomized study, 23 patients having mild chronic asthma were treated with menthol vapour. Although menthol inhalation had no effect on acute bronchodilation, it showed an increase on airway hyper-responsiveness without changing the magnitude of airflow limitation in the long term (over four weeks). Therefore, menthol may be valuable remedy for the treatment of mild asthma (Tamaoki et al., 1995).

SAFETY

While there is no adverse reaction report on peppermint leaf, peppermint oil preparations when taken overdose are likely to have side effects such as heartburn, rashes, bradycardia, headaches, muscle tremor and ataxia (Bone & Mills, 2013).

Despite the fact that there is a lack of proven chronic toxicity studies of peppermint oil in humans, the German Commission E stated that patients with gallbladder, liver disorders, or bile duct obstructions should not use peppermint oil. Patients with hiatal hernia, gastrointestinal reflux or kidney stones are also advised not to take peppermint oil capsules (McKay & Blumberg, 2006).

Peppermint tea should be used with caution in people with iron deficiency, because it causes a decrease in the absorption of iron by 84% due to its tannin content; however, this compares to ubiquitous black tea, which impairs iron absorption by 79-94% (Bone & Mills, 2013).

When peppermint oil preparations are taken with other medications such as felodipine, simvastatin, antacids or cyclosporin, this may change the bioavailability of these drugs in the body as a result of the slowing effect of peppermint oil on intestinal transit (Bone & Mills, 2013; Edwards et al., 2015).

Bradycardia, fibrillation, laryngeal spasm or breathlessness may occur in the case of excessive menthol inhalation or sensitivity to menthol. Bronchial spasm risk is higher in children and babies (Bone & Mills, 2013). Although there is no toxicity report of peppermint oil use on breastfeeding and pregnant women, caution should be taken to be on the safe side (Bone & Mills, 2013).

Peppermint oil, particularly its menthol component, may cause allergic adverse reactions, gastrointestinal problems, severe esophagitis, stomatitis, pancreatitis, gastritis and diarrhoea as well as heartburn when taken in non-enteric-coated form. Pure peppermint oil may cause skin reactions and chemical burns (k2Bone & Mills, 2013). However, a patch study conducted by Kanerva et al. (2001) with 4000 patients showed that peppermint oil and menthol did not cause any irritation or allergic reactions (McKay & Blumberg, 2006). Peppermint oil seems to be a beneficial and safe treatment for the treatment of irritable bowel syndrome (Shams et al., 2015).

CONCLUSION

Peppermint is accepted by the British Pharmacopoeia, the European Pharmacopoeia and United States Pharmacopoeia-National Formulary and has been used for centuries to alleviate gastrointestinal discomfort such as spasm, irritable colon, bile duct issues; respiratory tract problems; oral mucosa inflammation; headache and allergic problems (Bone & Mills, 2013). Furthermore, peppermint is widely used in food industry as a flavouring agent and also in the cosmetic industry, particularly within oral care products such as mouthwashes, toothpastes and floss.

In the UK, peppermint oil has received Traditional Herbal Registration for the relieving the symptoms of minor digestive disorders and also Product Licence (PL) for the treatment of irritable bowel syndrome in the enteric-coated capsule form. (Bone & Mills, 2013; Edwards et al., 2015). Although likely to be efficient and safe for most conditions for which it is used, current available data is insufficient to support the effectiveness and safety of peppermint to gain general recognition for particular uses (Accessdata.fda.gov, 2016).

© Tugba Ucar, MSc student in Medicinal Natural Products and Phytochemistry

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