Menthol

Structure

As menthol (C10H20O, r = 156.3 g/mol) is the naturally occurring (-)- or L-menthol (levomenthol, levomentholum). The European Pharmacopoeia contains two monographs:

1. menthol Levomentholum
2. racemic menthol Mentholum racemicum

Menthol is a cyclic monoterpene alcohol. It has three asymmetric carbon atoms and occurs in four diastereomeric enantiomer pairs.

Stem plants

Menthol is found in plants of the genus. It is the major constituent of the essential oil of peppermint ( x L., Lamiaceae). It is produced synthetically or extracted from Japanese mint ( var. ).

Properties

Levomenthol (Figure) has a pleasant minty-fresh odor and is present as colorless, shiny prisms or needle-shaped crystals. Racemic menthol is present as a free-flowing or agglomerated crystalline powder or in the form of prismatic or acicular shiny crystals. Both substances are practically insoluble in water, very slightly soluble in ethanol 96% and petroleum ether, slightly soluble in fatty oils and liquid kerosene, and very slightly soluble in glycerol. Levomenthol melts at about 43°C, racemic menthol at 34°C. When menthol is mixed with camphor, thymol or borneol, liquid mixtures are formed. Safety statements: Xi irritant, R36: Irritating to eyes. Storage: keep well closed at room temperature between 15-25°C.

Effects

Preparations at low concentrations (dermal usually 1%, in nasal products 0.1%) have a cooling effect. High concentrations produce a warming to burning, irritating, and painful sensation and may result in increased sensitivity to cold.

  • Cooling and refreshing to the skin and mucous membranes.
  • Relieves itching
  • Analgesic
  • Local anesthetic
  • Antimicrobial
  • Carminative, antispasmodic, choleretic
  • Insecticidal (mosquitoes)

Mechanism of action

The sensation of cold is not triggered physically, but by binding of menthol to a cold receptor that is physiologically activated by cool temperatures. This is the cation channel TRPM8 of the TRP family of channels. TRPM8 is localized to free nerve terminals of afferent A and C fibers and is central to the sensation of cold. The same cold receptor is also activated by eucalyptol and icilin, leading to an increase in intracellular calcium concentration and the initiation of an action potential. Capsaicin also binds to a TRP channel, namely TRPV1 (transient receptor potential vanilloid subtype 1), which is activated by heat, but unlike menthol, it elicits a sensation of warmth.

Indications

For itchy skin conditions locally as a cooling and antipruritic agent, for example, for childhood diseases such as chickenpox, for cold sores, hemorrhoids, insect bites, and eczema. The preparations usually contain 1% menthol. For colds and nasal congestion in the form of inhaler pens, nasal remedies or cold balms. Menthol triggers a feeling of freshness in the nose when inhaled and has a subjective (but not objectifiable) relieving effect. It should not be used on infants or young children, as this can lead to respiratory failure. For cold symptoms and inflammation of the mouth and throat, for example in the form of lozenges, bronchial pastilles, in cold balms, baths and inhalants. For sports injuries, joint and muscle pain, for example, as a gel, cream, overlay or cold spray. For digestive problems and flatulence orally in small doses. Internally, however, peppermint oil or tea is usually used. As a deodorant for bad breath (e.g., Mints, Fisherman’s Friend). For headaches, applied locally as a headache oil or balm to the temples. Other uses: for example, in cigarettes, food, sweets, chewing gum, perfumes, cosmetics and toiletries.

Contraindications

Use in infants and young children. Children, pregnancy and lactation: according to the professional information. The preparations should not get into the eyes.

Interactions

Not known.

Adverse effects

Preparations

Menthol powder (Talcum cum levomentholo) is a mixture of talc and menthol and is used for weeping skin diseases and childhood diseases such as chickenpox or measles. Menthol Shake Brush (Suspensio alba cutanea aquosa cum levomentholo) is a mixture of white shake brush with menthol and is used for acute inflamed and itchy skin diseases. Menthol cold cream (Ungentum leniens cum levomentholo) is a mixture of cold cream with menthol and is used as a cooling agent in inflammatory and itchy skin conditions. Menthol spirit (Levomentholi solutio ethanolica, Spiritus mentholi) is a mixture of menthol, ethanol and water and can also be used for itchy skin conditions. The disadvantage is that it burns and dries out the skin in open house areas. The corresponding formulations can be found, for example, in the DMS. When prepared in the pharmacy, the menthol crystals can be crushed in a mortar or dissolved in a little ethanol 96%, so that they can be better incorporated into the bases.