Menthol is an organic compound with the chemical formula C10H20O that occurs naturally in mint and some other plants. It can be extracted from the leaves by distillation, but is more commonly made synthetically. Pure menthol is a crystalline solid, but it is often used in the form of peppermint oil. It produces a sensation of coolness in the mouth or when applied to the skin and can act as a mild anesthetic. The compound is widely used in cough and cold remedies because of its soothing effects and as a flavoring in candy, chewing gum, medical products and cigarettes.
Mint plants synthesize this compound in their leaves, possibly as a natural insecticide or to discourage predators, many of which appear to dislike the smell. The compound was isolated from peppermint oil in 1771 in Europe, but it may have been in use in Japan in much earlier times. Industrially, it can be obtained from mint leaves by steam distillation, but most menthol is now made synthetically by a complex, but more economical, process. Amateur essential oil enthusiasts can extract impure peppermint oil from the leaves of the plant using grain alcohol or vodka, separating the oil by freezing, but further processing would be required to isolate the pure compound.
Properties and Effects
Although it is a solid at room temperature, menthol can be melted with warm water and readily produces a strong-smelling vapor. It is only slightly soluble in water, but dissolves easily in many organic solvents, including alcohol. Although it is of very low toxicity, it has a number of noticeable effects on the body, which have led to a variety of therapeutic uses.
Menthol stimulates the body’s cold receptors, producing a cooling sensation when it is inhaled or applied to the skin. As with the chemical capsaicin found in chili peppers, which stimulates the heat receptors, the compound does not actually change the skin’s temperature, but merely produces a feeling of cold. The anesthetic properties of menthol are thought to be due to the fact that it binds to kappa opioid receptors. These are cells found in the brain, spinal cord and neurons; among other things, they help control the perception of pain.
Another important property of this compound is that it can act as a counterirritant, that is, a substance that causes mild irritation or inflammation in one place, distracting attention from pain in another. It also increases the penetration of medications applied to the skin. Menthol suppresses the urge to cough and creates a sensation of increased openness in the nasal passages in people suffering from nasal congestion, although it appears to have no actual decongestant effect.
The cooling sensation produced by this compound, along with its anesthetic and counterirritant properties, has led to its use in products intended to relieve skin irritation, sore throat, or nasal congestion. It may be used to treat sunburn, fever, or muscle aches as well. Most products used to relieve these conditions contain only small amounts of the compound. Pure crystals, however, can be used with warm water to relieve cold symptoms through the release of vapor. It should be used with caution, as inhalation in excessive amounts can cause pain in the nasal passages.
In traditional Asian medicine, menthol is sometimes prescribed to treat nausea, diarrhea, indigestion, headache, cold or sore throat. When used as a supplement for health reasons, it is usually taken in the form of peppermint oil. Products that commonly contain it include toothpaste, cough drops, lip balm, mouthwash and chewing gum.
Menthol is sometimes added to cigarettes to improve flavor and give a sensation of coolness. There is evidence that people who smoke these cigarettes are more likely to suffer from serious health problems, although it is not generally thought that this is due to any direct effects of the chemical. It has been argued that the flavoring may make cigarettes more palatable to young people. A study in 2012 suggested that youngsters who smoke menthol cigarettes are 80% more likely to become addicted. It is also possible that people who smoke these cigarettes may inhale more deeply and retain the smoke in their lungs for longer, increasing the quantities of toxic and carcinogenic compounds that are absorbed.
A relatively recent use for the compound is as a natural pesticide. It is an active ingredient in some products used to control mite infestations on honeybees. Used at the right concentration, it is effective in killing the mites, but not harmful to the bees. As of 2013, given the level of interest in the use of natural products in the control of pests, this compound may find further similar uses.