Peppermint and eucalyptus oil - two oils that are so often used in conjunction for muscle pain and soreness that I shoved em both together in one review.
Peppermint oil is extracted from the leaves of the peppermint plant, which is actually a hybrid mint - a cross between watermint and spearmint. It is originally native to Europe and the Middle East, but now cultivated and grown all over the world and almost all climates. Sometimes peppermint is found growing in the wild alongside its parent plants, but being a hybrid, it is typically sterile and produces no seeds. Peppermint spreads in vegetation quickly through its root system.
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Eucalyptus oil is extracted from the leaves of wide genius of flowering trees and scrubs in the myrtle family, Myrtaceae. There are more than 700 species of eucalyptus, almost entirely native to Australia, and now cultivated in many tropical or temperate climates.
The oil is an important component in Aboriginal medicine as a treatment to encourage the healing of wounds and fungal infections. Today eucalyptus is included in a variety of over-the-counter products that seek to relieve congestion, or relieve muscle and joint pain. It's also an ingredient in some mouthwashes due to its antiseptic properties.
RESEARCH ON PEPPERMINT OIL
At this time, there is limited research specifically on peppermint oil's topical applications. Much research is focused on its possible relieving effects of irritable bowl syndrome.
However, there is an established body of research on menthol, which is found in high concentrations in peppermint oil.
Menthol enhances cooling sensations via stimulating the cold-sensitive transient receptor potential channel melastatin 8 (TRPM8). This receptor is responsible for the well-known cooling sensation it provokes when inhaled, eaten, or applied to the skin.
It also has weak analgesic (pain-killing) effects due to its selective activation of our κ-opioid receptor. This protein mediates many effects including changing our perception of pain, consciousness, motor control, and mood.
RESEARCH ON EUCALYPTUS OIL
Cineole is the active therapeutic component in eucalyptus oil. It is a colorless, oily, slightly water-soluble liquid terpene ether with a wide range of uses. The eucalyptus plant boasts one is one of the highest concentrations of cineole, and therefore, the word cineole is also synonymous with eucalyptol. There are many studies on various effects of eucalyptus, but for the this article I'm going to focus on its potential topical pain relieving aspects.
A study in 2008 concluded eucalyptus oil, "is able to implement the innate cell-mediated immune response," and that, "this might drive development of a possible new family of immuno-regulatory agents, useful as adjutant in immuno-suppressive pathologies, in infectious disease and after tumour chemotherapy." An older study from 2000 that examined cineole's effects on mice also concluded there may be potential anti-inflammatory properties.
Although the oil has long been known for its anti-inflammatory properties via our transient receptor potential (TRP) channels (aka cell structures that mediate a variety of sensations like temperature, pressure to the brain)... until a study in 2017, it was unclear which of the channels mediates the oil's effects (aka how exactly it works). The control mice in the study were genetically altered to have their TRPM8 channels removed, and in those mice the anti-inflammatory results (effects comparable with those of ibuprofen) were abolished. The study also noted that TRPM8 channels are more sensitive in humans that mice, explaining the hyper potency of eucalyptus in people. TRPM8... why that sounds familiar! That is also the mechanism in which menthol (found in high concentrations in peppermint oil) helps to reduce pain. Ah, match made in heaven.
But wait! A study in 2017 indicated an interaction between eucalyptol and another transient receptor potential (TRP) channel - this one being TRPV1, which detects body temperature, scalding heat, and nociceptive (harmful) pain.
I found an established body of research that may suggest the use of peppermint oil (menthol) and eucalyptus (eucalytol) for mild pain relief and anti-inflammatory effects. Given the very low risk and very limited side effects, it would seem worth a try for individuals seeking a complementary and/or natural product for pain relief.