Menthol is, together with camphor, the signature ingredient of Chinese and Southeast Asian medicated oils. It is the sharp, minty, cooling smell of Tiger Balm White, Po Sum On, Axe Brand, Yu Yee Oil, and virtually every “cooling” balm on the Asian market. It is also in toothpaste, chewing gum, cough drops, cigarettes, aftershave, and most topical analgesics sold in Western pharmacies. Few molecules have travelled so widely through so many industries.
And yet most people who use menthol-containing products daily have no idea what it actually does. “It’s cool” is correct but incomplete — menthol does not lower skin temperature meaningfully. The cool feeling is a hallucination produced by a nerve channel. This article explains how that works, what the real pharmacology is, what concentrations are safe, what the risks are in infants and G6PD-deficient individuals, and how to use menthol-containing medicated oils sensibly.
Menthol (chemical name: (1R,2S,5R)-2-isopropyl-5-methylcyclohexan-1-ol; CAS 2216-51-5 for L-menthol) is a cyclic monoterpene alcohol. At room temperature it is a waxy, white, crystalline solid with a sharp, unmistakable mint odour. It melts at 36-38°C — just below body temperature — which is why a crystal of menthol held in the hand dissolves within seconds.
Natural sources. Menthol is the main active compound in peppermint oil (Mentha piperita) and cornmint oil (Mentha arvensis). Cornmint is the dominant commercial source, particularly from India, which supplies over 80% of the world’s menthol. Traditional peppermint oil contains 30-55% menthol; commercial extraction concentrates it further.
Synthetic sources. Since the 1970s, synthetic menthol produced from thymol or citronellal has become widely available. Modern pharmaceutical menthol is largely synthetic and chemically identical to natural menthol, though natural peppermint oil contains other minor terpenes (menthone, pulegone) that give it a subtly different profile.
Stereochemistry matters. Menthol has multiple stereoisomers; only L-menthol (the naturally occurring form) produces the characteristic strong cooling effect. D-menthol and racemic menthol mixtures feel much less cool. Pharmaceutical-grade menthol is always L-menthol.
Menthol’s primary pharmacological target is TRPM8 (transient receptor potential melastatin 8), a cation channel found on sensory neurons in the skin and mucous membranes. TRPM8 is the body’s cold sensor — it normally opens when skin temperature drops below about 25-28°C, sending a “cold” signal to the brain.
Menthol activates TRPM8 at normal skin temperature. When you apply a menthol balm, the menthol diffuses into the skin, binds to TRPM8 on cold-sensitive nerve endings, and causes the channel to open as if the skin had been chilled. The nerve fires. The brain receives a cold signal. You feel cool.
But no actual cooling has occurred. Your skin temperature is unchanged (or may even rise slightly due to mild vasodilation). The cooling is a perceptual illusion generated entirely by the nerve channel. This is one of the cleanest examples in pharmacology of a molecule producing a sensation through direct ion channel mimicry.
Secondary targets. Menthol also weakly activates TRPV3 (warm receptor) at higher concentrations, which is why very strong menthol products can produce a simultaneous “cool then warm” sensation. And menthol has a weak desensitising effect on TRPA1 (the irritation receptor), contributing to its mild anaesthetic and antitussive action.
On nasal mucosa, inhaled menthol vapour activates TRPM8 on the trigeminal nerve endings in the nose, producing a strong sensation of increased airflow — even when objective nasal airway measurements show no change. Multiple clinical studies of menthol inhalation in people with colds confirm subjective symptom improvement but no objective decongestion. This is the same sensory substitution story as camphor, but for cold instead of warmth.
On cough, menthol has a mild antitussive effect that appears to work via TRPM8 activation on airway sensory nerves, raising the threshold for cough initiation. This is the basis of menthol cough drops and menthol-containing cold medicines.
Commercial medicated oils and balms vary widely in menthol content:
Western regulatory bodies are generally more permissive about menthol than camphor. The FDA allows up to 16% menthol in OTC topical analgesic products. The UK MHRA has similar guidance. Hong Kong registered proprietary Chinese medicines frequently reach 15-20%.
The short answer: yes, modestly, and mainly through sensory substitution rather than anti-inflammatory or tissue effects.
The mechanism is counterirritant analgesia. Menthol activates TRPM8 and generates a strong “cool” signal that travels along the same sensory pathways that would otherwise carry pain from an underlying muscle or joint. This additional signal partially masks the pain signal — an effect known as gate control in classical pain theory.
Clinical evidence. Randomised controlled trials of topical menthol for:
Menthol is not a disease-modifying treatment. It does not reduce inflammation meaningfully, it does not heal tissue, and it does not alter the underlying course of arthritis or injury. What it does is provide a safe, cheap, low-risk symptomatic relief that most users find genuinely helpful, particularly when combined with rest, gentle movement, and other supportive measures.
Comparison with topical NSAIDs. Topical diclofenac (Voltaren Gel) is a true anti-inflammatory — it reduces prostaglandin production locally — and in osteoarthritis trials it outperforms menthol modestly. But topical NSAIDs carry some GI and renal risk (even topically) and are more expensive. Menthol is safer for long-term intermittent use, which is why medicated oils have remained popular despite the arrival of modern topical NSAIDs.
For adults using menthol-containing medicated oils on themselves or other adults:
Intact skin only. Menthol is a mucous membrane irritant and should not be applied to broken skin, wounds, or dermatitis. Absorption through broken skin is much higher and can cause systemic effects.
Avoid mucous membranes. No eyes, no inside-the-nose, no inside-the-mouth, no genital areas. The sensation is intense and can cause real pain on contact. Corneal contact causes immediate burning and should be washed out with copious water.
Small quantities. A pea-sized amount per application. Large amounts on large body surface areas can cause significant systemic absorption.
Not under occlusion or heat. A menthol rub under a heating pad or occlusive bandage can cause chemical burns due to concentration effects.
Wash hands afterwards. Menthol residue on hands then transferred to eyes, nose, or children’s skin is a common source of accidental exposure.
Allergic sensitisation exists. Some people develop contact dermatitis to menthol or peppermint oil with repeated use. If a persistent rash appears under a menthol rub, stop and switch.
Menthol applied to the face, nose, or chest of a young infant can trigger laryngospasm — sudden closure of the vocal cords — which has caused respiratory arrest in infants. Several case reports document severe respiratory distress in babies rubbed with menthol-containing balms by well-meaning caregivers. The FDA, WHO, and most paediatric authorities recommend no menthol rubs on infants under 2 years at all.
The mechanism is thought to involve strong TRPM8 activation on laryngeal and tracheal sensory nerves, which in adults produces only a cooling sensation but in infants triggers a reflex protective closure. The infant respiratory tract is more reactive than the adult’s.
This applies equally to “baby” medicated oils that contain menthol at 4-6% — they are not safe for infants under 2. The traditional name 幼幼油 (literally “young-young oil”) is misleading; most formulations contain menthol and should be used only on older toddlers and children, not infants.
Menthol, like camphor, has been associated with haemolytic reactions in individuals with G6PD deficiency. The mechanism is oxidative stress on red blood cells. Reports are fewer and less dramatic than with naphthalene or high-dose primaquine, but caution is warranted. G6PD-deficient individuals and their caregivers should avoid menthol-containing products, particularly in infants and children. Hong Kong screens all newborns for G6PD, so parents of affected children are usually informed — but grandparents applying traditional oils may not realise the link.
Topical menthol at low concentrations is generally considered acceptable during pregnancy, but high-concentration medicated oils are not recommended. For breastfeeding women, avoid application to the breast (the infant can receive a dose through skin contact or oral ingestion from the nipple) and avoid the face before nursing.
Some people with asthma find that strong menthol inhalation triggers bronchospasm rather than providing relief. This is uncommon but documented. If menthol rubs worsen breathing, stop.
This question comes up because menthol is added to cigarettes, and there is evidence that menthol cigarettes are harder to quit than non-menthol cigarettes. Menthol itself is not pharmacologically addictive — there is no menthol withdrawal syndrome — but menthol appears to enhance the behavioural reinforcement of nicotine by making smoke easier to inhale and by pairing the sensory cooling with the nicotine reward. The US FDA has moved toward banning menthol cigarettes specifically for this reason.
Topical menthol in medicated oils carries none of this risk. Using Tiger Balm or Wong To Yick daily for chronic pain is not addictive in any meaningful sense, though users may develop psychological reliance on the sensation.
Menthol’s pharmacology outside of medicated oils is relevant context. In cigarettes, menthol’s local anaesthetic effect on the airway reduces the harshness of smoke and allows deeper inhalation — contributing to greater nicotine exposure per puff. In cough drops, it provides real sensory cough suppression plus the psychological reassurance of “doing something.” In chewing gum and toothpaste, it provides a flavour and a “cleanness” signal that is highly commercially valuable but has no dental benefit beyond what the toothpaste’s other active ingredients provide.
Understanding that all of these products share the same molecule helps users calibrate risk. A person who chain-chews menthol gum, smokes menthol cigarettes, uses menthol mouthwash, and applies menthol-containing medicated oil daily is receiving cumulative exposure that is worth thinking about, particularly if they also have reactive airways or G6PD deficiency.
Menthol is the molecule that makes cooling balms feel cool. It works by binding to the TRPM8 ion channel on skin and airway nerves, producing a cold signal without actual cooling. It has mild, genuine analgesic effects through counterirritation. In adults, topical use at 5-20% concentrations on intact skin is reasonably safe and widely beneficial for muscle aches, joint pain, and cold-symptom relief.
Do not use menthol products on infants under 2 years. Do not apply near eyes or mucous membranes. Do not combine with heating pads or occlusion. Wash hands after use. And if you are G6PD-deficient or caring for a G6PD-deficient child, avoid these products.
Used sensibly, menthol is one of the safest and most useful molecules in the traditional medicated-oil pharmacopoeia. Used carelessly, it has killed infants and caused chemical burns in adults. The rules are not complicated; they just need to be followed.
This article is part of the Medicated Oil Knowledge Hub, a free public reference on traditional Chinese and Southeast Asian herbal medicated oils. Information here is educational, not medical advice. For specific medical questions — especially involving children, asthma, or G6PD deficiency — consult a pharmacist or physician.