Isopulegol

Aroma and sensory profile

Isopulegol is menthol's rougher sibling. Fragrance references type it minty with medium strength and describe (−)-isopulegol at 10 percent as minty, cooling, medicinal, and woody, with a flavor of minty, cooling, mentholic, peppermint, grassy, herbal, and tropical [1]. The commercial isomer mixture reads wider: minty, cooling, eucalyptus, medicinal, woody, green, grassy, and herbal, or in a second panel's words minty, cooling, medicinal, woody with a green herbaceous undernote, with a flavor at 30 parts per million of minty cooling with a herbaceous peppermint nuance [2]. It cools the way menthol does but with a greener, more eucalyptus-like edge and less sweetness.

The molecule is a cyclic monoterpene alcohol with three stereocenters, so it has four diastereomers, each a pair of enantiomers: isopulegol, neoisopulegol, isoisopulegol, and neoisoisopulegol. The commercially important one is (−)-isopulegol, the (1R,2S,5R) form, because it is the intermediate from which most synthetic (−)-menthol is made [4]. Fragrance databases note that various stereoisomers occur in essential oils [2]. No panel description of (+)-isopulegol was found, so no claim about how the two enantiomers differ in odor is made here.

Effects and research

No human trial of isopulegol was found. The Research Institute for Fragrance Materials updated its safety assessment in 2022 [11], and the pharmacology is animal work, mostly from Brazilian groups.

The central nervous system data are the oldest. In mice, intraperitoneal isopulegol at 25 to 50 milligrams per kilogram produced anxiolytic effects in the elevated plus maze comparable to diazepam at 1 milligram per kilogram, without impairing motor activity, and it prolonged barbiturate sleep and increased immobility in depression models, a profile the authors called depressant and anxiolytic-like [5]. Against pentylenetetrazol convulsions it prolonged seizure latency similarly to diazepam and protected fully from mortality at higher doses, with GABA-A receptor involvement and reduced hippocampal lipid peroxidation [6].

The pain and inflammation data are more recent. Oral isopulegol at 0.78 to 25 milligrams per kilogram was dose-dependently antinociceptive in formalin, capsaicin, and glutamate models, reversed by naloxone, atropine, and methylene blue, implicating opioid, muscarinic, and nitric oxide pathways [7]. In paw edema and granuloma models it reduced albumin extravasation, leukocyte migration, interleukin-1 beta, and tumor necrosis factor alpha [8]. In paclitaxel-induced neuropathic pain, 25 to 100 milligrams per kilogram raised mechanical thresholds, an effect reversed by flumazenil and potentiated by MK-801, pointing to GABA-A and NMDA receptors [9]. These are model results at oral or injected doses, not evidence for inhaled or topical use.

Natural sources

Isopulegol is a mint-family and citrus-eucalyptus terpene. Fragrance databases record the (−) form in cornmint oil, lemon eucalyptus, Hesperozygis species, lemon balm, lemon leaf and petitgrain oils, lemongrass, mandarin, oregano, black pepper, and rosemary [1], and the isomer mixture in angelica seed, bergamot, several eucalyptus species, geranium, grapefruit, lemongrass, mandarin, pennyroyal, sage, and wormseed oils [2]. It is a minor constituent everywhere; its main route into the world is not a plant but a factory, where it is made from citronellal on the way to menthol [4]. In cannabis it is a trace terpenoid, occasionally reported in minty chemotypes and more often absent.

Boiling point and vaporization

PubChem compiles isopulegol's boiling point as 218 °C from the Joint FAO/WHO Expert Committee on Food Additives and 216 to 218 °C at 760 mmHg from fragrance references [3]. This profile uses 218 °C (424 °F), six degrees above the 212 °C the site previously listed. That is close to menthol and just above linalool, in the heart-note band, so it persists on skin and in a warm room rather than flashing off. In a still it is taken at 91 °C and 12 mmHg [1]. It is a combustible liquid with a flash point near 91 °C [1] and, as a monoterpene alcohol with one double bond, it oxidizes slowly in air. No thermal degradation study of isopulegol was found.

Safety and regulation

Isopulegol is listed by the US Food and Drug Administration as a permitted synthetic flavoring substance at 21 CFR 172.515, as "Isopulegol; p-menth-8-en-3-ol" [10]. The European Food Safety Authority found it safe in animal feed at 5 milligrams per kilogram, or 1 for cats [12]. The Research Institute for Fragrance Materials updated its assessment in 2022 [11]. No cancer classification applies.

Its hazard record is that of a mild irritant. Under Europe's classification inventory the main registration block flags it as harmful if swallowed in nearly all notifications and as a skin and eye irritant in about 85 percent; no sensitization code clears the threshold. A single separate notification carries suspected-carcinogen and organ-damage codes, which no other registrant echoes [3]. In practice isopulegol behaves like menthol: cooling and mildly irritating on skin, safe as a flavor at parts-per-million levels, and not a recognized fragrance allergen.

Industries and uses

Fragrance: isopulegol supplies a cooling minty-medicinal note in mint, eucalyptus, and herbal accords and is stocked mainly as the (−) enantiomer [1][2]. Culinary: it holds FDA flavoring status and contributes cooling peppermint nuances at 30 parts per million; it is a natural constituent of cornmint and lemongrass flavor [2][10]. Aromatherapy: cornmint and lemon eucalyptus oils are its carriers; the irritation profile is the relevant caution for topical use [3].