Eugenol

Research & Evidence

Eugenol is an aromatic phenylpropene — a volatile phenol — best known as the principal component of clove oil, but found across many of the herbs in this database. Among the source-herb monographs it is one of the better-characterised single constituents, with several distinct activities described:

• Analgesic (GABA-A modulation) — eugenol has been shown to interfere with GABA-A receptor signalling, inhibiting GABA-induced currents in trigeminal-ganglion neurons and in α1β2γ2 subtype-expressing cells ^{1Reference 1Lee SH et al. · 2015Eugenol Inhibits the GABAA Current in Trigeminal Ganglion NeuronsView study →}. Because GABA is central to how pain is perceived and modulated in the central nervous system, this is proposed as a mechanism behind eugenol’s long-standing use as a pain reliever — a use reflected in its history in dentistry and in the traditional use of clavo huasca root resin for toothache.
• Antibacterial and antibiotic-synergist — eugenol is active against pathogens including Escherichia coli, Listeria monocytogenes, Campylobacter jejuni, Salmonella enterica, Staphylococcus aureus and Helicobacter pylori, acting primarily by disrupting the cytoplasmic membrane ^{2Reference 2Hemaiswarya S · 2009Synergistic interaction of eugenol with antibiotics against Gram negative bacteria}. By damaging the bacterial plasma membrane it also potentiates conventional antibiotics: bacteria pre-treated with eugenol become more susceptible to hydrophilic antibiotics that are otherwise ineffective against gram-negative organisms, and eugenol acts synergistically with carvacrol and cinnamaldehyde ^{2Reference 2Hemaiswarya S · 2009Synergistic interaction of eugenol with antibiotics against Gram negative bacteria}. This membrane-disrupting, “resistance-modifying” behaviour is also reported alongside Turneraceae extracts against methicillin-resistant Staphylococcus aureus (MRSA) ^{5Reference 5Coutinho et al. · 2009Herbal therapy associated with antibiotic therapy: potentiation of the antibiotic activity against methicillin – resistant Staphylococcus aureus by Turnera ulmifolia LView study →}.
• Antifungal — eugenol produces broad antifungal effects against wood-decaying, plant-pathogenic and human-pathogenic fungi (including Botrytis cinerea) by disrupting fungal cell walls and membranes ^{3Reference 3Wang C et al. · 2010Antifungal activity of eugenol against Botrytis cinereaView study →}. More specifically, it has been shown to perturb the dual aromatic and branched-chain amino-acid permeases in the cytoplasmic membrane of yeast ^{4Reference 4Darvishi et al. · 2013The Antifungal Eugenol Perturbs Dual Aromatic and Branched-Chain Amino Acid Permeases in the Cytoplasmic Membrane of YeastView study →}.
• Aphrodisiac / smooth-muscle relaxant — in a related eugenol-rich species (Tynanthus micranthus), the constituent is suggested to underlie improvements in sexual potency, with researchers pointing to eugenol’s reported vasodilating and smooth-muscle-relaxant effects and noting that other eugenol-containing plants such as clove and nutmeg share documented aphrodisiac use ^{6Reference 6Cansian F et al. · 2014AnimalAphrodisiac properties of Tynanthus micranthus Corr. & Mello exView study →}.

This is a representative summary; the references below are the studies cited on the source-herb pages that genuinely pertain to eugenol.

Toxicity & Safety

At the concentrations found in herbal preparations, eugenol is well tolerated, and it has a long record of human exposure through cooking spices (clove, nutmeg, cinnamon) and through dental use. It is the activity that makes it useful — membrane disruption and phenolic reactivity — that also sets its limits: eugenol is a concentrated volatile phenol and can irritate skin and mucous membranes when applied undiluted, and in large overdoses (most relevant to ingestion of clove oil rather than whole-herb preparations) it has been associated with liver injury. Used as a minor constituent of the herbs listed here, no source flags it as more than low in toxicity, but concentrated eugenol or clove oil should be handled with the care given to any potent essential-oil phenol.

References

1. Lee SH, Moon JY, Jung SJ, Kang JG, Choi SP, Jang JH. (2015). Eugenol Inhibits the GABAA Current in Trigeminal Ganglion Neurons. PLOS ONE, 10(1), e0117316. doi:10.1371/journal.pone.0117316
2. Hemaiswarya S, Doble M. (2009). Synergistic interaction of eugenol with antibiotics against Gram negative bacteria. Phytomedicine. 16. 997-1005.
3. Wang C, Zhang J, Chen H, Fan Y, Shi Z. (2010). Antifungal activity of eugenol against Botrytis cinerea. Tropical Plant Pathology. 35. 3. http://dx.doi.org/10.1590/S1982-56762010000300001
4. Darvishi, E., Omidi, M., Bushehri, A. A., Golshani, A., & Smith, M. L. (2013). The Antifungal Eugenol Perturbs Dual Aromatic and Branched-Chain Amino Acid Permeases in the Cytoplasmic Membrane of Yeast. PLoS ONE, 8(10), e76028. doi:10.1371/journal.pone.0076028
5. Coutinho, H. D., Costa, J. G., Lima, E. O., Falcão-Silva, V. S., & Siqueira, J. P. (2009). Herbal therapy associated with antibiotic therapy: potentiation of the antibiotic activity against methicillin – resistant Staphylococcus aureus by Turnera ulmifolia L. BMC Complementary and Alternative Medicine, 9(1), 13. doi:10.1186/1472-6882-9-13
6. Cansian F. C, Merino F. J. Z, Amaral V. L. L, Salvador R. A, Campos P. M, Montrucchio D. P, Miguel O. G, and Miguel M. D. (2014). Aphrodisiac properties of Tynanthus micranthus Corr. & Mello ex. Schum in male mice. African Journal of Pharmacy and Pharmacology. 8. 47. DOI: 10.5897/AJPP2014.4149