Hierarchy of Evidence in Essential Oil and Aromatherapy Research

by Jade Shutes

by: Erika Galentin, MNIMH, RH (AHG), Clinical Herbalist and Aromatherapist

In medical research, as interpreted by evidence-based healthcare practices, there is recognition of the fact that not all types of research are of equal value in regards to informing therapeutic opinions or making clinical decisions.1 Arguably the same is true for essential oil and Aromatherapy research; some research designs are more powerful than others in their ability to answer research questions on the efficacy of essential oil and/or aromatherapy interventions in humans. The ability of a research study to answer such questions rests upon multiple contributing factors, the study design and the quality of that study being front and center.2

Within the evidence-based practice interpretation of medical research, the quality, or robustness, of evidence is often regarded in terms of a ‘Hierarchy of Evidence’, the pyramidal graphical depiction of which indicates that not all research methodologies are considered equal in regards to the evidence they provide. This is especially the case with experimental studies, in contrast to observational studies, which seek to evaluate the efficacy of a pharmacological or therapeutic intervention, such as an essential oil, an essential oil component, or Aromatherapy as a practice.

The bottom of the pyramid demonstrates less robust and clinically relevant but more plentiful evidence than the top. In contrast, the top of the pyramid represents more robust and clinically relevant evidence which tends to be in much shorter supply. It is worth noting that there have been criticisms against such a rigid hierarchical structure, for example, that it underestimates the limitations of research designs higher up the pyramid and vice versa lower down the pyramid.3 That being said, it is important to note that this ‘Hierarchy of Evidence’ is meant to provide a general map or set of parameters for evaluating clinical relevance, rather than conceptualizing different types of research absolutely. In addition, there is no one universally agreed upon version of the hierarchy or its pyramidal depiction. Lastly, available research into essential oils, essential oil components, and Aromatherapy is predominantly represented by specific tiers within the evidence pyramid; in vitro studies, in vivo animal studies, randomized controlled trials (RCTs), and systematic reviews.

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Essential Oil Research and Aromatherapy Research are Different

It is pertinent to note that research into essential oils and their components (as chemical or pharmacological substances) and aromatherapy (as a therapeutic intervention) are two divergent yet interconnected research trajectories utilizing different research methodologies and procuring different research outcomes that vary widely in their clinical relevance.4 Research into the properties or pharmacology of essential oils and/or their isolated components focuses on the substance itself as the subject of inquiry. This is also the case with essential oil and component research coming from industries such as food flavoring and preservatives, perfume and fragrance, animal feed, and agronomy and tobacco.

The most common type of pharmacological research available in regards essential oils and/or their components is in the form of in vitro research or in vivo animal studies.5 Although these are important starting places in our journey towards the scientific understanding of the clinical efficacy essential oils and their components, these types of studies are widely discussed within the pharmacological sciences as lacking clinical relevance.6 & 7 In other words, results from these types of studies are just the starting line; they are too far a distance away from replicating human therapeutic use to solely support clinical or therapeutic decisions. A possible exception to the relevance of animal studies as applicable to humans is in regards to toxicology, whereby animal studies are considered the norm as testing toxicology, including lethal dose, is generally not possible or ethically permitted in humans.8

On the other hand, scientific research into Aromatherapy and its use of essential oils as a therapeutic intervention is generally represented by in vivo methods. This refers to both pre-clinical animal studies most commonly involving mice and other rodents (murine models) and human clinical studies (randomized controlled trials, or RCTs). Human RCTs, considered the ‘gold standard’ of medical research, are incredibly expensive endeavors, and finding funding sources that do not present a conflict of interest can be difficult.9 In addition, high quality RCTs are complex to design to ensure that bias has been minimized and the study is powered enough to produce statistically significant results that can be used to inform the practice of Aromatherapy.10 Due to these two factors, expense and design flaws, high quality randomized controlled trials represent the smallest body of research available to the essential oil and Aromatherapy industry.11 & 12 This is unfortunate, as high quality RCTs provide the data required to replicate clinical trials and/or design future clinical trials as well as perform strong systematic reviews and meta-analyses. Replicated high quality RCTs, and the systematic reviews and meta-analyses they inform, represent the highest tiers in the hierarchy of evidence and hence provide the most scientific support for the efficacy, application, and safety of essential oils and the practice of Aromatherapy as a therapeutic intervention.


  1. Hall, H.R. & Roussel, L.A. 2016. Evidence-Based practice: An integrative approach to research, administration, and practice (2nd ed.). Burlington, VT: Jones & Bartlett Learning.
  2. Man-Son-Hing, M., Laupactis, A., O’Rourke, K., Molnar, F.J., Mahon, J., Chan, K.B.Y, & Wells, G. 2002. Determination of the clinical importance of study results. Journal of General Internal Medicine, 17, 469-476. doi:  10.1046/j.1525-1497.2002.11111.x
  3. Concato, J. 2004.  Observation versus experimental studies: What’s the evidence for a hierarchy?  NeuroRx, 1(3), 341-347. https://doi.org/10.1602/neurorx.1.3.341
  4. Edris, A.E. 2007. Pharmaceutical and therapeutic potentials of essential oils and their individual volatile constituents: A review. Phytotherapy Research, 21, 308-323. doi:10.1002/ptr.2072
  5. Adorjan, B. & Buchbauer, G. 2010. Biological properties of essential oils: An updated review. Flavor and Fragrance Journal, 25, 407-426. doi:10.1002/ffj.2024
  6. Thiese, M.S. 2014. Observational and interventional study design types: An overview. Biochemia Medica, 24(2),199-210. doi:  10.11613/BM.2014.022
  7. Rothman, S. 2001. Lessons from the living cell: The limits of reductionism. New York, NY: McGraw Hill.
  8. Chow, P.K.H., Ng, R.T.H., & Ogden, B.E. 2008. Using animal models in biomedical research: A primer for the investigator. Singapore: World Scientific Publishing.
  9. Smith, R. 2005. Medical journals are an extension of the marketing arm of pharmaceutical companies. PLoS Medicine, 2(5), e138. https://doi.org/10.1371/journal.pmed.0020138.g001
  10. Jakobsen, J.C., Gluud, C., Winkel, P., Lange, T., & Wetterslev, J. 2014. The thresholds for statistical and clinical significance: A five-step procedure for evaluation of intervention effects in randomized clinical trials. BMC Research Methology, 14(34). https://doi.org/10.1186/1471-2288-14-34
  11. Koo, M. 2017. A bibliometric analysis of two decades of aromatherapy research. BMC Research Notes, 10:46. https://doi.org/10.1186/s13104-016-2371-1
  12. Lee, M.S., Choi, J., Posadzki, P., & Ernst, E. 2012. Aromatherapy for health care: An overview of systematic reviews. Maturitas, 71, 257-260. doi: 10.1016/j.maturitas.2011.12.018

About Erika Galentin

Erika is a Clinical Herbalist and an ITEC certified Clinical Aromatherapist consulting from Sovereignty Herbs in Athens & Columbus, OH. She holds a degree in Herbal Medicine from the University of Wales, Cardiff, UK and Scottish School of Herbal Medicine, Glasgow, UK. She is a professional member of the National Institute of Medical Herbalists(UK) and the American Herbalists Guild (USA).

A decade of clinical practice has provided a platform for a deep and influential understanding of the efficacy of medicinal plants within a clinical environment. It is through this clinical practice that Erika is able encourage positive, learned relationships between plants and people. She is a firm believer in celebrating the role humanity plays in the ecology of our landscapes and that our relationships with the natural world should be based upon reciprocity and exchange. In addition to clinical practice, Erika is both a student and teacher of horticulture and native medicinal plant conservation and ecology.

Erika teaches, lectures and writes on native medicinal plant conservation and applied ecology, propagation, herbalism, aromatherapy, and clinical efficacy. Former Course Development Director for the Herbal Academy, Erika is a guest instructor at the Ohio Herb Education Center, Gahanna, Ohio, Faculty at the Eclectic School of Herbal Medicine, Lowgap, NC, and Course Contributor for the School of Aromatic Studies. She also participates as a member of the Stewardship Committee for Appalachia Ohio Alliance, a non-profit organization dedicated to the conservation of land and water in Southeast Ohio.

You can follow Erika on her personal websitefacebook, and instagram.