Scientific research struggles with testing essential oils for a range of reasons.
First, no generalized statement can be made about the efficacy of the oils in combatting any microbe, because there are well over 100 different oils on the market today. Essential oils are distilled or expressed from flowers, leaves, stems, bark, trunks, and resins, each with its own chemical composition. Even the oils from a specific plant can be different: for example, oil distilled from lavender is not the same as oil distilled from lavandin, as these are two distinct plant varieties from one botanical family.
Second, oils labeled from the same plant of origin may come from different geographic areas. Lavender gathered from a farm in Albuquerque, New Mexico, may not have the same properties as lavender grown in Provence, France. Differences in soil composition, fertilizer, and agricultural techniques may change the chemical structure of the oil.
The complexity of each oil’s composition also creates challenges for laboratory research. One bottle of lavender essential oil may have a different chemical makeup from another, even if they come from the same supplier. Plants gathered and processed in the morning may be altered somewhat when compared to plants harvested the night before. The components will be the same, but they may vary in concentration.
The oils’ effectiveness often varies from one study to the next, because the oil used in each study may be markedly different. With no federal regulation of what actually goes into each bottle, scientists struggle to be sure that the oil they test in a laboratory in California is the same as the one tested in India. Replicating results from one study to the next becomes an obstacle to success.
Researchers have also found it particularly challenging to conduct blinded studies with scented substances. Scents trigger emotions and memories, so if one group is tested using lavender and the control group is tested with a different scent, the second scent may inadvertently jostle a memory loose or create an emotional response in a control group subject. If the study is about cognitive ability or emotional reaction to a scent, the random triggered memory may distort the results of the study. If, on the other hand, the study is meant to test the physical therapeutic value of the lavender, the control group may be tested using an alternative scent that is known to have no positive or negative effect on the body. This may or may not lead to a calculable outcome.
To date, most studies have been phase I: in vitro (in a test tube), determining an oil’s effect on microbes or tissue; or phase II: on laboratory animals. A few studies have involved human subjects, but these have only a few participants, not the large populations involved in true phase III, double-blind testing of a drug or vaccine. These studies are too small to produce statistically significant results, so their findings are considered interesting but anecdotal. Currently, the best research involves reviews of many small studies, bringing together all the studies around the world and performing a meta-analysis of the results to draw conclusions. Such papers often reveal correlations that isolated studies cannot see.
A 2019 review of many studies, published in the June issue of the peer-reviewed journal Molecules, determined that while many essential oils do demonstrate antimicrobial properties against a wide range of diseases and infections, the success of one variety of an essential oil cannot be generalized to others of the same species. There are more than 400 species of thyme, for example, and while some of them have performed well in a test tube (not yet in humans) against viruses like herpes simplex and influenza H1N1, as well as both Gram-positive and Gram-negative bacteria, only specific thyme species have this capability. Such results could be used by unscrupulous sales channels to dupe the public into thinking that their thyme oil—whether or not it is the correct species—will help them ward off these viruses and bacteria. It will be some time before there is enough research to determine the correct species and the industry’s ability to produce large quantities of it, as well as the dosage and method of use that will make any specific essential oil marketable as a pharmaceutical.
This brings us to the biggest challenge involved in scientific research on essential oils: someone has to pay for the research, and the usual channels—pharmaceutical companies—have not taken a great deal of interest in doing so. As essential oils are natural substances, they cannot be patented as stand-alone drugs. This limits the ability of Big Pharma to make a profit with them in their pure form. If the limited research in progress does produce a positive result, pharmaceutical companies may eventually find ways to combine the oil with other products to create a patentable drug. This may accelerate the pace of continued study.
Essential oils have been in use for thousands of years, so researchers often want to take a shorter route to producing test results that reveal their effects on humans—that is, skipping the in vitro and animal testing steps and going right to testing them on humans. This makes traditional funding sources like the National Institutes of Health balk, as they prefer to fund research that follows the standard guidelines regardless of the substance being tested. Proposed studies are abandoned if the researchers can’t get the funds they need to move them forward, so essential oils languish, becoming the neglected stepchildren of medical research.