As is the case with many other complementary and alternative therapies in the United States, the use of essential oils or aromatherapy has increased in recent years. The term “aromatherapy” can be confusing because it is used to describe a wide range of practices involving odorous substances. In order to advise and better inform patients, physicians and other health professionals should be able to differentiate between aesthetic applications of odors and clinical uses of essential oils (essential oil therapy). They also should be able to identify key issues regarding safety and efficacy. There is a growing body of evidence in the scientific literature suggesting that plant essential oils, alone or in combination with other therapies, may be beneficial in treating a number of health conditions. This article describes essential oil therapy and identifies key issues for practice.   

The term “aromatherapy” has become common household language in the United States. It is used in association with a wide variety of products, including coffee beans, scented candles, potpourri, and personal lubricants. For many people, aromatherapy can refer to the use of anything that has a pleasant odor, and the clinical definition of the term - specific and intentional health and disease applications of plant essential oils - is all but lost. Perhaps aromatherapy was never a good descriptor for the ways essential oils are used by health professionals because essential oils do not necessarily smell good nor are they always inhaled. For these reasons, I prefer the phrase “essential oil therapy.”

Essential Oils

Essential oils are secondary metabolites produced by and stored in certain plants for specific biological purposes. Each essential oil is composed of many different chemical constituents, primarily terpenes and terpenoid molecules. We do not fully understand the oils’ purposes in plants, but we know they include preventing and treating infections, healing wounds, and attracting or repelling insects, birds, and other animals through complex chemical mimicry.1,2 For example, certain plants appear to protect themselves from overgrazing by producing sesquiterpene lactones that are highly antimicrobial. If eaten by grazing mammals, these substances affect the normal gastrointestinal flora and interfere with digestion.

Essential oils are extracted from plant material by steam distillation of the plant part that contains the oil or, in the case of citrus fruits, by expression. Some clinical-grade essential oils are also obtained by CO2 extraction. The medicinal properties of essential oils may not parallel those of the plants they are derived from because essential oils are far more concentrated and contain largely those chemical constituents that are lipophilic, nonpolar, and fall within a specific molecular weight range.1,3,4

Essential Oil Therapy

In the context of Western health care, essential oil therapy refers to the intentional, evidence-based use of plant essential oils for preventive, therapeutic, or health promotion purposes. Essential oils also have cultural or spiritual applications, such as the perennial use of frankincense essential oil in spiritual practices or meditation. 

Plants and plant products such as essential oils have been used therapeutically for millennia. Such natural products fell out of favor in the last century, although there was renewed and continued interest in the clinical properties of essential oils during the 1930s in some Western countries, notably France and Australia.5,6 In the United States, essential oils were produced and used primarily by the cosmetics, food, and flavoring industries, rather than therapeutically.

One of the most important factors driving interest in essential oils is the increased public use of complementary therapies in general.7,8 In response, health sciences education and training in complementary therapies has expanded to allow professionals to stay abreast of practices and advise and treat patients safely, as well as to allow integration of such practices where appropriate.9-11 The University of Minnesota Academic Health Center (AHC) is involved in several initiatives aimed at curricular change and research training in complementary therapies. For information about the NIH-sponsored curriculum initiative see www.csh.umn.edu/NIHGrant/index.html, and for information about the Complementary and Alternative Medicine Clinical Research Fellowship Program, see www.csh.umn.edu/ Research/mnconsort.html, both Web sites of the university’s Center for Spirituality and Healing. 

In addition to growing public interest in essential oils, there is a growing body of scientific literature on the demonstrated health effects of plants and plant products as well as increased access to essential oils research conducted in other parts of the world. This has contributed to growing provider and researcher interest as possibilities for safe, inexpensive, and effective treatment approaches emerge. In my experience, health professionals who begin to explore the available scientific evidence on essential oils are surprised to learn that there are hundreds of different essential oils, each containing many chemical constituents; the oils can act singly and interactively to affect human health.12 

How Essential Oil Therapy Works

As far as we know scientifically, essential oils are absorbed and metabolized according to their chemical composition, dose, and route of administration, as is the case with other substances. Each essential oil is different, but there is considerable overlap in their actions. For example, most essential oils high in sesquiterpenes, such as German chamomile (Matricaria recutita) and myrrh (Commiphora myrrha) have some anti-inflammatory properties; most that are high in monoterpene alcohols, such as tea tree (Melaleuca alternifolia) or true lavender (Lavandula angustifolia), or that are high in phenols, such as thyme (Thymus vulgaris), have antimicrobial properties; and those high in esters, such as clary sage (Salvia sclarea) have calming and antispasmodic effects.1,3 The route of administration is chosen based on pathophysiology, desired outcome, safety and toxicity data, professional practice parameters, and cultural preferences. For example, although inhalation may be the best route of administration for treating respiratory symptoms and for affecting mood or cognition, topical application is likely to work best for burns, wound care, and most skin conditions. Some essential oils, such as Eucalyptus globulus, can be toxic even when a teaspoon is ingested; thus a provider would choose other routes of administration or other species of Eucalyptus that have similar therapeutic effects but present less danger of toxicity.4 Internal application of essential oils by suppository or oral ingestion is more common in places such as France, where essential oils are prescribed by physicians, while topical preparations and inhalation are more common in places such as the United Kingdom and the United States, where they are employed by nurses and other non-M.D. health care professionals.1,13-15

Applications

In general, public interest in the therapeutic uses of essential oils has grown much faster than the available research evidence to support it. There is, however, a growing body of published laboratory and human studies on specific essential oils used for selected health outcomes that can aid practitioners in advising patients and can provide direction for future clinical research. Two examples of essential oils that have been shown in some studies to be beneficial to patients are described below. A body of English-language research and a lengthy tradition of therapeutic use also exist for other essential oils, such as Lavandula angustifolia (true lavender). 

Mentha piperita (peppermint) essential oil has a long history of use as flavoring in the food industry and for therapeutic purposes.16 The complete essential oil and one of its major constituents, menthol, have been studied for their antimicrobial properties17-19 as well as their potential roles in reducing fatigue, pain, and nausea.20,21 Peppermint herb has been used for centuries as a digestive aid; recent studies support the use of M piperita essential oil in reducing spasm and pain related to gastrointestinal conditions and procedures.22-27 This action is thought to be related to its action as a calcium antagonist22 and may partially explain peppermint’s value in the treatment of nausea. Results of a study on the brain activity effects of peppermint odor as measured by electroencephalogram suggest that peppermint oil, when inhaled, acts as a mild stimulant independent of conscious mental associations.28 Although peppermint essential oil is a good candidate for further study because there is existing published research on its clinical properties, peppermint oil and its major constituents have been shown to produce toxic effects in animals and human infants, underscoring the need for caution and informed practice.4 As we know, natural does not equal safe.

Melaleuca alternifolia (tea tree) is another promising essential oil for research and clinical practice. Over the past 30 years, numerous studies have reported the efficacy of M alternifolia essential oil against a variety of pathogenic microorganisms.29-34 Many Staphylococcus aureus isolates (including methicillin-resistant S aureus) as well as other bacteria and fungi have been found to be susceptible to M alternifolia essential oil.35-42 Although there are some case studies and preliminary reports of the safety and efficacy of topically applied M alternifolia in treating infections in the health sciences literature, further clinical research is needed.33,34,39,43-45 It is also important to note that many plants are referred to as “tea tree,” but that the above applies only to M alternifolia. 

Considerations for Practice 

• It is important to ask patients about their use of essential oils when taking patient histories. Essential oils are most often used for infection control, wound care, pain relief, insomnia, nausea, inflammation, and anxiety, but they are also used for other conditions.1 

• Synergistic and antagonistic effects are possible with blends of essential oils as well as when combining essential oils and medications, although there are few published studies. In one laboratory study, a eucalyptus essential oil enhanced skin absorption of 5-fluorouracil 34-fold.46

• With few exceptions, essential oils are never applied in 100% concentrations. They are almost always used in low concentrations (1% to 10%), diluted in an appropriate carrier oil, gel, or aqueous preparation.

• Therapeutically intended essential oils should be identified on the label by genus and species (and sometimes chemotype or variety), part of the plant used, country where the plants were grown, and batch identifier. Therapeutic-quality essential oils are hard to locate and identify in the retail market, but some hospital departments and clinic pharmacies in the Twin Cities have developed relationships with companies they have found reliable and have begun to sell a limited selection of essential oils.

• Product source and chemical identification is of utmost importance in the clinical use of essential oils, as adverse effects have been linked to oxidized, synthetic, or adulterated products. Periodic gas chromatography and mass spectrometry analysis should be performed for quality assurance when essential oils are used in practice or research. As with any other natural product, designation as organically grown does not ensure that product integrity was maintained throughout processing and distribution.

• Essential oils should be stored in airtight, dark glass containers with integral drop dispensers in a cool place out of reach of children, much like other products that are used medicinally. Some essential oils, such as citrus oils, oxidize more readily than others, resulting in altered chemical composition and thus altered effects, including skin irritation and sensitivity. 

• Some essential oils, notably a few of the citrus oils such as bergamot (Citrus bergamia), contain furanocoumarins that can cause phototoxicity when applied to the skin prior to sun exposure. Other essential oils contain components that are suspected or known carcinogens, such as safrole, the major component of sassafras essential oil.4,47

Potential Promise

Essential oil therapy has great potential as a complementary approach to symptom management and, in some cases, treatment of disease for a wide range of health problems. Along with other treatments and, in some cases, alone, essential oils can add to the range of available therapies that can be integrated into Western biomedicine. Reports of sensitivity, irritation, and toxicity associated with essential oil use can be found in the medical literature, and it is not known whether these reports represent increased awareness and reporting or other factors. Essential oils not ingested and used in appropriate concentrations and applications have resulted in few known complications. With increasing public and professional interest in essential oils, however, there is a great need for laboratory and clinical research to expand and clarify the evidence base as well as for additional research on safety and toxicity. The World Health Organization (WHO) Guidelines for the Assessment of Herbal Medicines allows variation in the usual clinical trial path in the case of therapeutic substances that have a long history of apparently safe use. 48 some essential oils may fit this criterion.

Role of Essential Oils in Current Medical Practice

Essential oils should be approached using the same guidelines and cautions the physician or practitioner would use for other types of therapies. Physicians who want to include the use of essential oils in their practice might prepare to do so through independently reviewing the literature, participating in established courses, or partnering with a qualified and experienced individual. Conservatism is vital to protect patients; but it is also important to recognize that exposure to essential oils is neither new nor rare. Peppermint oil is a good example of an essential oil that has a history of use and holds promise for medicine but also warrants caution because of its potential side effects. Peppermint in a variety of forms can be found in toothpaste, chewing gum, teas, cigarettes, and countless other products and foods. Although it should be used in low concentrations on mucous membranes, overall, peppermint is considered a very safe substance. In high oral doses, however, peppermint oil has produced severe toxic reactions, and its major component, menthol, has been associated with apnea and collapse in infants, severe jaundice in babies and others with G6PD deficiency, and hepatotoxicity.4 This oil has been extensively studied by the food, cosmetics, and tobacco industries, but much of that research may be proprietary or in databases not usually searched for health information and thus not readily available to the health care community. Other essential oils are also ubiquitous as additives to foods and cosmetics, drawing on their well-known preservative (antimicrobial) properties.

Essential oils are already part of the therapeutic milieu and research programs in some long-term care facilities and hospitals in Minnesota, especially those with integrative medicine departments. Staff in facilities where there are essential oils programs report that lavender (Lavandula angustifolia) and Roman chamomile (Chamomaelum nobile) essential oils seem helpful in promoting sleep and reducing the need for nighttime sedation in elderly patients. They report that lavender also seems to help reduce agitation among some patients with dementia, resulting in lowered use of antipsychotic medications, although there are mixed reports in the literature.49 Tea tree (Melaleuca alternifolia) and other essential oils have reportedly been used to successfully treat chronic wounds in some facilities, and results are beginning to appear.50 Mandarin (Citrus reticulata) and ginger (Zinziber officinalis) have been used to stimulate appetite before meals. I include the above anecdotal accounts in order to give examples of current practice and to stimulate interest in further clinical research and safety studies.

There is currently no essential oils certification specific to the health professions, although there are proprietary aromatherapy programs that offer their own certification. Through the independent Aromatherapy Registration Council, individuals can be tested on basic knowledge of essential oils at independent testing sites throughout the United States, including the University of Minnesota; they can then use the title Registered Aromatherapist. Essential oils coursework at the University of Minnesota aims to provide foundational knowledge to health professionals who want to better inform their patients or use essential oils in practice. MM

Linda Halcón received her Ph.D. in epidemiology from the University of Minnesota in 1998. She is currently an assistant professor in the School of Nursing at the University of Minnesota and the 2002-2004 A. Marilyn Sime Faculty Research Fellow of the Center for Spirituality and Healing. 

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