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Home > Health Library > Aromatherapy With Essential Oils (PDQ®): Integrative, alternative, and complementary therapies - Health Professional Information [NCI]
This information is produced and provided by the National Cancer Institute (NCI). The information in this topic may have changed since it was written. For the most current information, contact the National Cancer Institute via the Internet web site at http://cancer.gov or call 1-800-4-CANCER.
This cancer information summary provides an overview of the use of aromatherapy with essential oils primarily to improve the quality of life of cancer patients. This summary includes a brief history of aromatherapy, a review of laboratory studies and clinical trials, and possible adverse effects associated with aromatherapy use.
This summary contains the following key information:
Many of the medical and scientific terms used in the summary are hypertext linked (at first use in each section) to the NCI Dictionary of Cancer Terms, which is oriented toward nonexperts. When a linked term is clicked, a definition will appear in a separate window.
Reference citations in some PDQ cancer information summaries may include links to external websites that are operated by individuals or organizations for the purpose of marketing or advocating the use of specific treatments or products. These reference citations are included for informational purposes only. Their inclusion should not be viewed as an endorsement of the content of the websites, or of any treatment or product, by the PDQ Integrative, Alternative, and Complementary Therapies Editorial Board or the National Cancer Institute.
Aromatherapy is a derivative of herbal medicine, which is itself a subset of the biological or nature-based complementary and alternative medicine (CAM) therapies. Aromatherapy has been defined as the therapeutic use of essential oils from plants for the improvement of physical, emotional, and spiritual well-being.
Essential oils are volatile liquid substances extracted from aromatic plant material by steam distillation or mechanical expression. Essential oils produced with the aid of chemical solvents are not considered true essential oils because the solvent residues can alter the quality of the essential oils and lead to adulteration of the fragrance or to skin irritation.
Essential oils are made up of a large array of chemical components that consist of the metabolites found in various plant materials. The major chemical components of essential oils include monoterpenes, esters, aldehydes, ketones, alcohols, phenols, and oxides, which are volatile and may produce characteristic odors. Different types of essential oils contain varying amounts of each of these compounds, which are said to give each essential oil its particular fragrance and therapeutic characteristics. Plant species may have different chemovarieties (variations of subspecies that produce essential oils with different chemical compositions, as a result of genetic variation and growth conditions). Thus, their essential oils can occur as several chemotypes that differ in chemical composition and may produce different clinical effects. It should be noted that essential oils are distinctly different chemically from (fatty) oils, such as those used as food. As essential oils age, the chemical composition changes. Many essential oils do not have an expiration date.
Synthetic odors are often made up of many of the same compounds that are components of the essential oils. These compounds are synthesized and typically combined with other odor-producing chemicals. However, synthetic fragrances frequently contain irritants, such as solvents and propellants, that can trigger sensitivities in some people.[2,3,4] Most aromatherapists believe that synthetic fragrances are inferior to essential oils because they lack natural or vital energy; however, this has been contested by odor psychologists and biochemists.
Aromatherapy is used or claimed to be useful for a vast array of symptoms and conditions. A book on aromatherapy in children suggests aromatherapy remedies for everything from acne to whooping cough. Published studies regarding the uses of aromatherapy have generally focused on its psychological effects as a stress reliever or anxiolytic agent or its use as a topical treatment for skin-related conditions.
A large body of literature has been published on the effects of odors on the human brain and emotions. Some studies have tested the effects of essential oils on mood, alertness, and mental stress in healthy participants. Other studies investigated the effects of various (usually synthetic) odors on task performance, reaction time, and autonomic parameters or evaluated the direct effects of odors on the brain via electroencephalogram patterns and functional imaging studies. Such studies have consistently shown that odors can produce specific effects on human neuropsychological and autonomic function and that odors can influence mood, perceived health, and arousal. These studies suggest that odors may have therapeutic applications in the context of stressful and adverse psychological conditions.
Practitioners of aromatherapy apply essential oils using several different methods, including the following:
Other direct and indirect applications include mixing essential oils in bath salts and lotions or applying them to dressings.
Different aromatherapy practitioners may have different recipes for treating specific conditions, involving various combinations of essential oils and methods of application. Differences seem to be practitioner dependent, with some common uses more accepted throughout the aromatherapy community. Training and certification in aromatherapy for lay practitioners is available at several schools throughout the United States and United Kingdom; however, there is no professional standardization in the United States and no license is required to practice in either country. Thus, there is little consistency among practitioners in the specific treatments used for specific illnesses. This lack of standardization has led to variability in therapeutic protocols used in research on the effects of aromatherapy. Anecdotal evidence alone or previous experience has driven the choice of essential oils and different researchers choose different essential oils when studying the same applications. However, now there are specific courses for licensed health professionals that give nursing or continuing medical education credit hours, including a small research component and information about evaluating and measuring outcomes.
The National Association for Holistic Aromatherapy (NAHA) (www.naha.org/) and the Alliance of International Aromatherapists (www.alliance-aromatherapists.org) are the two governing bodies for national educational standards for aromatherapists. NAHA is taking steps toward standardizing aromatherapy certification in the United States. Many schools offer certificate programs approved by NAHA. A list of these schools can be found on the NAHA website (https://naha.org/index.php/education/approved-schools/). National examinations in aromatherapy are held twice per year.
The Canadian Federation of Aromatherapists has established standards for aromatherapy certification in Canada (www.cfacanada.com/). They also have standards for safety and professional conduct and a public directory of certified aromatherapists. Other countries may have similar organizations.
Although essential oils are given orally or internally by aromatherapists in France and Germany, their use is generally limited to inhalation or topical application in the United Kingdom and United States. Nonmedical use of essential oils is common in the flavoring and fragrance industries. Most essential oils have been classified as GRAS (generally recognized as safe), at specified concentration limits, by the U.S. Food and Drug Administration (FDA). See the International Federation of Aromatherapists website (www.ifaroma.org/) for a list of international aromatherapy programs.
Aromatherapy products do not need approval by the FDA.
Proponents of aromatherapy report that aromatic or essential oils have been used for thousands of years as stimulants or sedatives of the nervous system and as treatments for a wide range of other disorders. They link it historically to the use of infused oils and unguents in the Bible and ancient Egypt, remedies used throughout the Middle Ages and the Renaissance, and the burning of aromatic plants in various religious rites. The current applications of aromatherapy did not come about until the early 20th century when the French chemist and perfumer Rene Gattefosse coined the term aromatherapy and published a book of that name in 1937. Gattefosse proposed the use of aromatherapy to treat diseases in virtually every organ system, citing mostly anecdotal and case-based evidence.
Although Gattefosse and his colleagues in France, Italy, and Germany studied the effects of aromatherapy for some 30 years, its use went out of fashion midcentury and was rediscovered by another Frenchman, a physician, Jean Valnet, in the latter part of the century. Valnet published his book The Practice of Aromatherapy in 1982, at which time the practice became more well-known in Britain and the United States. Through the 1980s and 1990s, as patients in Western countries became increasingly interested in complementary and alternative medicine (CAM) treatments, aromatherapy developed a following that continues to grow to this day. In addition to the use of essential oils by nurses and aromatherapy practitioners for specific medical issues, the popularity of aromatherapy has also been exploited by cosmetics companies that have created lines of essential oil-based (although often with a synthetic component) cosmetics and toiletries, claiming to improve mood and well-being in their users.
Despite the growing popularity of aromatherapy in the latter part of the 20th century (especially in the United Kingdom), little research on aromatherapy was available in the English-language medical literature until the early or mid-1990s. The research that began to appear in the 1990s was most often conducted by nurses, who tended to be the primary practitioners of aromatherapy in the United States and United Kingdom (although it is dispensed by medical doctors in France and Germany). Aromatherapists now publish their own journal, the International Journal of Essential Oil Therapeutics. Also, many studies regarding the effects of odor on the brain and other systems in animals and healthy humans have been published in the context of odor psychology and neurobiology (and in the absence of the specific term aromatherapy).
In addition to topical antimicrobial uses, aromatherapy has also been proposed for use in wound care [5,6] and to treat a variety of localized symptoms and illnesses such as alopecia, eczema, and pruritus.[7,8,9] Aromatherapy has also been studied via inhalation for airway reactivity.
Studies on aromatherapy have examined a variety of other conditions, including the following:
Published articles have described the use of aromatherapy in specific hospital settings such as cancer wards, hospices, and other areas where patients are critically ill and require palliative care for the following symptoms:
In addition, several observational studies provide examples of the clinical uses of aromatherapy (and other CAM modalities), although they are generally not evidence based. Participants have included hospitalized children with HIV, homebound patients with terminal disease, and hospitalized patients with leukemia. Aromatherapy has also been used to reduce malodor of necrotic ulcers in cancer patients.
Studies of aromatherapy use with mental health patients have also been conducted. Most of the resulting articles describe successful incorporation of aromatherapy into the treatment of these patients, although outcomes are clearly subjective.
Theories about the mechanism of action of aromatherapy with essential oils differ, depending on the community studying them. Proponents of aromatherapy often cite the connection between olfaction and the limbic system in the brain as the basis for the effects of aromatherapy on mood and emotions; less is said about proposed mechanisms for its effects on other parts of the body. Most of the aromatherapy literature, however, lacks in-depth neurophysiological studies on the nature of olfaction and its link to the limbic system, and it generally does not cite research that shows these links. Proponents of aromatherapy also believe that the effects of the treatments are based on the special nature of the essential oils used and that essential oils produce effects on the body that are greater than the sum of the individual chemical components of the scents.
These assertions have been contested by the biochemistry and psychology communities, which take a different view of the possible mechanism of action of odors on the human brain (most do not differentiate the odors produced by essential oils from those of synthetic fragrances). This neurobiological view, which focuses mostly on the emotional and psychological effects of fragrances (as opposed to the other symptomatic effects claimed by aromatherapists), takes into account what is known about olfactory transduction and the connection of the olfactory system to other central nervous system functions, including memory; however, it is primarily theoretical because of the lack of significant research addressing this topic.
Numerous studies on the topical antibacterial effects of essential oils have been published; most have found the essential oils to have significant antimicrobial activity. Some essential oils are antiviral and inhibit replication of the herpes simplex virus. Other essential oils are fungistatic and fungicidal against both vaginal and oropharyngeal Candida albicans.
Studies on rats in Europe and Japan have shown that exposure to various odors can result in stimulation or sedation, as well as changes in behavioral responses to stress and pain. A study  on the sedative effects of essential oils and other fragrance compounds (mostly individual chemical components of the essential oils) on rat motility showed that lavender oil (Lavandula angustifolia Miller [synonyms: Lavandula spicata L.; Lavandula vera DC.]) in particular had a significant sedative effect, and several single-oil constituents (as opposed to whole essential oils) had similarly strong effects. The authors do not comment on the presumed mechanism for this effect. The differences in bioavailability are ascribed to different levels of lipophilicity, with the more lipophilic oils producing the most sedative effects. The researchers also found significant plasma levels of the fragrance compounds after inhalation, suggesting that the effects of aromatherapy result from a direct pharmacological interaction rather than an indirect central nervous system relay.
Other studies have investigated the effects of aromatherapy on rats' behavioral and immunological responses to painful, stressful, or startling stimuli. In two European studies, rats exposed to pleasant odors during painful stimuli exhibited decreased pain-related behaviors, with some variation in response between the sexes.[5,6] Two studies from Japan showed an improvement in immunological and behavioral markers in rats exposed to fragrances while under stressful conditions.[7,8]
No studies in the published peer-reviewed literature discuss aromatherapy as a treatment for cancer specifically. The studies discussed below, most of which were conducted in patients with cancer, primarily focus on the following:
These studies purport to test the efficacy of aromatherapy, implying that the products used contain essential oils; however, only an occasional reference article includes significant descriptive information about the product(s) used (e.g., composition, source) thereby greatly limiting the ability of interested clinicians and researchers to compare or duplicate studies or produce meaningful meta-analyses of the research results.
Anxiety and Depression
A major review published in 2000  focused on six studies investigating treatment or prevention of anxiety with aromatherapy massage. Although the studies suggested that aromatherapy massage had a mild transient anxiolytic effect, the authors concluded that the research done at that time was not sufficiently rigorous or consistent to prove the effectiveness of aromatherapy in treating anxiety. This review excluded trials related to other effects of aromatherapy (such as pain control) and did not include any studies looking at the effects of odors that were not specifically labeled as aromatherapy.
Another randomized controlled trial investigated the effects of massage or aromatherapy massage in 103 cancer patients who were randomly assigned to receive massage using a carrier oil (massage group) or massage using a carrier oil plus the Roman chamomile essential oil (Chamaemelum nobile [L.] All. [synonym: Anthemis nobilis L.]) (aromatherapy massage group). Two weeks after the massage, a statistically significant reduction was found in anxiety in the aromatherapy massage group (as measured by the State-Trait Anxiety Inventory) and improvement in symptoms (as measured by the Rotterdam Symptom Checklist [RSCL]). The subscales with improved scores were psychological, QOL, severe physical, and severe psychological. The massage-only group showed improvement on four RSCL subscales; however, these improvements did not reach statistical significance.
A study that evaluated an aromatherapy service following changes made after an initial pilot at a U.K. cancer center, also reported on the experiences of patients referred to the service. Of the 89 patients originally referred, 58 patients completed six aromatherapy sessions. Significant improvements in anxiety and depression (as measured by the Hospital Anxiety and Depression Scale [HADS]) were reported at study completion in comparison with measurements taken before the six sessions.
A placebo-controlled, double-blind, randomized trial conducted in Australia investigated the effects of inhalation aromatherapy on anxiety during radiation therapy. A total of 313 patients who received radiation therapy were randomly assigned to one of three groups (carrier oil with essential oils, carrier oil only, or pure essential oils of lavender, bergamot (Citrus aurantium L. ssp. bergamia [Risso] Wright & Arn. [Rutaceae]; [synonym: Citrus bergamia Risso]), and cedarwood (Cedrus atlantica [Endl.] Manetti ex Carriere [Pinaceae])). All three groups received the oils by inhalation during radiation therapy. There were no significant differences reported in depression (as measured by HADS) or psychological effects (as measured by the Somatic and Psychological Health Report) between the groups. The group that received carrier oil only showed a statistically significant decrease in anxiety (as measured by HADS) compared with the other two groups.
Health-Related Quality of Life
A randomized, controlled, pilot study examined the effects of adjunctive aromatherapy massage on mood, QOL, and physical symptoms in patients with cancer. Forty-six patients were randomly assigned to either conventional day care alone or day care plus weekly aromatherapy massage using a standardized blend of essential oils (1% lavender and chamomile in sweet almond carrier oil) for 4 weeks. Patients self-rated their mood, QOL, and the intensity of the two symptoms that were the most concerning to them at the beginning of the study and at weekly intervals thereafter. Of the 46 patients, only 11 of 23 patients (48%) in the aromatherapy group and 18 of 23 patients (78%) in the control group completed all 4 weeks. Patient-reported mood, symptoms, and QOL improved in both groups, and there were no statistically significant differences between the two groups in any of these measures.
A placebo-controlled, double-blind, crossover, randomized trial compared an essential oil (choice of lavender, peppermint, or chamomile) with a pleasant-smelling placebo (rose water) administered by diffuser overnight for 3 weeks in 50 adult patients with newly diagnosed acute myeloid leukemia who were hospitalized for administration of intensive chemotherapy. Most patients reported poor quality sleep on the Pittsburgh Sleep Quality Index (PSQI) at baseline, with a mean score of 12.7. During the aromatherapy week, the mean PSQI score decreased to 9.7, but returned to a near-baseline score of 12.4 during the washout week. The difference in mean PSQI score and mean placebo score was statistically significant (P = .0001). Aromatherapy also reduced the weekly average Edmonton Symptom Assessment Scale–Revised (ESASr) score by 6.06 points, which was statistically significant (P = .0006). All subscales demonstrated a reduction in ESASr score with six symptom score changes showing a statistically significant benefit from aromatherapy:
Another randomized controlled trial examined the effects of aromatherapy massage and massage alone in 42 patients with advanced cancer over a 4-week period. Patients were randomly assigned to one of the following groups:
No significant long-term benefits of aromatherapy or massage in pain control, QOL, or anxiety were reported, but sleep scores (as measured by the Verran and Snyder-Halpern sleep scale) improved significantly in both groups. A statistically significant reduction in depression scores was also reported (as measured by the HADS) in the massage-only group.
A randomized controlled trial of lavender, tea tree oil, or no-treatment control in adult patients who received outpatient chemotherapy with paclitaxel reported that trait anxiety and sleep quality improved with lavender, and that tea tree oil led to the highest change in sleep quality. However, changes in anxiety were observed only on the trait anxiety scores, not on the state anxiety scores, which may reflect short term changes associated with an aromatherapy intervention. In addition, there were no significant differences in sleep scores between the two aromatherapy groups and the controls, which the study was designed to detect.
Radioactive iodine damage to normal salivary glands may be minimized by increased saliva production during the period of treatment. Inhalation aromatherapy was evaluated for its ability to increase saliva production during this administration period. An aromatherapy intervention consisting of a 2:1 mixture of lemon and ginger essential oils versus a distilled water (no smell) control inhaled for 10 min/d during a 2-week hospitalization for administration of radioactive iodine therapy for differentiated thyroid cancer was investigated in a randomized controlled trial of 71 patients. Salivary gland function was assessed by scintigraphy. Compared with placebo, those in the aromatherapy group showed a significantly higher rate of change of the maximum accumulation ratio in the parotid and submandibular glands (P < .05) and a significantly increased rate of change of the washout ratio before and after therapy in the bilateral parotid glands (P < .05). Although an increasing trend was observed for the submandibular glands in subjects receiving aromatherapy, no significant differences were noted between the groups. These results suggest that by increasing saliva production during radioactive iodine treatment with inhalation aromatherapy with a lemon and ginger combination, increased iodine clearance in salivary glands may lead to reductions in long-term damage to saliva production.
Nausea and vomiting
A randomized, controlled, crossover trial investigated the effects of inhaled ginger essential oil on alleviating chemotherapy -induced nausea and vomiting in Asian women with breast cancer. Aromatherapy administered as inhaled ginger essential oil for 5 days was associated with small statistically significant, but not clinically significant, reductions in acute nausea and had limited effects on reducing vomiting or delaying nausea in 60 evaluable patients.
In a placebo-controlled, double-blind, randomized trial of bergamot inhalation aromatherapy compared with a pleasant-smelling shampoo that did not contain essential oils administered at the time of stem cell infusion in 37 children and adolescents undergoing stem cell transplant, aromatherapy was not found to be beneficial in reducing nausea, anxiety, or pain. As administered in this study, bergamot inhalation aromatherapy may have contributed to persistent anxiety after the infusion of stem cells. Although no more effective than placebo, parents receiving aromatherapy showed a significant decrease in their transitory anxiety during the period between the completion of their child's infusion and 1 hour after infusion. Nausea and pain subsided over the course of the intervention for all children, although nausea remained significantly greater in patients who received aromatherapy. These findings suggest that the diffusion of bergamot essential oil may not provide suitable anxiolytic and antiemetic effects among children and adolescents undergoing stem cell transplantation. The double-blinding of the trial may explain the results, as single-blinded or nonblinded trials in general supported the aromatherapy intervention.
A similar study evaluated the efficacy of an aromatherapy intervention for reduction of symptom intensity of nausea, retching, and/or coughing among adult patients who received stem cells preserved in dimethyl sulfoxide. The study found that an intervention of tasting or sniffing sliced oranges was more effective at reducing symptom intensity compared with orange essential oil inhalation aromatherapy.
In a randomized placebo-controlled study of two different types of external aromatherapy tabs (lavender-sandalwood and orange-peppermint) compared with a matched placebo-controlled delivery system in 87 women undergoing breast biopsies, there was a statistically significant reduction in self-reported anxiety with the use of the lavender-sandalwood aromatherapy tab compared with the placebo group (P = .032).
In a three-arm randomized trial of 123 patients that compared lavender, eucalyptus, and no essential oil administered via inhalation, procedural pain after needle insertion into an implantable central venous port catheter was significantly decreased in the lavender oil inhalation aromatherapy group compared with the control group. Inhalation of eucalyptus oil did not reduce procedural pain levels during needle insertion.
A four-arm randomized trial compared aromatherapy, music therapy, a combination of the two, and a placebo in breast cancer patients before and after a mastectomy. The aromatherapy arm utilized a mixture of lavender, bergamot, and geranium essential oils, while the music arm offered a choice of music styles. Measured endpoints included pain intensity and anxiety. The combination of treatments was superior to either single treatment with all endpoints.
Current Clinical Trials
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Safety testing on essential oils has shown minimal adverse effects. Several essential oils have been approved for use as food additives and are classified as GRAS (generally recognized as safe) by the U.S. Food and Drug Administration; however, ingestion of large amounts of essential oils is not recommended. In addition, a few cases of contact dermatitis have been reported, mostly in aromatherapists who have had prolonged skin contact with essential oils in the context of aromatherapy massage. Some essential oils (e.g., camphor oil) can cause local irritation; therefore, care should be taken when applying them. Phototoxicity has occurred when essential oils (particularly citrus oils) are applied directly to the skin before sun exposure. One case report also showed airborne contact dermatitis in the context of inhaled aromatherapy without massage. Often, aromatherapy uses undefined mixtures of essential oils without specifying the plant sources. Allergic reactions are sometimes reported, especially after topical administration. Individual psychological associations with odors may result in adverse responses. Repeated exposure to lavender and tea tree oils by topical administration was shown in one study to be associated with reversible prepubertal gynecomastia. The effects appear to have been caused by the purported weak estrogenic and antiandrogenic activities of lavender and tea tree oils. Therefore, avoiding these two essential oils is recommended in patients with estrogen -dependant tumors. However, this is the first published report of this type of adverse effect when using products containing tea tree or lavender oils.
To assist readers in evaluating the results of human studies of integrative, alternative, and complementary therapies for people with cancer, the strength of the evidence (i.e., the levels of evidence) associated with each type of treatment is provided whenever possible. To qualify for a level of evidence analysis, a study must:
Separate levels of evidence scores are assigned to qualifying human studies on the basis of statistical strength of the study design and scientific strength of the treatment outcomes (i.e., endpoints) measured. The resulting two scores are then combined to produce an overall score, with a score of 1 being the strongest evidence and a score of 4 being the weakest design (or sometime similar). For an explanation of the scores and additional information about levels of evidence analysis for people with cancer, see Levels of Evidence for Human Studies of Integrative, Alternative, and Complementary Therapies.
The PDQ cancer information summaries are reviewed regularly and updated as new information becomes available. This section describes the latest changes made to this summary as of the date above.
Added text about a four-arm randomized trial that compared aromatherapy, music therapy, a combination of the two, and a placebo in breast cancer patients before and after a mastectomy (cited Deng et al. as reference 15).
This summary is written and maintained by the PDQ Integrative, Alternative, and Complementary Therapies Editorial Board, which is editorially independent of NCI. The summary reflects an independent review of the literature and does not represent a policy statement of NCI or NIH. More information about summary policies and the role of the PDQ Editorial Boards in maintaining the PDQ summaries can be found on the About This PDQ Summary and PDQ® - NCI's Comprehensive Cancer Database pages.
Purpose of This Summary
This PDQ cancer information summary for health professionals provides comprehensive, peer-reviewed, evidence-based information about the use of aromatherapy with essential oils in the treatment of people with cancer. It is intended as a resource to inform and assist clinicians in the care of their patients. It does not provide formal guidelines or recommendations for making health care decisions.
Reviewers and Updates
This summary is reviewed regularly and updated as necessary by the PDQ Integrative, Alternative, and Complementary Therapies Editorial Board, which is editorially independent of the National Cancer Institute (NCI). The summary reflects an independent review of the literature and does not represent a policy statement of NCI or the National Institutes of Health (NIH).
Board members review recently published articles each month to determine whether an article should:
Changes to the summaries are made through a consensus process in which Board members evaluate the strength of the evidence in the published articles and determine how the article should be included in the summary.
Any comments or questions about the summary content should be submitted to Cancer.gov through the NCI website's Email Us. Do not contact the individual Board Members with questions or comments about the summaries. Board members will not respond to individual inquiries.
Levels of Evidence
Some of the reference citations in this summary are accompanied by a level-of-evidence designation. These designations are intended to help readers assess the strength of the evidence supporting the use of specific interventions or approaches. The PDQ Integrative, Alternative, and Complementary Therapies Editorial Board uses a formal evidence ranking system in developing its level-of-evidence designations.
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The preferred citation for this PDQ summary is:
PDQ® Integrative, Alternative, and Complementary Therapies Editorial Board. PDQ Aromatherapy With Essential Oils. Bethesda, MD: National Cancer Institute. Updated <MM/DD/YYYY>. Available at: https://www.cancer.gov/about-cancer/treatment/cam/hp/aromatherapy-pdq. Accessed <MM/DD/YYYY>. [PMID: 26389313]
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Last Revised: 2022-06-07
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