There is a major difficulty in studying aromatherapy by inhalation: how to conduct a double-blind, placebo-controlled trial. For the results of a study to be truly reliable, both participants and researchers must be kept in the dark regarding participants who received real treatment and who received placebo. (For more information on why this is so crucial, see
Why Does This Database Depend on Double-blind Studies?
.) Although it may be possible to keep researchers in the dark regarding which group is which, participants will certainly be aware of whether they smell something or not! This is a problem because it has been shown that when researchers create expectations about the effects of certain aromas, those effects may occur simply because of those expectations.
Researchers have used various clever compromises in an effort to partially solve this problem. For example, some studies used a control group that received an aromatic substance believed to be ineffective, without informing the members of the control group that this alternate aromatic substance will not work. Unfortunately, it is just as hard to prove that an aromatic substance is ineffective as it is to prove that it is effective! If the placebo in a study is just as effective as the tested treatment, the study will falsely indicate that the tested treatment is ineffective.
Furthermore, many odors already have associations attached them, based on cultural patterns. Lavender oil, for example, conjures up for many people memories of their grandmother. It simply is not possible to remove such expectations.
In other studies, researchers tricked participants in the control group and told them that they might be receiving an active but odorless treatment, when in fact they were simply given an inactive treatment without much in it. Still other studies managed to find ethical ways of keeping their study participants in the dark regarding whether they were enrolled in a study at all, and then introduced the odors surreptitiously. Partially effective compromises such as these are necessary. Unfortunately, most published studies on aromatherapy fail even to achieve this level of rigor, falling far below minimal scientific standards of reliability.
Thus, everything written below about true aromatherapy—that is, inhalation of an aroma—must be taken with a grain of salt.
These problems do not arise to the same extent in studies of essential oils taken by mouth or applied directly to the skin.
Inhalation of Essential Oils
Alzheimer's Disease and Other Forms of Dementia
Preliminary controlled trials suggest that various forms of aromatherapy might be helpful for calming people with
and other forms of dementia
For example, in one interestingly designed, but very small, study, a hospital ward was suffused with either
or water for two hours.
An investigator who was unaware of the study’s design and who wore a device to block inhalation of odors entered the ward and evaluated the behavior of the 15 residents, all of whom had dementia. The results indicated that use of lavender oil aromatherapy modestly decreased agitated behavior. A less rigorous study also reported benefit with lavender.
However, people with dementia tend to lose their sense of smell, making this approach seem somewhat limited in its usefulness.
Essential oil of
has also shown promise for this purpose; in a double-blind study of 71 people with severe dementia, use of a lotion containing essential oil of
reduced agitation compared to placebo lotion.
Here, absorption through the skin may have played a role.
Several relatively poorly designed studies hint that aromatherapy combined with
may help to relieve
in people without Alzheimer's disease.
Another study suggests that aromatherapy with geranium oil might modestly reduce anxiety levels (again in people without Alzheimer's).
Researchers have also studied aromatherapy as a potential treatment for the cognitive (eg, memory) impairments caused by dementia.
In a small study, 28 elderly people with dementia (including 17 people with Alzheimer's disease) were exposed to rosemary and lemon oil in the morning and lavender and orange in the evening for 28 days. When researchers compared the dementia assessment scores during the treatment period to the scores from the previous month (control period without aromatherapy), they found that all of patients experienced an improvement in their symptoms.
A controlled study suggests that inhalation of black pepper vapor may reduce the craving for cigarettes.
In this trial, a total of 48 smokers used cigarette substitute devices that delivered black pepper vapor, menthol, or no fragrance. The results showed that use of the black pepper-based dummy cigarette reduced symptoms of craving for the first morning cigarette.
Weak evidence hints that
oil applied to the forehead might relieve
A topical ointment known as Tiger Balm has also shown promise for headaches. Tiger Balm contains camphor, menthol, cajaput, and clove oil. A double-blind study enrolling 57 people with acute tension headache compared the application of Tiger Balm to the forehead against placebo ointment as well as the drug acetaminophen (Tylenol).
The placebo ointment contained mint essence to make it smell similar to Tiger Balm. Real Tiger Balm proved more effective than placebo and just as effective and more rapid-acting than acetaminophen.
In a trial of 66 women waiting to undergo abortions, 10 minutes of inhaling the essential oils of vetivert, bergamot, and geranium failed to reduce
significantly more than placebo treatment.
In another study,
oil failed to reduce tension during an anxiety-provoking task and might have actually
However, other studies have shown more favorable effects. In one such trial, researchers assessed the anxiety level in 340 dental patients while they were waiting for their appointment.
Those that inhaled the scent of lavender showed lower levels of anxiety compared to the control group. In another study, 150 patients were randomized to one of three treatment groups: control (standard care), standard care plus lavender, or sham (standard care plus another kind of oil).
Those that were in the lavender group did experience a reduction in their level of anxiety.
Taking a different approach, researchers evaluated the effects of
done with essential oils on people suffering from anxiety and/or
while undergoing treatment for cancer.
The treatment did appear to provide some short-term benefits.
Weak evidence suggests that inhaled
Peppermint was associated with improved nausea symptoms in a small randomized trial of 35 women after nonemergency cesarean section compared to placebo aromatherapy and standard antiemetic drugs.
Inhaled peppermint oil may also be helpful for relieving mucus congestion of the lungs and sinuses; however, there is only weak supporting evidence for this use.
In one study, abdominal massage with lavender, rose, and clary sage reduced
to a greater extent than an almond oil placebo.
In another study,
combined with lavender, rosemary, and peppermint aromatherapy was more effective than acupressure alone for treating the shoulder pain caused by a certain form of
Controlled studies have evaluated proprietary-inhaled aromatherapy preparations for treating the
and preventing flare-ups of
but the results were marginal at best.
A study involving vapor rub found a more positive effect, though. One hundred and thirty-eight children (aged 2-11 years old) with upper respiratory infection were randomized to receive vapor rub (camphor, menthol, and eucalyptus oils), petroleum jelly, or no treatment.
The children who had vapor rub applied before bedtime experienced an improvement in their nighttime symptoms (eg, less coughing, less nasal congestion) compared to the other two groups.
A controlled study evaluated rosemary and also lavender aromatherapy for
enhancing memory and mental function
, but found results that were mixed at best.
However, lavender oil has shown a bit of promise for
In an interesting trial, 145 nursing home residents were randomized to wear a lavender-scented patch or an unscented patch for 1 year. Those who wore the lavender patch experienced fewer falls than the control group.
Aromatherapy has also been studied as a potential treatment for pain during and after
with mixed results. A randomized controlled study involving 251 pregnant women found evidence that aromatherapy may help reduce the perception of pain during labor and possibly reduce the risk of a newborn needing intensive care.
However, a review of 2 randomized trials did not find a positive effect. In the larger of the two studies, 513 women were randomized to receive aromatherapy (
, or mandarin essentials oils) or standard care. Different methods were used to expose the women to the oils, like applying a compress, giving a massage, or using a footbath. There were no differences between the two groups in level of pain, rate of cesarean section, or use of pain medication.
Similarly, in a large, controlled trial involving more than 600 participants, lavender oil in bathwater failed to improve pain after
Another large study failed to find aromatherapy more helpful than placebo for reducing psychological distress among people undergoing
radiation therapy for cancer
Oral Use of Essential Oils
is a standard ingredient in cough drops and cough syrups, as well as in oils added to humidifiers. A standardized combination of eucalyptus oil plus two other essential oils has been studied for effectiveness in a variety of respiratory conditions. This combination therapy contains cineole from eucalyptus, d-limonene from citrus fruit, and alpha-pinene from pine. Because these oils are all in a chemical family called monoterpenes, the treatment is called
essential oil monoterpenes
Most, though not all, double-blind studies, some of which were quite large, indicate that oral use of essential oil monoterpenes can help
, sinus infections, and
For example, a 3-month, double-blind trial of 246 people with chronic bronchitis found that consumption of essential oil monoterpenes helped prevent the typical worsening of chronic bronchitis that occurs during the winter.
Another study evaluated 676 male and female outpatients with
bronchitis and found that essential oil monoterpenes were more effective than placebo.
Essential oil monoterpenes are thought to work by thinning mucus, though they may have other effects.
oil alone may be helpful for respiratory problems as well. In a double-blind trial, 32 people on steroids to control severe asthma (steroid-dependent asthma) were given either placebo or essential oil of eucalyptus for 12 weeks.
The results showed that people using eucalyptus were able to gradually reduce their steroid dosage to a greater extent than those taking placebo.
In another study, eucalyptus oil proved helpful for the treatment of "head cold" symptoms (technically, nonpurulent rhinosinusitis).
In this double-blind, placebo-controlled study of 152 people, use of cineole at a dose of 200 mg three times daily markedly improved cold symptoms as compared to placebo.
While not technically classified as an aromatherapy, a
double-blind, placebo-controlled study of 39 people found that an enteric-coated peppermint-
oil combination taken 3 times daily by mouth for 4 weeks significantly reduced
pain as compared to placebo.
Of the treatment group, 63.2% of participants were pain-free after 4 weeks, compared to 25% of the placebo group. Similarly, results from a double-blind comparative study of 118 people suggest that the combination of peppermint and caraway oil is comparably effective to the standard drug cisapride, which is no longer available.
After 4 weeks, the herbal combination reduced dyspepsia pain by 69.7%, whereas the conventional treatment reduced pain by 70.2%.
A preparation of peppermint, caraway, fennel, and
oil was compared to the drug metoclopramide in a double-blind study enrolling 60 people.
After 7 days, 43.3% of the treatment group was pain-free, compared to 13.3% of the metoclopramide group.
Oral use of
has shown considerable promise for
irritable bowel syndrome
. However, most studies were relatively poorly designed.
Silexan is a capsule that contains a preparation of lavender oil. Two hundred and twenty-one adults diagnosed with having an anxiety disorder were randomized to receive 80 mg a day of silexan or placebo for 10 weeks.
Those who were in the treatment group had a reduction in their anxiety, slept better, and did not have any unwanted side effects compared to those receiving the placebo.
Lorazepam (Ativan) is a common medication prescribed to treat anxiety. In one study, adults with generalized anxiety disorder were randomized to receive lorazepam or silexan.
At the end of the 6-week period, silexan was as effective as lorazepam in reducing anxiety symptoms. Again, silexan does not appear to have any side effects and, unlike lorazepam, it does not have the potential for abuse.
Other Oral Uses for Essential Oils
One study found preliminary evidence that a complicated mixture of essential oils (taken by gargle or mouth spray) might be helpful for reducing snoring symptoms.
A thorough review of 11 randomized, controlled trials found that the use of mouth rinses containing essential oils is effective against
and dental plaque formation when used in combination with regular oral hygiene.
Topical Use of Essential Oils
Tea tree oil
, an essential oil from the plant
, possesses antibacterial and antifungal properties.
It has been tried for various forms of
, but the only supporting evidence for this use comes from an
There is slightly better evidence to support the use of tea tree oil for the treatment of
, and related fungal infections.
hints that oil of
, a flavoring agent from dried bitter orange peel, might have some effectiveness against athlete's foot when applied topically.
Topical essential oils might be helpful for
, a form of hair loss that can occur in men and women. In a 7-month, double-blind, placebo-controlled trial, 84 people with alopecia areata massaged either essential oils or a non-treatment oil into their scalps each night for 7 months.
The treatment oil contained essential oils of thyme,
, lavender, and cedarwood. The results showed that 44% of the treatment group experienced new hair growth, compared to only 15% of the control group.
Cineol (from eucalyptus) has shown some effectiveness for repelling
In a preliminary double-blind blind study, coriander oil applied topically protected the skin from the harmful effects of ultraviolet radiation more than a placebo cream (
experience muscle pain in many parts of the body. A pilot double-blind study found that topical application of a proprietary mixture containing camphor oil, rosemary oil, eucalyptus oil, peppermint oil, aloe vera oil, lemon oil, and orange oil could reduce fibromyalgia pain more effectively than placebo.
Another study found that massage combined with the topical application of ginger and orange essential oils was no better than massage plus olive oil at relieving pain, reducing stiffness, or improving function in patients with
of the knee.
One study in rats indicates that under some circumstances essential oils instilled into the ear may be able to penetrate the eardrum.
While this supports the idea of treating otitis media (the typical ear infection of childhood) with herbal ear drops, it also raises concerns about possible harm to the middle ear.
Finally, for literally hundreds of essential oils,
test tube studies
show antimicrobial effects (activity against fungi, bacteria, and/or viruses).
Presumably, essential oils are part of the plants own defenses against such organisms. However, contrary to widespread claims, such studies do
indicate that these essential oils can work as antibiotics; innumerable substances kill microorganisms in the test tube but not when taken orally by people. (Bleach would be one good example!)