Inhaling the scents of lavender or peppermint oils might help to reduce stress, but the evidence supporting that claim is too weak to rely on, according to a new analysis.
Researchers reviewed the handful of previous clinical trials testing whether aromatherapy can reduce stress, and found most to be biased, although most also did show a benefit.
"Aromatherapy seems to be effective for reducing stress in healthy people. However, there (is) still some possibility that it is a placebo or not beneficial for reducing stress," said Dr. Myeong Soo Lee of the Korea Institute of Oriental Medicine in Daejeon, the study's senior author.
If aromatherapy does reduce stress, it could be an important health intervention, the study team writes in the journal Maturitas. They note that stress can lead to heart disease, stomach ulcers, hypertension and other illnesses.
To assess the evidence for aromatherapy as a stress-management tool, the researchers analyzed results from five randomized controlled trials testing aromatherapy against dummy substances or against no treatment at all. All the trials measured participants' feelings of stress and three of them also looked for changes in the so-called stress hormone cortisol.
Lee's team chose to include only studies testing healthy people, and the five studies had a total of 147 participants.
Overall, the analysis found that "aroma inhalation has favorable effects on stress management." But the studies that tested cortisol levels found no differences with aromatherapy.
Most of the experiments were also subject to bias, Lee's team notes, because of the way the trials were designed.
For example, four of the five studies failed to conceal from both the subjects and the experimenters whether or not the subjects were receiving aromatherapy or were in the control group. This makes it harder to rule out the placebo effect as the cause of the difference in stress level, especially when subjects are asked to assess their own stress levels.
Because of such weaknesses, the researchers were unwilling to draw a firm conclusion about whether or not aromatherapy is effective.
Lee said that the studies asking patients to evaluate their own stress levels "showed beneficial effects of aromatherapy on reducing stress level compared to no treatment."
Dr. Wolfgang Steflitsch, a pulmonologist at the Otto Wagner Hospital in Vienna who wasn't involved in the research, told Reuters in an email that aromatherapy "is a valuable option for reduction of stress, anxiety, depression, sleep disturbance and burnout."
He agrees the studies to date are limited by the potential for bias, but notes that study design has been improving over the last decade.
Although this review raises some doubts about the effectiveness of aromatherapy, Steflitsch said he thinks there is no harm in the practice.
Patients with an allergic history should do a tolerance test beforehand, he noted, but said otherwise, "If high quality essential oils are used from an aromatherapy expert, there will be no harm."
Other researchers have raised some concerns about whether aromatherapy used in environments like spas can cause injury. For example, a study in Taiwan found that essential oils can combine with air particles and irritate eyes and air pathways.
Lee said that participants in the trials her team reviewed did not experience any harm, however other reviews have indicated that essential oils can be toxic at high concentrations, especially when taken orally.
Steflitsch recommends that people planning to use aromatherapy should first see a medical doctor for diagnosis. If aromatherapy is used, he said, it should be used along with any other necessary medical interventions.
The researchers conclude that the current evidence does not conclusively show aromatherapy to be an effective method for managing stress and future studies should focus on designing proper control groups.
SOURCE: bit.ly/1rtDnD1 Maturitas, online August 20, 2014.