The results suggest that aroma inhalation may be effective than no treatment or odorless inhalation in reducing self-reported stress and cortisol level. However, the risk of bias was high in all of the included trials. Collectively, the existing trial evidence shows limited effectiveness of aromatherapy in reducing stress. Future studies should be of high quality with a particular emphasis on designing an adequate control intervention.
Most of the included RCTs did not employ blinding of patients, practitioners, and assessors. Furthermore, none of them reported the methods of random sequence generation and allocation concealment. Trials with inadequate blinding and inadequate allocation concealment may be subject to selection bias and are likely to generate exaggerated treatment effects [24] and [25]. There were low risks of bias in incomplete outcome data. One trial showed high risk of bias in selective reporting. Most of the RCTs had a small sample size, and their results were therefore prone to a type II error. Small trials may also overestimate treatment effects by approximately 30% [26].
The mechanisms by which aromatherapy reduces stress may be related to sympathetic nervous system and parasympathetic modulation. Aromatherapy may decrease to sympathetic nervous system activity and increase parasympathetic nervous system [27]. These may promote relaxation and reduce stress level. However, more basic research is needed to fully understand the mechanisms underlying the effects of aromatherapy.
One recent systematic review reported that aromatherapy had possible adverse events [28]. However, none of the reviewed studies reported any adverse events related with aromatherapy. Aromatherapy may appear to be generally safe; regardless, the safety of aromatherapy requires further research.
Our review has a number of important limitations. Although strong efforts were made to retrieve all RCTs on the subject, we cannot be absolutely certain that we succeeded. Moreover, selective publishing and reporting are other major causes for bias [29], [30] and [31], which have to be considered. It is conceivable that several negative RCTs remain unpublished and thus the overall picture may be distorted. Another possible source of the bias could be the fact that most of the included trials were carried out in Korea and Japan, regions which have been shown to produce almost no negative studies [32]. Further limitations include the paucity and the often suboptimal methodological quality of the primary data. In total, these facts limit the conclusiveness of this systematic review considerably. To establish the role of aromatherapy in the stress management, adequately designed trials are required.
Collectively, the existing trial evidence is not convincing and does not show aromatherapy to be an effective modality for managing stress. Future studies should be of high quality, with a particular emphasis on designing adequate and appropriate control groups.
In conclusion, these results provide limited evidence for the effectiveness of aromatherapy in reducing stress level of healthy subjects. However, the number, size and quality of the RCTs are too low to draw firm conclusions. Further rigorous RCTs seem warranted but need to overcome the many limitations of the current evidence.