Socio-demographic characteristics
Of a total of 310 subjects available, 150 did not meet our eligibility criteria, and 100 individuals refused to participate in the study (Figure 1). Of those participating in the study, 80% in both LEO and placebo groups were housewives; 40% of the participants in the LEO group had elementary and middle school education, and 33.3% of the subjects in the placebo group had high school education. Most of the participants in the LEO (73%) and placebo (80%) groups had average income (self-reported) (Table 1).
Positive and negative mood between groups and within groups
Regarding the positive and negative affect scores before the intervention, no statistically significant difference was observed between the two groups according to the independent t-test.
Within-group comparison (comparing each time point with the baseline) in the LEO and placebo groups based on the ANOVA test showed an increase in the mean positive affect score and a decrease in the mean negative affect score at 28 and 56 days after the intervention compared to the baseline, which was statistically significant in both groups (p<0.001). Also, the repeated-measure ANOVA test showed that there was a statistically significant difference between the three-time points of measurement in both the LEO and placebo groups (p<0.001), showing a significantly more elevation in the mean positive affect score and a significantly less decline in the mean negative affect score over time in the intervention compared to the placebo group.
The means of positive and negative mood scores were compared between the LEO and placebo groups at two separate time points after the intervention using the ANCOVA test and after adjusting for the baseline values and age. The results showed that at 28 and 56 days after the intervention, the mean of the positive mood score was significantly higher in the LEO group compared to the placebo group (p<0.001). Although according to within-group comparisons, the mean positive mood score showed a significant elevation in both the LEO and placebo groups, this elevation was more prominent in the LEO group, resulting in a statistically significant difference between the two groups at 28 and 56 days after the intervention.
Based on the repeated-measure ANOVA test, the increase in the mean positive mood score and the decrease in the mean negative mood were statistically significant comparing the LEO and placebo groups after controlling for the effect of age (P<0.001).
After controlling for the effect of age, comparing the means of positive and negative mood scores between the LEO and placebo groups based on the Greenhouse-Geisser or Sphericity Assumed test (i.e., the interaction of time and group) showed a statistically significant difference between the two groups (p<0.001, Table 2).
Stress, anxiety and depression scores within groups and between groups
Within-group comparison based on the ANOVA test (comparing post-intervention time points with the baseline) in the LEO group revealed a significant decrease in the average score of stress and depression on days 28th and 56th after the intervention compared to the baseline (p<0.05); however, no significant changes were noticed in these scores in the placebo group at none of the time points. Repeated-measure ANOVA for within-group comparisons revealed that the declining trends in the average scores for stress and depression were statistically significant over time in the LEO group (p<0.05), but not in the placebo group. According to the independent t-test, there was no statistically significant difference between the two groups (LEO and placebo) in terms of the baseline average scores for stress and depression.
After controlling for age and baseline values, and based on the ANCOVA test, a significant decline was observed in the mean scores of stress and depression 28 and 56 days in the intervention compared to the control group after the intervention (p<0.05). Based on the repeated-measure ANOVA test, after controlling the effect of age, the reduction of the mean score of stress in the LEO group (but not the mean score of depression) was observed to be statistically significant compared to the placebo group (p=0.031).
According to the Greenhouse-Geisser or Sphericity Assumed test (i.e., the interaction between time and group) and after adjustment for age, the reductions in the mean scores of stress (p < 0.001) and depression (p<0.003) in the LEO group were statistically significant compared to the placebo group.
According to the Wilcoxon test, the mean score of anxiety in the LEO group showed a significant decrease on days 28 and 56 after the intervention compared to the baseline (p<0.05); however, no statistically significant change was observed in the anxiety score in the placebo group.
According to the Mann-Whitney test, there was no statistically significant difference in anxiety scores between the LEO and placebo groups at any time point after the intervention.
According to the Friedman test, the reduction in anxiety over time was statistically significant in the LEO group (P = 0.001), but not in the placebo group (Table 3).
During the follow-up and after 28 days, two individuals in the intervention group and three people in the control group discontinued taking the capsules due to headaches and dizziness. Since our approach was intention-to-treat, all participants completed the study questionnaires and entered in the final analyses. No specific adverse effects were reported by other participants, as evidenced by reviewing the relevant checklist.
Discussion
The results of this study showed that the use of LEO by COCs consumers significantly improved their mood compared to the placebo group that this effect had increased over time. Also, stress and depression significantly declined in the participants using LEO at 28 and 56 days after the intervention compared to the placebo group. Although the level of anxiety also decreased significantly in the LEO group during the follow-up, no statistically significant difference was observed between the LEO and placebo groups.
In a study on 24 healthy young women, Sazawa et al. (2022) compared the effects of aromatherapy cutaneous patches containing lavender applied during sleep for seven nights with that of the placebo on mood and physiological indicators of stress, reporting a significant improvement in the total of score of mood disorders in the intervention group over time [27]. These findings were consistent with our observations in the present study.
The results of a study in Iran showed that the mean scores of sleep quality, life quality, and mood significantly improved in individuals inhaling lavender compared to the placebo group [28], supporting our observation in terms of mood.
Vaziri et al. (2017), in a randomized clinical trial on 56 women, investigated the effects of lavender oil aromatherapy during the early hours after labor on pain, fatigue, and mood one hour and six hours after delivery; the results showed that perineal pain, physical pain, fatigue, and depression gradually and markedly alleviated in the intervention group compared to the control group. After controlling for the interaction between time and group, there was still a significant difference between the two groups, highlighting the efficacy of lavender aromatherapy in the alleviation of postpartum ailments. Moreover, the recent study showed that after adjusting for the interaction between time and group, a significant increase in the mean positive affect score and a significant decline in the mean negative affect score in the lavender aromatherapy group; however, the differences between the two groups were not statistically significant when adjusting for the time-group interaction [14]. This finding was in line with that of the present study as we noticed a significant improvement in both positive and negative affect scores, as well as depression, in the LEO group. However, in the present study, we witnessed a statistically significant decrease in the negative affect score in both groups, which was inconsistent with the report of Vaziri et al. and can be related to women’s psychological changes in the postpartum period. It is noteworthy that our participants consumed COCs for contraception.
Dong et al. (2016) conducted a study to investigate the effects of an optical-olfactory stimulant on blood pressure and mood in 18-28-year-old people. In this study, light therapy, along with the inhalation of lavender, peppermint, and lemon extracts, was delivered in 5 days The results of the recent study showed that the 5-day administration of the light-olfactory stimulant (i.e., lavender) significantly ameliorated mood disorders in the intervention group compared to the control group [29]; which is in agreement with the results of the present study; but it was not consistent with Shandong's study in terms of between-group effects (The lack of effect of light and lavender scent alone on mood disorders). A possible reason for this observation could be the different forms of lavender used in these studies (lavender aromatherapy plus light therapy vs. LEO oral supplements). Moreover, the participants in the study of Dong et al. constituted both men and women, but only women participated in the present study. Also, mood assessment indicators measured in the two studies were different, and the duration of follow-up was short in the recent study (i.e., 1, 3, and 5 days) compared to our research (28 and 56 days).
Linalool has been noted to be the most important constituent of lavender, which can act as a sedative by inhibiting glutamate binding [30]. On the other hand, Pennine is another major component of lemon and lavender that has been noted to boost sedative responses in the presence of gamma-aminobutyric acid (GABA) [31].
The results of a clinical trial demonstrated that lavender promotes slight anti-anxiety effects in humans; however, these effects were not observed in patients suffering from severe anxiety [32]. This notion somehow agrees with our observation in the present study, revealing that although anxiety was reduced in the LEO group, the difference in anxiety score reduction was not statistically significant compared to the placebo group.
A review study showed that standardized essential oil extract of L. angustifolia (SLO) was well tolerated and effectively alleviated moderate to severe anxiety and depression [33], which was consistent with the results of the present study in terms of a reduction in depression, but inconsistent about anxiety. A possible explanation for the discrepancy observed could be the difference in the type of lavender extract used.
In vitro, studies have been conducted to scrutinize the neuropharmacological mechanisms of essential oils and their chemical ingredients [34]. Studies have delineated the effects of LEO on the central nervous system at the molecular level, indicating an inhibitory effect on the activity of voltage-gated calcium channels [35]. In contrast to some other monoterpenes available in LEO, linalool and linalyl acetate can exert profound inhibitory effects on voltage-gated calcium channels [36]. On the other hand, the active ingredients of LEO have shown affinity toward the N-methyl-D-aspartate (NMDA) receptor and serotonin transporters (SERT). The binding to and inactivating the NMDA receptor and neuronal messages are mostly attributed to linalool and linalyl acetate as linalool can strongly bind to the SERT receptor [36]. On the other hand, a study on rats reported that linalool could hinder glutamate binding in the cerebral cortex [30]. These observations partly explain the molecular function of lavender in alleviating neurological disorders.
Among the strengths of the present study are its triple-blinded design, the small rate of withdrawals during the follow-up, and being a multi-center trial, including clinics covering people with different socio-economic statuses, which boosted the generalizability of our findings.
One of the disadvantages of the present study was the short duration of the intervention (56 days) due to financial restrictions and completing the PANAS questionnaire by self-reporting, which could have affected their responses due to the possibility of different mindsets among the participants.