Citrus aurantium is widely used throughout the world. Having originated in China, it has been used as a medicinal plant in traditional Chinese medicine for thousands of years [1]. Various plant parts (fruit, peel, leaves, bark, flowers) and preparations have been used for a wide range of conditions including: digestive complaints, respiratory infections, various psychological symptoms, and central nervous system disorders [1]. Citrus aurantium is reputed to have thermogenic, bronchospasmolytic, anticonvulsant, anxiolytic, and sedative actions [2]. The anxiolytic effects of Citrus aurantium have been demonstrated using the distilled essential oil from the peel as an aroma therapy treatment in clinical studies in a range of conditions [3–6]; however, clinical studies on the effects of whole plant extracts of Citrus aurantium have primarily focused on weight loss (as a herbal combination product), or assessing the safety of the herb—due to the presence of pyrrolizidine alkaloids [7]. Despite concerns about the pyrrolizidine alkaloid content in bitter orange extract, studies have demonstrated no adverse effects [7]; however, more research is needed.

The essential oil of Citrus aurantium has been shown to be responsible for anxiolytic effects, with the monoterpene limonene identified as the primary constituent responsible for this action [8]. The anxiolytic effect has been found to occur in the serotonergic system via the 5HT_1A serotonin receptor [8].

Studies have demonstrated anxiolytic effects using the essential oil of Citrus aurantium in mice and rodents [9–11]; however, few studies have investigated these effects using whole plant extracts of bitter orange. One study on mice used the elevated plus maze (EPM) and the open-field test to investigate the anxiolytic effects of bitter orange in preparations of both the distilled essential oil from the peel and various hydroethanolic extracts of the leaves [2]. Mice were orally administered either essential oil (1 g/kg or 0.5 g/kg) or 1 g/kg of a hydroethanolic extract (crude extract, hexanic fraction, dichloromethanic fraction, aqueous fraction) or diazepam (1.2 mg/kg), or a control solution. Results revealed that only the essential oil and the diazepam groups significantly increased the number of entries into the open arms of the EPM compared to placebo, with the diazepam group demonstrating the greatest reduction in anxiety-like behaviors. No reduction in anxiety-like behaviors was observed for any treatment in the open-field test.

The acute anxiolytic effects of bitter orange was assessed in a double-blind RCT [12]. Sixty preoperative patients (n = 60) with anxiety who were having a minor lower limb operation were randomized into two groups and orally administered either Citrus aurantium (blossom petals and stamens) distillate (1 ml/kg; standardized for linalool, total phenolic acids, and flavonoids); or placebo (saline solution 1 ml/kg). Treatments were administered 2 hours prior to the operation. Anxiety symptoms were measured using the State-Trait Anxiety Inventory (STAI)-state and the Amsterdam Preoperative Anxiety and Information Scale (APAIS), with physiological measures for heart rate and blood pressure at baseline (prior to treatments) and 2 hours later (directly before surgery). Patients in the bitter orange group demonstrated a significant reduction in anxiety scores on both the STAI-state and APAIS, while the placebo group did not show a significant reduction in anxiety. There were no significant differences in physiological measures for either group. However, the reporting of statistical results was problematic, with no group × time interaction stated. For this reason, it is difficult to establish whether reductions in anxiety observed in the bitter orange group were significantly greater than those observed in the placebo group.

Echinacea species are best known for their immunomodulatory and anti-inflammatory effects [13]. The flowers and roots of Echinacea spp. Have a long history of traditional use by indigenous North Americans who have used this medicine to treat a range of conditions including headaches, snake bite, toothache, and the common cold [14]. A variety of species have been used for their medicinal actions; however, Echinacea purpurea, Echinacea angustifolia, and Echinacea pallida species are most commonly used in modern herbal preparations [15]. The primary bioactive constituents in Echinacea spp. are alkylamides and polysaccharides, which are present at varying levels depending on the plant part used, with the roots having the greatest concentration of alkylamides [15].

The anxiolytic effect of Echinacea spp. may be due to a range of lipophilic alkylamide constituents that bind to cannabinoid B1 receptors predominantly located on neurons [13]. Cannabinoid B1 receptors regulate neurotransmitter function and are involved in GABA and glutamate modulation [16]. In addition, E. angustifolia extract has been demonstrated to reduce the release of glutamate and decrease synaptic excitation in the hippocampus; however, the specific constituents responsible for this action have not been identified [17].

Two animal studies explored the anxiolytic activity of Echinacea spp. Anxiolytic-like effects were studied in rats using a variety of extraction types of either E. purpurea root, E. purpurea herb, or E. angustifolia root [18]. Various doses were tested depending on the extract used, and all doses were administered in a volume of 2 mL/kg. Three extracts decreased anxious behaviors to various degrees in the EPM, social interaction, and social avoidance tests. Significant anxiolytic effects were only observed at doses of 1.5 and 2 mg/kg for the E. purpurea root ethanol extract (4% echinacoside), 4 mg/kg for the E. purpurea root hydroalcohol extract (unstandardized), and 1 mg/kg for the E. angustifolia root ethanol extract (4 % echinacoside). Interestingly, the anxiolytic effects were not dose-dependently related to alkylamide content, although the three extracts demonstrating anxiolytic effects had considerably higher amounts of alkylamides compared to the ineffective extracts. Further investigation of the E. angustifolia extract found a dosage range of 3–6 mg/kg effective at 30 min following treatment, and 4–8 mg/kg at 1 hour after treatment. No locomotor-suppressant effects were seen for any extract at any dose.

A second study by the same group used the E. angustifolia extract from the previous study to investigate various behavioral effects including anxiolysis in rats [13]. They found a significant reduction in anxiety-like behaviors for the extract, with rats spending more time exploring open arms in the EPM, while displaying significantly reduced conditioned fear.

The researchers from the above study conducted a RCT in 33 adults [13]. This clinical trial investigated the anxiolytic effects of E. angustifolia root (ethanol extract, 4 % echinacoside) in a healthy sample without a psychiatric diagnosis, but with high scores on the STAI (both state and trait). This was an open-label design with a 3-day observation phase, followed by a 1-week treatment phase and a 2-week washout period. Participants were randomly assigned to receive either one or two E. angustifolia tablets (20 mg extract) per day. Within 3 days, there was a significant decrease in both state and trait STAI scores for the higher dose of 40 mg per day that remained stable until day 7 (end of treatment) and for the 2-week follow-up observation period. No anxiolytic effect was found for the lower dose, and there were no serious adverse effects reported. Further research is needed using more robust methodology with a placebo comparison to support the use of Echinacea spp. in treating anxiety symptoms.

Echium amoenum is an Iranian plant medicine with a long history of traditional use. The flowers and leaves have been used traditionally to treat heart disease, the common cold, and other pulmonary conditions [19]. The petals are used for their effects on the nervous system, as they have analgesic, anxiolytic, and sedative actions [19]. In traditional Iranian medicine, decoctions of the petals are consumed for the duration that the anxiety or stress symptoms are experienced [19], suggesting that this herbal medicine may be suitable for more chronic forms of anxiety. A range of constituents have been identified in the petals, which include saponins, flavonoids, unsaturated terpenoids, sterols [20], alkaloids (including pyrrolizidine alkaloids), and volatile oils [19, 21].

The mechanism of the anxiolytic action of Echium amoenum has yet to be firmly established. It is hypothesized that flavonoids may be responsible for the anxiolytic effect of Echium amoenum due to their ability to bind to benzodiazepine receptors [20].

Five animal studies have investigated the anxiolytic effects of Echium amoenum flowers using either aqueous or hydroalcoholic extracts; with each type of extract demonstrating both acute and chronic anxiolytic-type effects [19–23]. For example, one study in mice explored both the acute and chronic anxiolytic effects of a hydroalcoholic extract of Echium amoenum flowers using the light/dark test [19]. Three doses (i.p.: 12.5, 25, and 50 mg/kg) were used and compared to diazepam and control treatments; each were administered as a single treatment for the acute test, and one dose per day for 1 week for the chronic test. Acute anxiolytic effects were found for 25 and 50 mg/kg Echium amoenum doses only, with mice spending significantly more time in the illuminated area. Chronic anxiolytic effects were found with 12.5, 25, and 50 mg/kg of the Echium amoenum extract per day over 1 week, with the greatest effect for the 50 mg dose. Anxiolytic effects were found for all active treatments with increased time spent in the illuminated zone, but not increased number of transitions.

Two clinical trials to date have used an aqueous extract of Echium amoenum flowers to investigate the effects on chronic forms of anxiety. Forty-four patients with DSM-IV diagnosed OCD, and scores of 21 or over on the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) were included in a 6-week, double-blind RCT investigating the efficacy and safety of Iranian borage [24]. Participants received either 500 mg/day of Echium amoenum (4 × 125 mg capsules) or placebo. Patients were assessed with the Y-BOCS and the Hamilton Anxiety Rating Scale (HAM-A) at baseline and weeks 1, 2, 4, and 6. A gradual reduction in OCD symptoms was observed for both groups, with a greater reduction in Y-BOCS for Echium amoenum compared to placebo at weeks 4 and 6 (6.27 point reduction in mean Y-BOCS scores at end point). A similar pattern was found for anxiety symptoms, with a significant reduction in HAM-A scores for Echium amoenum at weeks 4 and 6 (10.06 point reduction at end point), although this decrease was not significantly different than placebo. No significant difference in adverse effects was found between the two groups, although increased constipation was reported in the placebo group (talcum powder tablets).