Thuja occidentalis (Arbor vitae): A Review of its Pharmaceutical, Pharmacological and Clinical Properties

Arbor vitae (Thuja occidentalis L.) is a native European tree widely used in homeopathy and evidence-based phytotherapy. Many reviews and monographs have been published on the herbal substance's description, mode of action and clinical use. However, no comprehensive evidence-based review is available. Therefore, our aim was to search MEDLINE databases and survey manufacturers for further details or unpublished data. This review presents the botany, ethnobotany and phytochemistry, especially the different contents of essential oil (Thujone) in relation to different extraction procedures of this medicinal plant. Thuja's antiviral action and immunopharmacological potential, such as stimulatory and co-stimulatory effects on cytokine and antibody production and activation of macrophages and other immunocompetent cells, have been evaluated in numerous in vitro and in vivo investigations. Although no controlled trials have been conducted on Thuja occ alone, many clinical studies have been performed with a herbal medicinal product containing a special extract of Thuja occ and other immunostimulants, demonstrating its therapeutic efficacy and safety in respiratory tract infections.

lower side showing a brighter green where resin glands also reside. Small, 1-2 cm long green to brown coniferous pins contain the seeds.

Ethnobotany
Previously, the genus Thuja (family: Cupressaceae) was considered to consist of the species Thuja koraiensis Nakai, Thuja occidentalis, Thuja orientalis L. and Thuja plicata D. Don (15,18) which are commonly cultivated in Central Europe.
Macroscopically, the cut herb shows small flattened, green twigs bearing paired, decussate leaves ~3 mm long. The keeled alternately lateral pairs partly conceal the flat rhomboidal, facial pairs pressed close together. The odour and taste are strongly camphoraceous (3). Only the dried twig tips of the species Thuja occ are used in the drug Thujae occidentalis herba. This consists of the varieties of Thuja occ: Thuja occ cv. Aureospica, Thuja occ cv. Lutea, Thuja occ cv. Vervaeneana and Thuja occ cv. Wareana (20). The cut herb should contain no more than 2% of the stem over a 4 mm diameter (Fig. 2). Table 1 presents the general description and distinguishing features of the four varieties.
A critical factor for Thuja's use as a medicinal herb is its content of essential oil. Indeed, the content of thujone seems to be significantly affected by different extraction procedures. The highest content of essential oil was found in extracts obtained by distillation, whilst percolation with purified water reduced the thujone content in the extract to the lowest level (29). Using purified water as a solvent, an average of 0.6 mg of thujone was extracted from 1 g of drug during percolation   Thuja polysaccharides and proteins (~4% of drug) Figure 3. Thujone, which consists of 85% ␣-thujone and 15% ␤-thujone, is the major constituent in the essential oil from the dried herbal substance Thujae occidentalis herba. (30). In contrast, when 30% (v/v) ethanol was used, 2.8 mg of thujone was extracted from 1 g of Thujae occidentalis herba, and Ͼ2.5-fold higher amounts of thujone (7.9 mg) were attained with high ethanol concentration (90% v/v) (30). From the result of 12 batches, the mean value for the thujone content was 1.16 mg/g of herbal substance and the maximum value was 2.95 mg/g, corresponding to 0.30%. Using a special extraction procedure with 30% (v/v) ethanol, the content of thujone in the extract can be reduced to one-third of the amount obtained by extraction with ethanol 90% (v/v) (Fig. 4).

Pharmacological Properties
The immunopharmacological potential of Thuja has been investigated in various in vitro and in vivo test models (33)(34)(35).
Of the many existing studies on this subject, only the most relevant in vitro and in vivo investigations will be described in greater detail in the following.

HIV-1 Activity
Thuja polysaccharides (TPS) inhibited human immunonodeficiency virus (HIV)-dependent cell death at a final concentration of 625 g/ml. At this concentration, TPSg was shown to be completely non-toxic for MT-4 cells, which had not been infected with HIV-1. TPS were shown to inhibit HIV-1-specific antigen expression on freshly infected MT-2 cells in a dosedependent manner (36,37).

Spleen Cell Proliferation
The Thuja retentate fraction (obtained by dialysis of an aqueous-ethanolic extract of Thuja occ cut-off: 10 000 Dan, Amicon SP20, S10Y10) caused an increase in the proliferation rates of spleen cells from NMRI and C3H/HeJ mice. In comparison with the medium control, a significantly higher incorporation rate was found up to a concentration of 7.8 g/ml. The maximum stimulation indices were 5.3 in the tests with spleen cells from NMRI mice. In the tests with C3H/HeJ spleen cells, stimulation index (SI max ) values of 3.4 were reached (38).

CD4-positive T Cells
Polysaccharide fractions with mol. wts ranging from 20 000 to Ͼ1 000 000 Da were isolated from the aqueous alkaline extract of the herbal parts of Thuja occ by ethanol precipitation and fractionation by ultrafiltration. High molecular weight subfractions of TPS proved to be highly mitogenic on peripheral blood leukocytes. It was demonstrated that the mitogenic and cluster-forming activity of TPS causes T-cell induction particularly of the CD4-positive T-helper/inducer cells in connection with an increased production of interleukin-2 (IL-2). This indicates that not only proliferation but also a differentiation to fully functional T-helper cells takes place. Only T cells, not B cells, were stimulated by TPS. The activation occurred only in the presence of the monocyte/macrophage fraction and was neutralized by antibodies against interferon (IFN)-␥ and IL-1. TPS-induced IFN-␥ production in CD4 ϩ cells was assumed to be the mode of action, whereby the IFN-␥ produced stimulated monocytes/macrophages to produce IL-1. As a second signal of the T-cell activation, this triggered the expression of IL-2 receptors and the production of IL-2 in CD4 ϩ T-helper cells (39,40).

Cytokine Induction
The incubation of P388C mouse macrophages with 10-100 g/ml of the retentate fraction from Thujae occidentalis herba led to an increased secretion of the cytokines IL-1, IL-6 and tumor necrosis factor-␣ (TNF-␣) in the cell culture supernatant. With regard to TNF-␣, the retentate fraction from Thuja showed concentration-dependent activity (38). These results were confirmed in cell culture assays with human blood monocytes, where incubation with the Thuja retentate fraction also induced a significant increase in IL-1,

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Properties of Thuja occidentalis (Arbor vitae)  IL-6 and TNF-␣ secretion. The cytokine titers in the supernatants were higher than the titers induced by phytohemagglutinin (PHA; 10 g/ml). After incubation of NMRI spleen cells with the Thuja retentate fraction, an increase in the production of IFN ␣ ␤ was observed. After application of the Thuja retentate, a maximum of 19 U/ml IFN ␣ ␤ was determined in the cell culture supernatants, whereas no IFN ␣ ␤ activity was detected in the supernatants from the medium control. Activity was also shown for the positive control substance concanavalin A which induced an IFN ␣ ␤ production of 19 U/ml at a concentration of 2 g/ml.

Antibody Production
The retentate fraction produced a concentration-dependent increase in the number of antibody-producing lymphocytes in the hemolytic plaque assay in vitro. The number of anti-SRBC-(sheep red blood cell)-IgM-producing plasma cells rose, as did all Ig-secreting plasma cells, as registered by the 'reverse' technique using protein A-labeled SRBCs. Incubation with lipopolysaccharide (LPS) as positive control also led to a concentration-dependent increase in the number of plaqueforming cells (41).
The influence of Thujae occidentalis herba on the oxidative metabolism of macrophages was determined by measuring the NO 2 Ϫ production of mouse alveolar macrophages (MH-S) after 48 h incubation with the respective test fractions. The NO 2 Ϫ production of the MH-S cells was increased by incubation with the retentate fraction in a concentration-dependent fashion from 2 Ϯ 0.3 mol/l (medium control) to a maximum of 42.5 Ϯ 2.7 mol/l. The combined addition of IFN and LPS (1 ϩ 5 ng/ml) served as a positive control in this test system (Fig. 5) (38).
The results of the in vitro investigations indicate an immunomodulatory and antiviral potential of Thuja occ. The main target of the immunologically active constituents seems to be the macrophage/monocyte fraction. The activation of macrophages manifested itself in increased cytokine secretion and the stimulation of NO 2 Ϫ production. The presence of macrophages was a prerequisite for the activation of CD4positive T cells, thus indicating a decisive role for macrophages also for the observed effects in spleen cell cultures i.e. stimulation of cell proliferation and antibody production.

Leukocyte Counts in Mice
A single dose of Thuja extract (i.v.) led to a dose-dependent left shift in white blood counts within 4 h. The segmented and stab granulocytes in the blood of the animals receiving the extract increased 3-to 4-fold compared with the control animals. The percentage of segmented granulocytes increased from 9 to 36%, and the percentage of stab granulocytes from 3 to 15%. Correspondingly, the number of lymphocytes decreased in comparison with the control mice (from 89 to 48%). The effects were transitory, i.e. the blood count had normalized after 15 h at the latest (38).

Cytokine Induction
The i.v. administration of a retentate fraction of Thujae occidentalis herba induced a significant increase in TNF-␣, IL-6 and IL-1 activity in the serum of treated mice compared with control mice that had received physiological NaCl only. The only dose dependency was found with regard to the induction of IL-6 and IL-1 (38).
Stimulation of cytokine production could also be seen in irradiated mice after i.v. application of Thuja TPSg fraction. After oral application of an aqueous-ethanolic extract of a mixture of Thujae occidentalis herba, Baptisiae tinctoriae radix, Echinaceae purpureae radix and Echinaceae pallidae radix, there were no recognizable differences in the cytokine titers [IL-2, IFN-␥, granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-1 and TNF-␣] between sera of NMRI mice that had been treated perorally with the herbal extract for a period of 1-9 days and the sera of the control mice. Concentrations of cytokines in sera of both groups ranged around the detection limit of the respective assays. Spleen cells from NMRI mice, however, that had been treated with the extract for 1-9 days produced higher amounts of IL-2, GM-CSF and IFN-␥ ex vivo in comparison with those from the control mice especially after additional stimulation by LPS or concanavalin A in vitro. Application of the extract also triggered the production of IL-1 and TNF-␣ by peritoneal macrophages ex vivo, particularly after co-stimulation with LPS ex vivo, thus mimicking the action of microbial antigens, such as bacterial LPS, in the case of infections (43).
Thus the peroral administration of the Thuja-containing preparation did not induce any systemic increase in cytokine titers, but probably caused a local activation of cytokineproducing cells for 'priming'. Thus, after local contact with the immunomodulatory active components of the extract in the gut or in the upper part of the bronchial system, cells are able to react to an additional stimulation causing increased cytokine secretion. Under physiological conditions, such a local modulatory cytokine induction is to be assessed more positively than a systemic one, which in the case of an overexpression could cause considerable side effects (43).

Antibody Response in Normal and Immunosuppressed Mice
Oral application of the extract significantly enhanced antibody response to SRBCs in healthy NMRI mice, inducing an increase in numbers of splenic plaque-forming cells (PFCs) and titers of specific antibodies in sera from treated mice. In mice immunosuppressed by old age or additional treatment with hydrocortisone, treatment with the extract resulted in a normalization of antibody response to SRBCs.
To prove the hypothesis of a gut-associated lymphoepithelial tissue (GALT) activation, the effects of Thuja on immunoreactivity of Peyer's patches (PP) cells was investigated by measuring the anti-SRBC antibody response of PP cells ex vivo using the hemolytic plaque technique. For this purpose, a Thuja retentate fraction was administered by gavage to five mice at a dose of 1 mg. Mice in the control group received a corresponding amount of matrix without test substance. After 15 h, mice were sacrificed and the PP cells were isolated and incubated with medium, LPS or the Thuja retentate fraction on cell culture plates. The number of antibodyproducing cells was determined after 7 days using the reverse hemolytic plaque technique. The extract significantly enhances antibody response to SRBCs in PP, thus indicating that immunologically active macromolecules can contact cells of GALT and modulate the mucosal immune response (43,44).

Immune Response in Mice after Long-term Treatment
The effects of an orally administered aqueous-ethanolic extract of a mixture of Thujae occidentalis herba, Baptisiae tinctoriae radix, Echinaceae purpureae radix and Echinaceae pallidae radix on the immune response in mouse were investigated over the short and long term. In all investigated mouse strains, the extract induced an increase in the numbers of PFCs and in the titers of specific antibodies in the sera of treated mice. Administration of the extract over several months also stimulated the PFC response without affecting spleen weight, total cell yield per spleen or white blood cell count (41).

Influenza Virus Type A Infection in Mice
The effects of oral application of an aqueous-ethanolic extract of a mixture of Thujae occidentalis herba, Baptisiae tinctoriae radix, Echinaceae purpureae radix and Echinaceae pallidae radix on the course of influenza A virus infection was investigated in Balb/c mice. The mixture extract was administered to mice via their drinking water for 14 days, starting 6 days before intranasal infection with influenza A virus. The extract induced a statistically significant increase in survival rates, prolonged mean survival time and reduced lung consolidation and virus titers. The experiment demonstrated that the plant immunomodulator given 6 days before exposure is a potent inhibitor of influenza A virus pathology in vivo.
Because of the proven immunomodulatory efficacy of the preparation, it was assumed that the positive effect of this extract exerted on infected mice is mediated first of all by its immunostimulating activity and that the direct antiviral activity is rather of secondary importance (45).

Clinical Efficacy
While there are many pre-clinical investigations in the literature, no data on clinical trials using Thuja occ as a single herbal substance are available. In contrast, many clinical studies have been conducted with a herbal medicinal product containing an aqueous-ethanolic extract of a mixture of Echinacea purpurea, Echinacea pallida, Baptisia tinctoria and Thuja occ [Esberitox ® , Schaper & Brümmer, Salzgitter, Germany (33,34)]. In most of these studies, the therapeutic efficacy of the Thuja-containing herbal product was proven in the treatment of acute respiratory tract infections and the common cold. Two of these pre-GCP (Good Clinical Practice) clinical trials showing good therapeutic efficacy of the herbal product on viral upper respiratory tract infections (URIs) are particularly noteworthy. One randomized, placebo-controlled, double-blind study enrolled 100 subjects (10). After just 3 days, the patients from the active

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Properties of Thuja occidentalis (Arbor vitae) treatment group showed significant improvements in their common cold symptoms. In the other randomized, placebocontrolled, double-blind study (9), 150 patients with URIs were treated with either Esberitox or placebo. In this latter study, Esberitox showed a similar efficacy result on URI symptoms to that in the former study. In addition, a significant enhancement of the immune response was observed in recall antigens. In four other studies, the herbal product was also found to be beneficial when used as an adjuvant to antibiotic treatment of bacteria-induced URI (11,12,46,47). In the following section, only the more recent and GCP-compliant studies are discussed in detail.

Common Cold
In a double-blind, placebo-controlled multicenter study (15 practitioners in Germany), patients suffering from acute common cold were randomized to receive Esberitox (Thuja daily dose corresponding to 6 mg three times daily (t.i.d.)] or placebo for 7-9 days (48). On a daily basis, the patients documented the intensity of 18 cold symptoms and their overall cold status on a 10-point scale and rated their general wellbeing using Welzel-Kohnen color scales. Additionally, a physician assessed the severity of illness on days 4 and 8 using the Clinical Global Impression (CGI-1). The main and confirmatory outcome measure was expressed as a total efficacy value. This was gauged from the z-standardized area under the curve (AUC) values of the primary end-points (rhinitis score, bronchitis score, CGI-1 and general well-being). Adverse events, overall tolerability, vital signs and laboratory parameters were documented. A total of 259 patients were eligible to be evaluated (intention to treat; ITT). The primary efficacy parameters demonstrated the herbal product's superiority over placebo (P Ͻ 0.05). The effect size was 20.6% of the SD [90% confidence interval (CI) 0.04-41.1] in the ITT population and 23.1% (1.7-44.5) in the valid case population. In relation to general well-being, the effect size was 33.9% of the SD (12.5-55.3%, VC). The efficacy of the drug was most prominent in the subgroup of patients who started therapy at an early stage of their cold or with a symptom score of Ն4 (Fig. 6). In the drug group, patients suffering symptoms of at least moderate intensity at baseline showed a response rate (at least 50% improvement of the global score, day 5) of 55.3%, and those in the placebo group showed a response rate of 27.3% [P ϭ 0.017; number needed to treat (NNT) ϭ 3.5]. This study showed the therapeutic benefit and superiority of Esberitox versus placebo for decreasing the duration of the common cold and alleviating symptoms. A greater benefit can be expected if patients with the common cold are able to start taking the drug as early as practicable after manifestation of initial symptoms.
In another double-blind, placebo-controlled study, 94 adults (age Ͼ18 years) suffering from the common cold for no longer than 1 day were randomized to two different treatment groups: active substance (the Thuja daily dose corresponding to 6 and 12 mg t.i.d) or placebo (49). The results of the primary efficacy criteria for evaluation (reduction of total number of tissues used throughout the entire observational period) were confirmed to be statistically significant and clinically relevant. This was true for both the ITT analysis (P ϭ 0.0259) and the per protocol analysis (P ϭ 0.0146). Subsequent pairwise group comparisons showed a statistically significant superiority of the high dose compared with placebo (P ϭ 0.0323). Similar results were obtained for the secondary efficacy parameters such as the number of tissues used on day 2 (P ϭ 0.0071) and day 5 (P ϭ 0.0495). From these results, it can be concluded that the clinical efficacy of the Thujacontaining herbal medicinal product has been confirmed for the treatment of acute common cold.

Supportive Therapy to Standard Antibiotics
Many antibiotics are known to have a suppressive effect on the immune system. A total of 53 patients scheduled to receive antibiotics for treatment of acute exacerbation of chronic bronchitis (as an example of a severe bacterial infection requiring antibiotics) were enrolled in a prospective, multicenter, p=0.045 p=0.040 Figure 6. Improvement of rhinitis scores (congested nose, runny nose, sniffling, frequency of handkerchief use, frequency of sneezing). Mean symptom scores (baseline adjusted mean and SE) are shown for placebo (n ϭ 57) and the Thuja-containing herbal remedy (n ϭ 58). P-values for the differences between treatment groups are given for the patients with a symptom score of Ն4 at day 1 (repeated measures; co-variates at day 1).
double-blind, placebo-controlled study (50). The aim of this research was to confirm any beneficial effects of the co-medication of antibiotics with the drug in subjects with acute exacerbation of chronic bronchitis. The main inclusion criteria were chronic bronchitis, staged to forced expiratory volume of the first second (FEV 1 ) with a value between 35 and 75%. Patients were randomly assigned to receive antibiotics plus either the herbal product (Esberitox, n ϭ 25; Thuja daily dose corresponding to 12 mg t.i.d.) or placebo (n ϭ 27). Antibiotic therapy was administered according to generally accepted guidelines and the study medication was given concomitantly for 28 days. The baseline-adjusted means for FEV 1 (%) on day 10 were 68.7 points for the drug group and 59.2 points for the placebo group (P ϭ 0.0303), showing a faster improvement in the drug group. For FEV 1 , the difference between the two treatment groups was 267 ml (P ϭ 0.0499). The time to halfmaximal improvement was 5.7 days in the drug group compared with 12.8 days in the placebo group.

Clinical Safety
Symptoms of intoxication from the fresh Thuja plant include vomiting, stomach ache, diarrhea and gastroenteritis followed by absorption disorders, headache, nervous agitation and chronic convulsions, and symptoms of liver and renal toxicity extending to yellow liver atrophy, arrhythmia and myocardial bleeding (51). In cases of overdose and abuse, the oral intake of Thuja extracts induced severe metabolic disturbances. Intoxication was accompanied by an irritant effect on the gastrointestinal tract, uterus, liver and kidney. Infants who ingested leaves and twigs of fresh plant showed mild gastrointestinal disorders and vomiting (24). These reactions can be explained by the high contents of thujone in the fresh plant. This ingredient occurs in significant quantities in the essential oil fraction Thuja occ (see 'Phytochemistry' above) and many other plants that are part of the human diet such as Salvia folium, widely used as spice and in teas, or Artemisia pontica, for the production of vermouth wine (52). Thujone is reported to be the toxic agent in absinthe, a liqueur popular in the 19th and early 20th centuries, and to have antinociceptive, insecticidal and anthelmintic activity (53). The acute toxicity information available for thujone was reported as follows (LD 50 in mg/kg): rats oral, 500; rabbit i.v., 0.031; rabbits dermal, 5000. The lethal dose appeared at 0.2 ml/kg body weight (BW) (54). In other investigations, porphyrogenic properties or mitogenic activities were reported (55,56). The Committee on Veterinary Medicinal Products noted that the LD 50 values stated for the constituent thujone were 87.5 mg/kg of BW after subcutaneous administration in mice and 240 mg/kg BW after intraperitoneal administration to rats (24). In an in vitro investigation, however, thujone was demonstrated to be non-toxic (57) and an oral dose of up to 1.25 mg thujone/kg BW is considered harmless in humans (58). Up to a single daily dose of 75 mg is reported to be safe in humans. In their opinion issued on 2 December 2002, the Scientific Committee of the European Commission stated that a 60 kg adult who consumes one litre of an alcoholic beverage containing 5 mg/l thujone, i.e. the maximum permitted level of thujone in alcoholic beverages with up to 25% alcohol, would ingest ~0.08 mg thujone/kg BW. This intake is ~100 times lower than the no effect level (NOEL) derived from a 14 week study in rats (59).

Adverse Events in Clinical Trials
The safety of Thuja-containing preparations especially those extracts percolated with ethanol 30% (v/v) can be regarded as good. In a randomized, double-blind, placebo-controlled study, 26 patients receiving Thuja and 23 patients receiving placebo showed adverse events (48). Adverse drug reactions (ADRs) were suspected in five patients in the placebo group and in two patients in the active treatment group [one adverse event in the drug group (sleeplessness) was deemed possibly related to the study medication]. No serious adverse event occurred. In another study, 19 mild to moderate adverse events (drug ϭ 8, placebo ϭ 11) in 14 subjects were documented (50). None of them was assessed as serious and no event was classified as an ADR. In a recent double-blind study, no adverse events were reported (49). The investigators concluded that the herbal product can be recommended as a safe treatment for acute viral respiratory tract infections. In addition, no adverse events were reported in another study (60). One study (9) showed that three patients of both study groups experienced mild nausea. Additionally, one patient treated with the Thuja-containing preparation complained of rheumatic symptoms and one patient from the placebo group reported heartburn. All adverse events were classified as 'mild' to 'moderate' and judged to be unrelated to the study medications.

Pregnancy and Lactation
Thujone-containing plants and preparations do not have direct or indirect abortive effects (52). Like many other herbs, decoctions of Thuja have traditionally been used for its abortive properties, presumably mixed with Savin tops (15). Such trials have been carried out repeatedly by laymen, but have failed nearly every time to prove any abortive properties and have often resulting in lethal toxicity. Deadly apnea can occur if large quantities of thujone are absorbed by the circulatory system (14). Over the many years of experience with Esberitox, no such safety concerns have been reported. For medical practice, however, it is recommended that Thuja not be used during pregnancy and lactation before consulting a doctor.

Post-marketing Data
Our search shows that Thuja is only used phytotherapeutically in the product Esberitox, a herbal medicinal product that has been marketed throughout the world for many decades. Considering the last 5 years of marketing (January 1999-August 2003), the distribution of the number of sold and/or prescribed packages of the herbal product corresponds to a total amount of 1150 million units (1 unit corresponds to ment with the maximum dosage (18-36 mg Thuja) as directed, this corresponds to 12.2 million treated patients in the covered period worldwide. During these periods, a total of 63 ADR reports with Esberitox were recorded. The majority of the ADRs were of a mild and transient nature. Classified by organ systems, skin and appendage disorders, such as skin rashes, were reported most frequently. This corresponds to an incidence of about five ADRs per 1 million treated patients. During this period, only one case of a serious and unexpected ADR of acute sarcoidosis (Löfgren's syndrome) was reported in the literature (61). Since the woman had injected a mistletoe preparation in addition to Esberitox, with a positive lymphocyte transformation test (LTT), a causal relationship with the Thuja-containing herbal product was assessed as 'unlikely'.
The recommended daily dose of Esberitox in adults is a dose corresponding to 18-36 mg of Thuja. Children should take 12-24 mg. This is equivalent to a daily dose of 18-36 g of thujone for adults and 12-24 g for children. This daily dose is several orders of magnitude below the threshold of 1.25 mg thujone/kg BW considered harmless for humans, and also below the maximum level of thujone permitted in alcoholic beverages (~0.08 mg thujone/kg BW for an adult).

Summary and Conclusion
Thuja occidentalis is widely used in homeopathy and evidencebased phytotherapy. Its immunopharmacological potential has been demonstrated in numerous in vitro and in vivo test models showing its immunostimulating and antiviral activities. A critical factor for Thuja's use as a medicinal herb is its content of thujone, which is reported to be the toxic agent of many fresh plants that are part of the human diet. However, the content of thujone in Thuja-containing finished products depends on the extraction procedure; thus it was shown that percolation with ethanol 30% (v/v) can significantly reduce the thujone content in comparison with the percolation by using ethanol 90% (v/v) for extraction. Three GCP-compliant, double-blind, placebocontrolled clinical studies have verified the efficacy and safety of a herbal medicinal product containing Thuja (Esberitox) for treatment of the common cold, and its efficacy as an adjuvant to standard antibiotic treatment of severe bacterial infections. In these studies, only mild to moderate adverse events were documented with a rare frequency, and none were assessed as serious. Based on the recommended daily dose and the manufacturing procedure, the amounts of thujone that would be ingested are well below the threshold limit values considered harmless for humans, and even below the maximum permitted level in alcoholic beverages. Over the past 5 years, Ͼ12 million patients have been treated with this herbal medicinal product worldwide. During the same period, ADRs of a mild to moderate nature were reported with a rare frequency, but none was judged to be relevant to the safety of the herbal product. Thus, it can be concluded that due to a special extraction procedure, this Thuja-containing preparation is a safe and effective herbal medicinal product for treatment of the common cold. The recommended daily dose of up to 36 mg of Thuja is within a safe and non-critical range. Notwithstanding these positive data, further comprehensive evidence from clinical trials, especially with the herbal substance alone, is still needed.