The Therapeutic Effects of Nigella sativa on Skin Disease: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

The aim of this systematic review was to identify randomized controlled trials that looked at the effects of Nigella sativa in any form on different skin diseases. Up to March 2022, the online databases of Scopus, Web of Science, PubMed, Embase, Google Scholar, and Cochrane trials were searched. This study included 14 records of people who had experienced different types of skin disease including atopic dermatitis, vulgaris, arsenical keratosis, psoriasis, vitiligo, acute cutaneous leishmaniasis, warts, eczema, and acne. The mean SD age of the patients was 28.86 (4.49); [range: 18.3–51.4], with females accounting for 69% (506 out of 732) of the total. The follow-up mean SD was 8.16 (1.3) (ranged: 4 days to 24 weeks). The odds ratio (OR) was found to be 4.59 in a meta-analysis (95% CI: 2.02, 10.39). Whereas the null hypothesis in this systematic review was that lotion had no impact, OR 4.59 indicated that lotion could be effective. The efficacy of N. sativa essential oil and extract has been demonstrated in most clinical studies. However, more research is needed to completely evaluate and validate the efficacy or inadequacy of therapy with N. sativa, although it appears that it can be used as an alternative treatment to help people cope with skin problems.


Introduction
Te skin is the largest organ and functions as a barrier to protect the underlying tissues against the elements and pathogens, while also fulflling many physiological roles and biochemical functions such as preventing excessive water loss [1]. Skin diseases have recently become a major concern among people of all ages due to their highly visible symptoms and persistent and difcult treatment that have a signifcant efect on quality of life [2].
Nigella sativa belongs to the Ranunculaceae family is an annual plant which distributed in southern Europe and some parts of Asia, including Syria, Turkey, Saudi Arabia, Pakistan, and India. Diferent active pharmaceutical ingredients have been identifed in the N. sativa seeds, including saponins, favonoids, cardiac glycosides, thymoquinone, thymol, limonene, carvacrol, p-cymene, alpha-pinene, 4-terpineol, longifolene, t-anethole benzene, isoquinoline, and pyrazole alkaloids, as well as unsaturated fatty acid such as linoleic acid, oleic acid, and palmitic acid [3]. Food and therapeutic uses of N. sativa oil seeds have a long history in Persian traditional medicine. Avicenna, in his famous book, Te Canon of Medicine, has reported several black cumin properties, such as fatigue improvement and energy recovery. It has been traditionally used for the treatment of asthma, bronchitis, and rheumatism.
Since traditional treatments have become widely popular in recent decades, it is imperative to provide patients with skin diseases enough evidence-based alternatives to help them manage their symptoms. Te aim of this systematic review and meta-analysis was to evaluate the overall efectiveness of N. sativa products for treating skin problems.

Methods
In this systematic review and meta-analysis, we preferred reporting items according (PRISMA) guideline (Supplementary fle S1).

Search Strategy and Study Selection.
Te MESH and non-MESH search terms applied were ("Nigella sativa * " OR "Kalonj * " OR "Black Cumin * ") AND ("Acne Vulgaris" OR Dandruf OR "Atopic Dermatitis" OR "Contact Dermatitis" OR "Exfoliative Dermatitis" OR "Perioral Dermatitis" OR "Seborrheic Dermatitis" OR Eczema OR Hirsutism OR Ichthyosis OR "Seborrheic Keratosis" OR "Cutaneous Lupus Erythematosus" OR "Discoid Lupus Erythematosus" OR "Phototoxic Dermatitis" OR "Phototoxic Dermatitis" OR "Hyperpigmentation" OR "Hypopigmentation" OR "Pruritus Ani" OR "Pruritus Vulvae" OR "Acne Vulgaris" OR "Seborrheic Dermatitis" OR "Psoriasis"). In our search strategy, study designs, participants, publication time, and language were deliberately not limited in order to facilitate fnding all the relevant studies. All searches were conducted by two researchers (NM and MI) independently. Duplicated studies were then eliminated. In general, these two authors had an agreement on selecting the studies, and possible variations were removed by discussion.

Inclusion and Exclusion
Criteria. If a study met the following criteria, it was considered for inclusion: (1) patients with mild to severe skin disorders were recruited, (2) using of any products of N. sativa in the forms of oral and topical (3) the type of skin disorders conditions experienced by the participants was not restricted, (4) utilized N. sativa in combination with other plants, phytochemicals, drugs, or supplements; (5) controlled clinical trials of either parallel or crossover design.
We excluded research that involved: (1) Publications without accessible English abstracts, as were reviews, case reports, comments, theses, book sections, and conference proceedings. (2) Employed animal models.

Data Collection.
Two authors (NM and MI) independently extracted data, including frst author, publication date, study type, location, total, sample size, age, sex, and dose, form (powder, oil, and extract), duration of treatment, main outcomes (mean and standard deviation), and adverse efects.

Quality assessment of the Evidence. Te Joanna Briggs
Institute critical appraisal technique was used to assess the quality of the studies included. Joanna Briggs proposed 13 criteria for evaluating the quality of randomization clinical trial trials. Te Joanna Briggs Institute critical evaluation was also used for case series and quasi-experimental investigations. Adjudication was utilized to resolve diferences in the grade of risk of bias studies.

Statistical
Analysis. Data were presented using descriptive statistics (mean ± SD) for continuous variables; frequency and percentage for categorical variables. We applied random-efectsmeta-analysis to calculate risk ratio and 95% confdence interval in Stata version 14.2.

Studies Characteristics.
Te list of included studies on skin disease therapeutic efects with N. sativa is shown in Table 1. In all, 14 records out of 300 unique articles were possibly eligible; ultimately, 4 papers were included in this meta-analysis [11,12,14,15] (Figure 1). Of the 14 included studies, one was conducted in Germany [10], the Czech Republic [11], Tukey [22], India [13], and Bangladesh [17], two were conducted in Iraq [14,18], and the other was carried out in Iran [12,15,16,[19][20][21]23]. Studies were done on individuals who had experienced diferent types of skin disease, as listed in Table 1. In the clinical trials studied in this review, N. sativa oil was administered in 12 studies, and in two N. sativa studies crude extract was administered [12,18].

Adverse Efects.
Out of the three studies that evaluated the adverse efects of treatment with N. sativa, Kalus et al. reported transient gastrointestinal problems [11]. One study reported that 62% of participants in the invention group had gastric irritation, including abdominal cramps, and indigestion [17], and the other 5 out of 75 patients in the N. sativa group (6.7%) reported topical side efects among patients [15].

Findings from the Meta-Analysis.
A total of 1159 patients were included in the systematic review. Te mean SD age of the patients was 28.86 (4.49); [range: 18.3-51.4], with females accounting for 69% (506 out of 732) of the total. Te follow-up mean SD was 8.16 (1.3) (ranged: 4 days to 24 weeks). Te odds ratio (OR) was found to be 4.59 in a metaanalysis (95% CI: 2.02, 10.39). Whereas the null hypothesis in this systematic review was that lotion had no impact, OR 4.59 indicated that lotion could be efective. Based on the Evidence-Based Complementary and Alternative Medicine  Figure 2) and Galbraith diagram (Figure 3), there was no signifcant heterogeneity between studies, but the value of T2 � 0.46 shows that there is signifcant heterogeneity within studies. Te results of the study do not appear to be impacted by publication bias, according to the funnel plot ( Figure 4) and the Egger's test (B = 3.54, p value = 0.36), although the assessment of the publication bias is unreliable because there were only four papers included in the meta-analysis. Based on the fndings of the sensitivity analysis, the results were infuenced by one study [14]. When the recent study was taken out of the sensitivity analysis, the results were 3.45 less than the estimated value. As can be seen from the subgroup analysis, it appears that this study had an infuence on the study's fndings.

Quality Assessment of Included Studies.
As a result, the quality of the included studies is assessed using the critical assessment tool for randomization clinical trials developed by the Joanna Briggs Institute. To evaluate the quality of case series and quasi-experimental research, please consult

Discussion
Te current meta-analysis revealed that supplementation with N. sativa can potentially be efective in the treatment of diferent skin problems including atopic dermatitis, eczema, warts, keratosis, psoriasis, vitiligo, infant skin infections, and acne. However, the fndings should be declared with caution because of heterogenicity. Te studies included in the metaanalysis were homogeneous, and the diferences between the studies did not signifcantly afect the estimated index, according to the value of the I2 index. However, there was heterogeneity within studies through using Galbraith diagram. Heterogeneity is an important consideration in systematic reviews, as high heterogeneity (more than 75%) indicates that it is not suitable to perform meta-analysis. To the best of our knowledge, there is no systematic review that has examined the efects of N. sativa on the improvement of symptoms of skin diseases. Te study of the various forms of N. sativa showed that the oil supplement in topical form is more commonly reported. Te pharmacological properties of N. sativa are more clearly observable in this form than in extract form because thymoquinone is a solvent in oil. A minimum of 4 weeks and a maximum of 24 weeks are recommended for the treatment period. Te dermatological treatments of N. sativa are attributed to its strong antioxidant, anti-infammatory, antimicrobial, and immunomodulatory potential, which altogether make it a promising skincare candidate. Since thymoquinone is one of the principal compounds of N. sativa and the concentration of it may be varied greatly depending on the storage and preparation of plant products, it is expected that the prescribed herbal products will be standardized according to the active ingredient thymoquinone. However, there is no information regarding the quantifcation or standardization of bioactive compounds among the clinical trials reviewed here.

Evidence-Based Complementary and Alternative Medicine
Standardization of herbal formulations is necessary in order to evaluate the quality of drugs based on the concentration of their active constituents or phytochemicals [24]. Standardization of herbal medicines carries an assurance of quality, efcacy, safety, and reproducibility [25]. Tymoquinone exists in tautomeric forms including the enol, keto, and mixtures in the oil of the plant. Te keto form is responsible for the pharmacological features of thymoquinone [26]. Several potential mechanisms can be proposed for the observed ameliorating infuences of N. sativa on infectious and noninfectious skin conditions including diferent types of allergies, autoimmunity, skin infammations and wounds, and vitiligo. Te fndings of Ali and Meitei showed that the extract of N. sativa, as well as its active constituent thymoquinone, mimics the action of acetylcholine in melanin dispersion leading to skin darkening via stimulation of cholinergic receptors of a muscarinic nature within the melanophores of wall lizard. Tis study opens new vistas for the use of N. sativa active ingredient, thymoquinone, as a novel melanogen for its clinical application in skin disorders such as hypopigmentation or vitiligo [9].    Evidence-Based Complementary and Alternative Medicine Generally, there is an agreement regarding the impressive efects of N. sativa on infammatory, oxidative, reactive oxygen species, and immunologic parameters in animal models. Houghton et al. showed that the antiinfammatory action of TQ resulted from the prevention of eicosanoids generation, such as thromboxane B2 and leukotriene (LT) B4, by inhibiting both cyclooxygenase and 5-lipoxygenase, and in part via nonenzymatic peroxidation of membrane lipids [27]. TQ induce a signifcant inhibition on LTC4 synthase activity [28]. Velagapudi et al. demonstrated that TQ treatment elevated the activation of the NrF2/ARE pathway leading to the suppression on NF-κB and following neuroinfammatory responses in microglia cells [29]. TQ was recently discovered to attenuate atopic dermatitis by reducing the levels of infammatory cytokines, such as IL-4, IL-5, and IFN-gamma, and immunomodulatory cells in the blood. However, Liang et al. indicated that a high dose of TQ (higher than 16 μM) possibly showed cytotoxicity on keratinocytes [8]. So, in clinical trials must be consider the standardization of the plant. 12 studies out of 14 in this review have reported the efcacy of essential oils of black cumin in skin disease. In light of the relatively low amount of TQ in the N. sativa essential oil, it seems that the skin healing efects of N. sativa are related to terpenoid compounds in addition to TQ. Terefore, the determination of these active constituents is recommended to achieve the N. sativa oil optimal dose to improve its efcacy. Te previous investigations have shown the essential oil immunostimulatory efects on T cells and meaningfully inhibited allergy-associated cytokines IL-4 and IL-13 [30]. Te antioxidant and anti-infammatory efects of some other constituents of N. sativa essential oil such as p-cymene, t-anethole, thymol, carvone, α-terpineol, longifolene, and β-caryophyllene have been demonstrated in various studies [30][31][32][33][34][35][36].

Limitations
Tere were some limitations on these clinical studies, including the lack of reporting of any herbal standardization, the lack of measurement of chemical constituents of the plant, and study quality. Te fndings of this review should be considered cautiously due to various limitations. Te fact that this study looked at skin disorders in general, and the number of clinical studies included in the meta-analysis was small (n � 4), so in the main analysis, therefore, limiting the sample size decreases the study's confdence level and increases the margin of error. Te protocol for this review was has not been preregistered with PROSPERO, so it is a limitation of this review.

Conclusions
Te efcacy of N. sativa essential oil and extract has been demonstrated in most clinical studies. Tis is the frst systematic review assessing the available literature on the efects of N. sativa on skin diseases in clinical studies. In this systematic review article, we tried to give persuasive clues on the efcacy of N. sativa in skin disorders management and its mechanisms of action. However, more research is needed to completely evaluate and validate the efcacy or inadequacy of therapy with N. sativa, although it appears that it can be used as an alternative treatment to help people cope with skin problems.

Data Availability
Te data that supports the fndings of this study are available in this article from the corresponding author upon request.

Conflicts of Interest
Te authors declare that there are no conficts of interest.