Thymoquinone: A Comprehensive Reference
1. Identity and Chemical Characterization
Chemical Names and Structure
Thymoquinone, a monoterpene molecule, is chemically known as 2-methyl-5-isopropyl-1,4-benzoquinone. It is also rendered in the literature as 2-isopropyl-5-methyl-1,4-benzoquinone, reflecting the same compound described from two different numbering perspectives. Its molecular formula is C10H12O2, with a molecular weight of 164.2. Among different chemical moieties, quinone is one of the most abundant groups, including naphthoquinone, anthraquinones, and benzoquinone. From this family of quinones, the benzoquinone represented by thymoquinone (TQ) has received enormous attention for its pharmacological properties and therapeutic potential.
TQ is a crystalline, yellow-colored, poorly water-soluble phyto-compound with excellent therapeutic efficacy in various diseases. Thymoquinone has been reported to possess potent lipophilicity and limited bioavailability, and exhibits light and heat sensitivity. Thymoquinone is also a major oxidation product of both thymol and its structural isomer, carvacrol.
Primary Botanical Source
Thymoquinone is abundantly present in seeds of Nigella sativa L., popularly known as black cumin or black seed, and belongs to the family Ranunculaceae. Nigella sativa L. is an annual herbaceous plant of the Ranunculaceae family, cultivated in the Middle East, Eastern Europe, and Western and Central Asia.
Most pharmacological properties of the whole seeds of nigella or their extracts are mainly attributed to its volatile oil, of which thymoquinone (2-isopropyl-5-methyl-1,4-benzoquinone), comprising about 27–57% of the oil, is the most abundant component. It was reported that the biological activities of N. sativa seeds are mainly ascribed to its essential oil constituent TQ (30–48%), and it was first extracted by El-Dakhakhny.
Secondary Botanical Sources
The presence of thymoquinone is not limited to N. sativa. When 50 species of Moroccan medicinal plants were evaluated for the identification of various phytochemicals, the presence of thymoquinone was identified in Piper longum extract by performing electron spin resonance (ESR) spectroscopy. Thymoquinone is also found in Nigella arvensis seeds; Thymus spp., Thymbra spp., Satureja spp., Monarda sp., Mosla spp., Origanum spp., Agastache spp., and Coridothymus spp. It is also found in select cultivated Monarda fistulosa plants, which can be steam distilled to produce an essential oil.
Co-occurring Active Constituents in Nigella sativa
Dithymoquinone, thymohydroquinone, and thymol are also the important active quinones found in N. sativa. Seed volatiles consist largely of olefinic and oxygenated monoterpenes, mainly p-cymene, thymohydroquinone, thymoquinone, γ-terpinene and α-thujene, with lower levels of sesquiterpenes, mainly longifolene. The seed is roughly 32–40% fixed oil, and that oil is dominated by linoleic and oleic fatty acids.
Common Forms and Preparations
Thymoquinone is encountered commercially and in research as a constituent of: (a) whole powdered Nigella sativa seeds; (b) cold-pressed or steam-distilled black seed oil; (c) standardized extracts enriched for TQ content; and (d) advanced pharmaceutical formulations. Though black cumin oil containing low TQ content (less than 1%) has been clinically investigated, a formulation containing 5% TQ (BCO-5) has exhibited significant clinical interest for alleviating sleep disorders and stress. HPLC analysis has been used to determine maximum accumulation of thymoquinone, and GC-MS analysis has been used to determine chemical composition of the seed oil. Variability in TQ content is well-documented: quantitative analysis of the seed (especially the volatile fraction) yields widely variable results, which may be due to one or a combination of different crop origins, possible varietal differences, contamination/adulteration, method of extraction, stage of maturation of the extracted seed, and other factors.
2. Traditional and Historical Use
Ancient and Cross-Cultural History
The whole or ground seeds of Nigella sativa L., known in Western culture as "black cumin" or "black caraway," have a three-millennial history of use in Middle- and Far-Eastern cultures as a food ingredient. N. sativa is native to southern Europe, North Africa, south and southwest Asia and has been traditionally used since ancient times as an important medicinal plant and spice. N. sativa is one of the most ancient known domesticated plants, and its seeds were reportedly found in Tutankhamun's tomb. Black cumin is referred to by its Hebrew name "Ketzah" in the Bible in the book of Isaiah 28:25–27, and is known for its curative properties.
The use of N. sativa seeds and oil in traditional remedies goes back more than 2,000 years, and the herb is described as "the Melanthion" by Hippocrates and Dioscorides. In the Holy Bible, it was identified as the curative black cumin, and Hippocrates described it as the Melanthion and Dioscorides as the Gith of Pliny.
Islamic, Indian, and Unani Medicine
N. sativa is included in the list of natural drugs in different medicines, including Tibb-e-Nabawi (the medicine of Prophet Mohammad), Unani Tebb, and Indian traditional medicine. Black seeds and their oil have a long history of folklore usage in the Indian and Arabian civilizations as food and medicine, and have been commonly used as treatment for a variety of health conditions pertaining to the respiratory system, digestive tract, kidney and liver functions, cardiovascular system, and immune system support, as well as for general well-being.
Traditional Therapeutic Indications
These valuable medicinal seeds have been used traditionally against a variety of diseases, such as dyspepsia, diabetes, headache, influenza, and asthma. The plant has been extensively utilized for the treatment of liver, lung, kidney, gastric, and psychological disorders. In traditional remedy, N. sativa seeds are commonly used as a spice and carminative.
Nigella sativa L. is a widely used spice and medicinal plant in Asia and the Middle East, renowned for its seeds and oil, which possess both culinary and therapeutic purposes.
3. Key Constituents and Established Mechanisms of Action
Antioxidant Mechanisms
Thymoquinone has been suggested to exert important health-beneficial effects, including antioxidant, anti-inflammatory, and anticancer effects. It has been shown to normalize glutathione levels and to increase the activities of antioxidant enzymes such as glutathione peroxidase, catalase, and superoxide dismutases. A central pathway by which TQ exerts antioxidant effects is activation of the Nrf2/ARE (Nuclear factor erythroid 2-related factor 2/antioxidant response element) signaling cascade. The activation of the Nrf2/ARE signaling pathway by TQ results in the inhibition of NF-κB-mediated neuroinflammation.
Anti-inflammatory Mechanisms
TQ exerts its anti-inflammatory and anti-oxidant effects via several molecular pathways, including the release of cytokines and activation of cyclooxygenase-2 (COX-2), nuclear factor erythroid 2-related factor 2 (Nrf2), phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT), and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Thymoquinone possesses immunomodulatory activities, in addition to its chemopreventive role, as it can target and modulate inflammatory molecules, like nuclear factor kappa B (NF-κB), interleukins, tumor necrosis factor-α (TNF-α), and certain growth factors. TQ inhibits LPS-induced IL-1β, IL-6, and IL-12p40/70 production, which suggests its potential in suppressing pro-inflammatory cytokines.
TQ inhibited inflammatory mediator production by blocking PI3K/Akt/NF-κB signaling pathway. Thymoquinone exhibited anti-inflammatory effects by decreasing several cytokines, including TNF-α, NF-κB, IL-6, IL-1β, IL-12p40/70, (CCL12)/MCP-5, (CCL2)/MCP-1, GCSF, and Cxcl10/IP-10, NO, PGE2, and iNOS. TQ also suppresses characteristics of airway inflammation by reducing the production of inflammatory mediators such as 5-lipoxygenase, leukotriene, and eosinophils.
Apoptosis and Anticancer Signaling
The anticancer effects of thymoquinone are mediated through different modes of action, including anti-proliferation, apoptosis induction, cell cycle arrest, ROS generation, and anti-metastasis/anti-angiogenesis. TQ reportedly induces apoptosis in tumor cells by suppressing NF-κB, Akt activation, and extracellular signal-regulated kinase signaling pathways, and also inhibits tumor angiogenesis.
The beneficial organoprotective activities of thymoquinone in experimental animal models of different human diseases are attributed to its potent anti-oxidant and anti-inflammatory properties. Thymoquinone has also been shown to alter numerous molecular and signaling pathways in many inflammatory and degenerative diseases, including cancer.
Ischemia and Cytoprotective Mechanisms
The primary protective mechanisms consistently identified in preclinical ischemia-reperfusion studies were the attenuation of oxidative stress, suppression of inflammation, and modulation of apoptosis and autophagy. These effects were mediated through key signaling pathways such as TLR4/NF-κB, MAPK, and Bcl-2/Bax. TQ achieves protection by modulating key intracellular signaling pathways, such as TLR4/NF-κB, MAPK, Bcl-2/Bax, LKB1/AMPK, TRPM, LC3II/p62, and JAK2/STAT3.
4. Scientific Evidence by Area of Use
4.1 Oncology (Cancer)
Preclinical Evidence (In Vitro and Animal Studies)
TQ has been reported to exhibit antiproliferative effects on cell lines derived from breast, colon, ovary, larynx, lung, myeloblastic leukemia, and osteosarcoma, and inhibited hormone-refractory prostate cancer by targeting androgen receptor and transcription factor E2F. TQ has anti-inflammatory effects, and it inhibits tumor cell proliferation through modulation of apoptosis signaling, inhibition of angiogenesis, and cell cycle arrest.
In the specific context of breast cancer, a systematic review following PRISMA 2020 guidelines found that TQ's ability to induce apoptosis, inhibit cell proliferation, enhance immune response, and reduce cell viability in cancer cells makes it a candidate for combination therapy in breast cancer treatment. The TQ–chemotherapy combination shows significant potential as a therapy for breast cancer, enhancing treatment efficacy while mitigating side effects. However, future clinical studies are needed to establish its safety and therapeutic applicability.
In glioblastoma multiforme (GBM), preclinical studies show that TQ inhibits key pathways involved in gliomagenesis, including PI3K/Akt/mTOR, NF-κB, and STAT3 pathways, thereby exerting antiproliferative, proapoptotic, and antiangiogenic effects; TQ also exhibits synergistic interactions with conventional chemotherapeutic agents, augmenting their efficacy while reducing treatment-associated toxicities.
Clinical Evidence (Human Trials)
Chemosensitization by TQ is mostly limited to in vitro studies, and it has potential in therapeutic strategy for cancer. A systematic search of PubMed, Web of Science, Scopus, and ClinicalTrials.gov identified ten completed clinical studies exploring the clinical utility of TQ in various human conditions, including type 2 diabetes, cancer, COVID-19, epilepsy, and chronic periodontitis. Overall, published trials suggest that TQ is generally well-tolerated and may provide therapeutic benefits in select conditions. The evidence base for TQ as a standalone anticancer agent in humans remains preliminary; no large-scale phase III clinical trials in oncology have been reported in the peer-reviewed literature as of the most recent systematic reviews.
Evidence strength (oncology): The bulk of the evidence is preclinical (in vitro and animal models). Clinical oncology data are limited to a small number of early-phase trials.
4.2 Metabolic Syndrome, Diabetes, and Glycemic Control
Human Clinical Evidence
According to a clinical trial by Najmi et al., N. sativa (500 mg/day) after 8 weeks was able to improve the efficacy of the therapeutic protocol (metformin + atorvastatin + aspirin) in patients with metabolic syndrome and poor glycemic control, so that NS-treated patients showed significant improvements in their fasting blood glucose (FBG), postprandial blood glucose (PPBG), HbA1c, and LDL-C levels.
A participant-blinded, placebo-controlled clinical trial of 60 type 2 diabetes mellitus patients taking oral hypoglycemic agents demonstrated that the administration of powdered N. sativa (2 g/day) for 1 year resulted in significant reduction of HbA1c. In addition, a single-blind, randomized controlled trial of 99 outpatients with metabolic syndrome who received 1.5 mL and 3 mL of oral N. sativa oil daily for 20 days reported a significant reduction in HbA1c levels. Furthermore, a prospective, comparative, open-label study of patients with chronic kidney disease (stage 3 and 4) due to diabetic nephropathy who received 2.5 mL of oral N. sativa oil daily for 12 weeks along with conservative therapy reported a reduction of blood glucose along with improvements in serum creatinine, blood urea, and elevated glomerular filtration rate (GFR), and hemoglobin levels.
Thymoquinone has demonstrated beneficial effects in the treatment of the different components of metabolic syndrome, with a good safety profile. The effects of N. sativa and TQ on different components of metabolic syndrome and cardiovascular disease risk factors, including high blood pressure, obesity, dyslipidemia, and high blood glucose, have been established.
Evidence strength (diabetes/metabolic syndrome): Multiple randomized controlled trials (though many are small and use whole seed or oil preparations rather than isolated TQ) support modest glycemic and lipid benefits. This is among the better-supported areas of TQ clinical evidence, though study sizes, durations, and preparations vary considerably.
4.3 Cardiovascular System
The main bioactive component thymoquinone is responsible for the pleiotropic pharmacological properties of the seeds, including anti-oxidant, anti-inflammatory, anti-hypertensive, anti-hepatotoxic, hypoglycemic, and lipid-lowering properties. In preclinical ischemia-reperfusion injury models, TQ treatment reduced myocardial infarct size, improved cardiac function, decreased the release of myocardial enzymes, and suppressed cardiac arrhythmias. Systematic preclinical evidence establishes TQ as a broad-spectrum agent against ischemia-reperfusion injury, though direct human cardiovascular intervention trials remain limited and small-scale. The Phase I human safety data (see Safety section) noted that TQ-enriched black cumin oil at 200 mg/day for 90 days was associated with a significant reduction in total cholesterol, LDL, VLDL, and triglycerides within normal range in healthy subjects.
Evidence strength (cardiovascular): Strong preclinical, including mechanistic data. Human evidence is indirect (lipid parameters in safety trials) or derived from metabolic syndrome trials. Dedicated cardiovascular endpoint trials in humans are lacking.
4.4 Neuroprotection
The neuroprotective effects of TQ may be related to its modulatory effects on inflammation, apoptosis, and oxidative stress. Thymoquinone may prevent neurotoxicity and Aβ1-40-induced apoptosis. Thymoquinone is therefore worth studying further for its potential to reduce the risks of developing Alzheimer's disease. In an animal model of temporal lobe epilepsy, thymoquinone pretreatment (10 mg/kg) decreased oxidative stress indices such as malondialdehyde and nitrate in the hippocampal tissue and severe seizure activity. Thymoquinone also ameliorated astrogliosis and reduction in neurons in cornu ammonis-1 (CA1), CA3, the hilar regions, and mossy fiber sprouting in the dentate gyrus of kainate-lesioned rats.
Epilepsy has been among the conditions studied in completed TQ clinical trials. However, this evidence remains preliminary.
Evidence strength (neuroprotection): Predominantly preclinical. A small number of human studies have been initiated, but robust clinical data are not yet available.
4.5 Antimicrobial Activity
Various studies on N. sativa have established a broad spectrum of pharmacological actions, which include antioxidant, antidiabetic, anticancer, antitussive, immunomodulator, analgesic, antimicrobial, anti-inflammatory, spasmolytic, and bronchodilator properties. Thymoquinone is held in great esteem for imparting antibacterial characteristics to black seed. It can also be used as an antibiofilm agent in the form of a bioactive.
Evidence strength (antimicrobial): Evidence is predominantly in vitro. Robust human trials for infectious indications are not yet available.
4.6 Respiratory Health and Allergy
Thymoquinone has anti-inflammatory, anti-oxidant, and anti-apoptotic properties in several disorders such as asthma, bronchitis, headache, eczema, fever, dizziness, and influenza. TQ suppresses characteristics of airway inflammation by reducing the production of inflammatory mediators such as 5-lipoxygenase, leukotriene, and eosinophils.
Evidence strength (respiratory/allergy): Largely preclinical and animal data. Very few clinical human trials for respiratory endpoints have been published.
4.7 Hepatoprotection and Nephroprotection
Thymoquinone is known to help protect against liver damage and is commonly mentioned as hepatoprotective, mainly being active against fibrosis. Thymoquinone has been shown in studies to have antioxidant, cardioprotective, neuroprotective, hepatoprotective, anti-inflammatory, antimutagenic, and antiproliferative properties.
Evidence strength (hepatoprotection/nephroprotection): Strong preclinical evidence. Some indirect human data from metabolic syndrome and diabetic nephropathy trials exist. Dedicated hepatoprotective clinical trials in humans are sparse.
4.8 Skin Conditions (Including Psoriasis)
TQ demonstrates diverse pharmacological properties, including antioxidant, anti-inflammatory, antiviral, anticancer, antibacterial, immunomodulatory, anticoagulant, antipsychotic, anxiolytic, antidepressant, and anticonvulsant activities. TQ, derived from Nigella sativa seeds, exhibits promising anti-inflammatory, antioxidant, and immunomodulatory properties that could prove beneficial in managing psoriasis. Preclinical studies employing mouse models have demonstrated that nano-TQ effectively mitigates inflammation, erythema, scaling, epidermal thickness, and cytokine levels in psoriatic lesions.
Evidence strength (skin): Predominantly preclinical and in vitro; human clinical trials for dermatological endpoints are lacking.
5. Pharmacokinetics, Bioavailability, and Formulation
A significant challenge for the clinical translation of thymoquinone is its biopharmaceutical profile. The delivery of TQ via oral administration is largely restricted by solubility-related poor oral bioavailability and the relatively low solubility of pure TQ in water. Physicochemical challenges include low solubility, low gastrointestinal stability, light and heat sensitivity, systemic metabolism, and fast uptake by healthy cells and tissue.
The oral bioavailability of TQ is low and requires a high dose for the management of a variety of diseases due to low stability in the GI environment. Clinical research evaluating the safety and efficacy of TQ in humans remains limited, partly due to biopharmaceutical limitations.
To overcome these barriers, researchers have investigated various nanotechnology-based delivery systems. Pharmacokinetic results with one chitosan-polymer lipid nanoparticle formulation (THQ-CPLNPs) depicted approximately 3.53-fold improved oral bioavailability of TQ compared to a simple suspension. Recent advancements in nanomedicine, particularly nanocarrier-based encapsulation of TQ, offer a promising solution. This approach aims to overcome the bioavailability challenges associated with TQ and unlock its full therapeutic potential.
6. Dosage Forms and Reported Dosages
The following dosages are reported from the studies cited and reflect experimental or clinical trial use — they do not represent clinical recommendations.
- In a placebo-controlled clinical trial involving 60 type 2 diabetes mellitus patients, powdered N. sativa was administered at 2 g/day for 1 year.
- In a clinical trial for metabolic syndrome, N. sativa was administered at 500 mg/day for 8 weeks alongside standard medications.
- A randomized controlled trial administered 1.5 mL and 3 mL of oral N. sativa oil daily for 20 days in patients with metabolic syndrome.
- A Phase I randomized, double-blinded, placebo-controlled trial evaluated TQ-enriched black cumin oil (BCO-5, containing 5% TQ) at a dose of 200 mg/adult/day for 90 days in 70 healthy subjects.
- In a preclinical (rat) study, TQ was administered at 50 mg/kg body weight/day for 4 weeks to assess glycemic and oxidative effects in diabetic animals.
- In an anti-obesity rat model, TQ was administered at 20 mg and 40 mg doses in high-fat diet-fed rats.
- Thymoquinone pretreatment at 10 mg/kg was used in a rat epilepsy model.
- In acute toxicity studies in rats, oral doses of 200, 300, and 500 mg/kg body weight were tested. The maximum tolerated dose for oral ingestion was determined to be 250 mg/kg in both male and female rats.
Though a convincing number of experimental studies are available, human studies with isolated thymoquinone are not available despite the long history of use of black cumin. Clinical studies including pharmacokinetic studies and regulatory toxicity studies are required to encourage the clinical development of thymoquinone.
7. Safety Considerations and Drug Interactions
General Tolerability in Human Studies
The Phase I clinical trial of TQ-enriched black cumin oil at 200 mg/adult/day for 90 days reported neither serious adverse side effects nor any significant alterations in hematological parameters. The absence of significant changes in biochemical parameters related to liver function (ALT, AST, ALP) and renal function (serum creatinine and urea) was also observed. Analysis of lipid profile showed a significant (p < 0.05) reduction in total cholesterol, LDL, VLDL, and triglycerides, but within the normal range. BCO-5 was concluded to be safe at 200 mg/adult/day for human consumption.
Preclinical Toxicity Data
Single oral ingestion of TQ at doses of 200, 300, and 500 mg/kg was given to male and female rats and observed for 5 days. Signs of weight loss, diarrhea, hypoactivity, slight abdominal distension, and shortage of breath were observed in 34% of the rats that received 300 and 500 mg/kg within the 48 hours post-dose. In a subacute toxicity study, oral administration of 100 mg/kg of TQ did not cause mortality in either male or female mice but resulted in toxicity to the liver. It is postulated that long-term consumption may cause toxicity to the liver but not to the extent of altering the functions of the organ. The no-observed adverse effect level (NOAEL) was found to be 10 mg/kg/day for mice in both sexes.
It has been demonstrated that thymoquinone causes side effects and interacts with drugs.
Cytochrome P450 Enzyme Interactions
A critical safety concern involves TQ's modulation of cytochrome P450 (CYP) drug-metabolizing enzymes. Both quercetin and thymoquinone have a moderate to strong inhibitory effect on CYP3A4 activity. Neither has any significant inhibitory effect on the activity of CYP1A2 or CYP2E1 enzymes. CYP3A4 is among the most important enzymes for drug metabolism and is responsible for the breakdown of a large proportion of pharmaceutical drugs.
Thymoquinone, the predominant bioactive compound in Nigella sativa oil, may inhibit the activity of cytochrome P450 2C9 (CYP2C9). Data demonstrated that thymoquinone could inhibit warfarin 7-hydroxylase activity with an IC50 value of 11.35 ± 0.25 μM. Kinetic analysis indicated that thymoquinone exhibited competitive inhibition on warfarin 7-hydroxylation with a Ki value of 3.50 ± 0.44 μM. Because S-warfarin is a CYP2C9 substrate and is the more pharmacologically active enantiomer, this interaction raises potential concerns about altered anticoagulant drug exposure.
When N. sativa was co-administered with cyclosporine in an animal model, the oral bioavailability of cyclosporine was significantly reduced, suggesting the possibility that N. sativa induces intestinal P-glycoprotein and/or hepatic CYP3A activity. This finding is relevant for transplant or immunosuppressed patients using cyclosporine.
Anticoagulation Concerns
As many patients taking prescription medications are concomitantly using herbal supplements, there is considerable risk for adverse herbal drug interactions. Such interactions can enhance the risk for an individual patient, especially with regard to drugs with a narrow therapeutic index such as warfarin, cyclosporine A, and digoxin. Given TQ's demonstrated inhibition of CYP2C9 (the primary warfarin-metabolizing enzyme) in vitro, potential pharmacokinetic interaction with warfarin is a specifically documented concern for this compound.
Light and Heat Sensitivity
TQ's hydrophobicity, low aqueous solubility, and photosensitivity limit its clinical application. These physical properties affect both the shelf-life and the bioavailability of preparations containing TQ, and are a key motivation for nanoformulation research.
Pregnancy and Reproductive Considerations
Studies specifically examining TQ's safety in pregnancy or lactation in humans are not present in the currently indexed clinical literature; preclinical data examining reproductive effects exist but are outside the scope of direct human evidence and are therefore not extrapolated here.
FDA Regulatory Status of the Parent Plant
In the United States, the FDA classifies Nigella sativa as Generally Recognized as Safe (GRAS) for use as a spice, natural seasoning, or flavoring (21 CFR 182.10). This GRAS status applies to the seed as a food ingredient and does not constitute a regulatory determination regarding isolated TQ as a therapeutic compound.
8. Current State of Research and Evidence Gaps
Thymoquinone, the lead compound of black seed, has been examined in hundreds of published papers — though most of that work is preclinical (cell or animal studies) rather than large human trials. Despite the long historical use of black seed in different health-related issues, human studies for the use of isolated thymoquinone are not as accelerated.
A 2025 systematic review identified ten completed clinical studies exploring TQ in various human conditions — including type 2 diabetes, cancer, COVID-19, epilepsy, and chronic periodontitis — with seven clinical studies ongoing. There are challenges such as limited clinical trials, low bioavailability, and the need for more research to understand its long-term safety and effectiveness.
The most promising areas for clinical translation, based on the convergence of preclinical mechanistic evidence and early human trial results, include: glycemic control and metabolic syndrome management; adjunctive oncology (chemosensitization); and neuroprotection. Formulation science — particularly nanoparticle and lipid-carrier delivery — is an active area that may ultimately improve bioavailability sufficiently to enable the design of controlled clinical trials with isolated TQ.
References
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