Black Seed

Thymoquinone (TQ), the primary bioactive compound in Nigella sativa, is a potent competitive inhibitor of CYP2C9 (IC50 = 0.9 µM; Ki = 3.50 µM in human liver microsomes). CYP2C9 is the main enzyme responsible for S-warfarin hydroxylation. At clinical doses (N. sativa >1 g/day), TQ concentrations sufficient to inhibit CYP2C9 are achievable, increasing warfarin plasma levels and anticoagulant effect. Additionally, N. sativa independently inhibits platelet aggregation and prolongs bleeding time, creating a dual pharmacokinetic and pharmacodynamic risk.

Monitor INR closely when black seed is added to or removed from warfarin therapy. In vitro-in vivo extrapolation suggests clinically significant inhibition at doses >1 g/day N. sativa or >18 mg/day TQ. Advise patients to report any unusual bruising or bleeding. Avoid use of black seed oil supplements in anticoagulated patients unless INR is closely monitored.

Thymoquinone competitively inhibits CYP2C9-catalysed phenytoin hydroxylation (p-HPPH formation) with a Ki value of 4.45 µM. In vitro-in vivo extrapolation predicts clinically significant inhibition at N. sativa >1 g/day. Phenytoin has a very narrow therapeutic index (10–20 mg/L) and is subject to saturable (Michaelis-Menten) kinetics; even minor reductions in clearance can produce disproportionate increases in plasma levels and toxicity (nystagmus, ataxia, encephalopathy).

Avoid concurrent use of black seed supplements with phenytoin. If co-administration occurs, monitor phenytoin plasma levels and signs of toxicity (ataxia, nystagmus, diplopia, confusion). Dose reduction of phenytoin may be required. Advise patients with epilepsy to disclose all supplement use to their neurologist.

An animal pharmacokinetic study co-administering Nigella sativa saline suspension with cyclosporine showed a significant decrease in cyclosporine Cmax and AUC0-∞ by 35.5% and 55.9% respectively, with 2-fold higher clearance. This is attributed to induction of CYP3A4 and/or P-glycoprotein at the intestinal level, increasing first-pass metabolism. The mechanism may differ from the CYP2C9 inhibition (thymoquinone) versus the induction effect seen with multiple doses, creating uncertainty about net clinical effect.

Black seed should be avoided in transplant patients on cyclosporine. Animal data suggests reduced cyclosporine exposure, which could precipitate acute rejection in transplant recipients. The bidirectional potential (inhibition vs induction depending on dose and duration) makes this combination unpredictable and dangerous in immunosuppressed patients.

Thymoquinone and N. sativa fixed oil reduce fasting blood glucose via multiple mechanisms: inhibiting hepatic gluconeogenesis, enhancing pancreatic beta-cell function, improving peripheral insulin sensitivity, and inhibiting intestinal glucose absorption. A randomised controlled trial demonstrated N. sativa (2 g/day for 12 weeks) significantly reduced HbA1c and fasting glucose in type-2 diabetic patients on standard hypoglycaemic therapy. Additive effects with insulin or sulfonylureas can cause hypoglycaemia.

Monitor blood glucose closely when black seed supplements are added to antidiabetic regimens. The glucose-lowering effect is modest but additive with hypoglycaemic drugs. Insulin and sulfonylurea doses may need downward adjustment to avoid hypoglycaemia. Advise patients to self-monitor blood glucose more frequently during initiation period.

Thymoquinone inhibits CYP1A2 (IC50 = 26.5 µM) and CYP3A4 (IC50 = 25.2 µM) in human liver microsomes, with the greatest effect at concentrations achievable from higher-dose supplements. Inhibition of CYP1A2 increases plasma levels of narrow-therapeutic-index drugs (theophylline, clozapine), while CYP3A4 inhibition affects tacrolimus and many other substrates. The IC50 values are near clinically achievable TQ concentrations at high N. sativa doses.

Monitor for dose-dependent toxicity of CYP1A2 and CYP3A4 substrate drugs in patients using black seed supplements, especially at high doses. For theophylline: check levels and respiratory status. For clozapine/olanzapine: monitor for anticholinergic toxicity and sedation. For tacrolimus: check trough levels. Black seed doses >1 g/day carry greater interaction risk.

Nigella sativa and its active constituent thymoquinone demonstrate antihypertensive effects via multiple pharmacological mechanisms: calcium channel blockade, angiotensin-converting enzyme (ACE) inhibition, nitric oxide preservation, diuretic activity, and cardiac depressant effects. Multiple RCTs confirm significant blood pressure reduction with N. sativa in patients with mild-to-moderate hypertension. A systematic meta-analysis confirmed consistent reductions in SBP and DBP. When co-administered with antihypertensive medications, additive pharmacodynamic hypotensive effects may lead to excessive blood pressure reduction and symptomatic hypotension.

Monitor blood pressure regularly when adding Nigella sativa to antihypertensive regimens. Dose reduction of antihypertensive medication may be required. Advise patients to report dizziness, fainting, or lightheadedness (signs of excessive hypotension). This interaction may also be used therapeutically as adjuvant hypertension management under medical supervision.

Animal pharmacokinetic studies with Nigella sativa seed oil (BSO) demonstrated a significant decrease in prednisolone Cmax and AUC0-last while increasing prednisolone clearance following single-dose co-administration. The proposed mechanism involves induction of P-glycoprotein (P-gp) and/or CYP3A4 at the absorption site, reducing systemic corticosteroid bioavailability. An earlier study of cyclosporine pharmacokinetics in rabbits showed a 2-fold increase in clearance following multiple N. sativa doses, consistent with P-gp and CYP3A4 induction. This interaction may reduce efficacy of corticosteroids used for immunosuppression or inflammatory disease control.

Patients on corticosteroid therapy for serious conditions (organ transplant, autoimmune disease, severe asthma) should avoid concurrent use of Nigella sativa supplements. If used together, monitor for signs of reduced corticosteroid effectiveness (worsening disease control, adrenal insufficiency symptoms). Separate administration timing and monitor relevant biomarkers (inflammatory markers, transplant function).

Thymoquinone (TQ), the principal bioactive constituent of Nigella sativa, demonstrates anticonvulsant properties in animal seizure models via an opioid receptor-mediated increase in GABAergic tone and partial benzodiazepine receptor modulation. In the pentylenetetrazole (PTZ) model, TQ (40-80 mg/kg IP) significantly prolonged seizure onset and reduced myoclonic seizure duration. These anticonvulsant effects may be additive with conventional AEDs, potentially enhancing therapeutic effects but also increasing the risk of CNS adverse effects (excessive sedation, ataxia, cognitive impairment). Independently, TQ inhibits CYP2C9, which metabolizes phenytoin and some other AEDs, potentially elevating plasma AED concentrations.

Monitor closely for enhanced anticonvulsant effects (excessive sedation, ataxia) when Nigella sativa is used with AEDs. If phenytoin is part of the regimen, see existing phenytoin interaction entry for specific CYP2C9 inhibition concerns. Start with lower Nigella sativa doses and titrate carefully in patients on multiple AEDs. Monitor drug levels where available.

Thymoquinone and the fixed oil of Nigella sativa inhibit eicosanoid generation in leukocytes and membrane lipid peroxidation, with demonstrated inhibition of thromboxane B2 synthesis and platelet aggregation. These anti-platelet effects are additive with antiplatelet medications including aspirin (COX-1 inhibitor) and clopidogrel (P2Y12 antagonist). Concurrent use increases the overall inhibition of platelet function, raising the risk of bleeding complications including gastrointestinal hemorrhage, prolonged bleeding after injury, and surgical hemorrhage.

Exercise caution when combining Nigella sativa with antiplatelet agents, particularly in patients with peptic ulcer disease, post-surgical states, or concurrent anticoagulation. Monitor for unusual bleeding or bruising. Advise patients to disclose Nigella sativa use before any invasive procedure or surgical intervention.

Thymoquinone has intrinsic CNS activity with sedative, anxiolytic, and GABAergic effects documented in animal studies. N. sativa extracts have been shown to enhance pentobarbital-induced sleep duration via potentiation of GABA-A receptor function. The GABAergic and partial opioid receptor-modulating activity of thymoquinone may potentiate the CNS depression produced by barbiturates, benzodiazepines, opioids, and alcohol. Animal studies confirm that thymoquinone impairs motor coordination and reduces locomotor activity, consistent with CNS depressant effects that would be additive with pharmacological sedatives.

Advise patients to avoid combining Nigella sativa supplements with sedatives, opioid analgesics, alcohol, or other CNS depressants. If concurrent use is unavoidable, start at lower doses and monitor closely for excessive sedation, respiratory depression, and impaired psychomotor function. Particular caution in elderly patients and those with sleep apnea.