Black seed oil is pressed from the seeds of Nigella sativa, a plant used across the Middle East, North Africa and South Asia for centuries, usually as a cooking spice and a household remedy. The oil's main active constituent is thymoquinone, and most trials use either the cold-pressed oil in capsules or the ground seed powder.

It is one of the more heavily studied herbal supplements. A 2025 GRADE-assessed meta-analysis counted 82 randomized trials with 5,026 participants, which is more than most entries on this shelf can claim. The trials run from one to 48 weeks, most of them short, and most of them small. What they report is consistent in direction: slightly lower blood pressure, blood sugar and lipids, and slightly lower weight. The question is how much to trust it, and a 2023 overview of twenty meta-analyses answered that bluntly.

Thymoquinone is the constituent the mechanistic literature concentrates on. In cell and animal work it scavenges free radicals, dampens inflammatory signaling, and acts on lipid and glucose handling in the liver. The oil also carries unsaturated fatty acids, and the seed powder carries fiber and other constituents the oil does not, which may be why the meta-analyses find oil and powder differ: oil was more effective for fasting glucose, total cholesterol and LDL, while only the powder raised HDL. None of these mechanisms has been shown to operate at supplement doses in people in a way that explains the trial results; they are the plausible story, not the demonstrated one.

The widest view. The 2025 meta-analysis of 82 randomized trials (5,026 participants, 1 to 48 weeks, 200 to 4,600 mg per day) found Nigella sativa improved body fat, BMI, waist circumference, weight, systolic and diastolic blood pressure, fasting glucose, HbA1c, CRP, LDL, total cholesterol and triglycerides, among a long list of markers. The abstract reports direction and significance, not effect sizes, so the earlier, narrower meta-analyses are where the numbers are.

Blood pressure. Pooled across randomized trials to 2022, systolic pressure fell by about 3 mmHg and diastolic by about 2.7 mmHg, with very high heterogeneity between trials.

Glucose. Seventeen trials pooled: fasting glucose down about 10 mg/dL, post-meal glucose down about 15 mg/dL, HbA1c down 0.57 percentage points. Oil outperformed powder for fasting glucose.

Lipids. Seventeen trials pooled: total cholesterol down about 16 mg/dL, LDL down about 14 mg/dL, triglycerides down about 21 mg/dL, no change in HDL overall.

Weight. Eleven trials pooled: 2.1 kg lighter than placebo, BMI 1.2 points lower, waist 3.5 cm smaller, with the authors themselves asking for caution given heterogeneity and the small number of high-quality trials.

Two trials worth reading on their own. An 80-patient, four-week, placebo-controlled asthma trial found a modest improvement in asthma control score (21.1 against 19.6) and a fall in blood eosinophils, with lung function trending better but not significantly. A 120-patient, three-month trial in fatty liver disease found less liver fat on ultrasound and larger drops in ALT and AST than placebo.

The quality check. A 2023 overview of 20 meta-analyses rated one as moderate quality, four low and 15 critically low. Of 110 outcome gradings, five were moderate, 17 low and 88 very low, downgraded for risk of bias, inconsistency and imprecision. The effects above are real in the sense that the trials found them; whether they would survive large, well-run trials is not known.

The trials report it as well tolerated. The obesity meta-analysis records no serious side effects, and the fatty liver trial observed no adverse effect over three months with blood counts and kidney markers monitored. Where side effects are reported at all they are minor. A toxicology overview of the animal literature found the seed extracts non-toxic at very high doses and the fixed oil slightly toxic only with chronic high dosing in rodents.

The practical cautions follow from what it does. If it lowers blood pressure and blood glucose, it does so on top of any medicine that lowers them, so anyone on antihypertensives or diabetes drugs should treat it as an addition to their regimen and say so to whoever prescribes. Long-term human safety data beyond a year do not exist, and the trials are too small to detect uncommon harms.

A food and a dietary supplement in the United States, the United Kingdom and the European Union, sold without review of its claims. It is a traditional medicine in several countries where the trials were run. No regulator has licensed it for blood pressure, glucose, lipids, asthma or liver disease, and the cardiometabolic claims the meta-analyses support are not claims a supplement label is permitted to make.