Black seed (Nigella sativa): evidence, safety and UK status

Nigella sativa seeds are small, black and angular. They are used in breads and curries, and traditionally in Islamic and Ayurvedic medicine, where Arabic names such as habbatus sauda are common. Online sellers sometimes describe the seed as a remedy for almost every condition, which is not how any regulator or review body describes it.

Products come as whole or ground seed, cold-pressed oil, capsules and extracts of thymoquinone, one of its active compounds. Because the content of these differs, any summary of "what black seed does" is really a summary of what particular preparations did in particular trials.

A 2015 systematic review of black seed in diabetes searched several databases and identified 19 eligible articles: two human trials, fourteen animal studies and three combined in vivo and in vitro studies. They suggested that the seed can modulate hyperglycaemia and abnormal lipids through several proposed mechanisms, including antioxidant effects and effects on insulin secretion and glucose absorption. The reviewers concluded that more clinical trials were needed to clarify benefits, the effective type and the dose.

A 2017 meta-analysis of seven trials in type 2 diabetes reported that supplementation significantly improved fasting blood sugar, with an average reduction of about 18 mg/dl, and also improved serum lipids. The authors cautioned that only a few studies were included and that future research was necessary. Neither review suggests black seed could replace medicines such as metformin, and trial participants were generally continuing standard treatment.

• Few human trials, with small numbers and short durations.

• Different preparations, doses and comparison groups across studies.

• Results for blood tests do not show reduced heart attacks, strokes or kidney disease.

• Publication bias is possible in small herbal trials.

Culinary use of the seed is long-standing and widely regarded as low risk. Concentrated oils and extracts are different, and the sources retrieved for this guide do not provide a defined safe dose, a list of established drug interactions or confirmed data for pregnancy. Formal pre-market safety assessment is limited in the UK because many such products are sold as foods rather than medicines.

Practical caution follows from the general principles in NHS advice: herbal products can interact with diabetes drugs, blood thinners, blood pressure medicines, statins and antidepressants, and are not recommended for children and should be avoided in pregnancy, breastfeeding and kidney or liver disease unless a clinician advises. Because trial data suggest an effect on blood sugar, a person taking glucose-lowering medicine should treat the seed as capable of adding to that effect and check with a pharmacist.

A black seed oil sold as a food supplement should not claim to prevent, treat or cure disease. If a product is presented as treating a condition, the MHRA may treat it as a medicine requiring authorisation, regardless of its natural origin, based on the claims, ingredients and manner of presentation, including social media and customer reviews. A product with a traditional herbal registration carries a THR number and mark; most black seed products on sale do not.

Sweeping claims that a seed treats almost every condition are not backed by the reviews and should be a prompt for scepticism. The NHS advises against buying unlicensed herbal medicines online, which includes imported oils and capsules with unclear ingredients.

Use it as a spice if you enjoy it. If you are considering a supplement for diabetes or cholesterol, discuss it with your GP, practice nurse or pharmacist first; they can check how it fits with your medicines and monitoring. Keep up regular HbA1c checks, cholesterol checks and any prescribed treatment, and tell the team if you see changes in glucose readings after starting. Stop and seek advice for rash, swelling, breathing difficulty or any new symptom, and report suspected reactions on a Yellow Card.