Cinnamon supplements: cassia versus Ceylon, coumarin and blood sugar

Cinnamon is the dried bark of various Cinnamomum trees. NIH distinguishes Ceylon cinnamon, known as "true" cinnamon and grown mainly in Sri Lanka, from cassia, grown in southeastern Asia and the most common type sold in North America. Many UK shoppers meet both without knowing, because jars and capsules often say only "cinnamon".

That ambiguity is not just a labelling quibble. NIH notes that cinnamon products may not clearly indicate which species or which parts of the plant, such as bark, leaves or roots, were used, and that research shows the types and parts differ in chemical composition in ways that can change their effect on people. A trial using one species may therefore say little about a capsule containing another.

The best-known chemical difference is coumarin. NIH reports that interactions between coumarin, found in cassia cinnamon, and the liver have been described. Eating cassia in ordinary culinary amounts does not usually supply enough coumarin to cause significant problems, but some cassia products contain high levels, and prolonged use of cassia could be an issue for sensitive people, such as those with liver disease. Ceylon cinnamon may contain only a trace of coumarin.

In practical terms a person who sprinkles cinnamon on porridge faces a very different exposure from someone taking daily high-strength cassia capsules for months. Anyone with a history of liver disease, abnormal liver tests, or heavy alcohol use should avoid concentrated cassia products unless a clinician has advised otherwise, and anyone who develops dark urine, yellowing of the skin or eyes, itching or upper abdominal discomfort while using cinnamon supplements should stop and seek medical review.

• Prefer a product that names the species (Cinnamomum verum or Ceylon) if you decide on a supplement.

• Spice-jar cinnamon at cooking quantities is not the concern raised by NIH.

• Keep the packaging; a clinician or the MHRA may want the species and manufacturer if a reaction occurs.

There have been many studies of cinnamon, especially in diabetes. NIH finds the results difficult to interpret because it is often unclear which species or plant part was tested, and concludes that it is unclear whether cinnamon supplementation helps diabetes or weight loss and that higher-quality research is needed. It also states plainly that research does not clearly support using cinnamon for any health condition.

A person with diabetes should therefore not treat cinnamon as an alternative to prescribed treatment, dietary advice or monitoring. If cinnamon capsules lower glucose at all, the effect is not established, and in combination with glucose-lowering drugs the sensible assumption is that any additive effect is unpredictable. Ask your diabetes team before starting, and keep checking glucose as advised. NIH also reports preliminary research suggesting that a nasal spray containing Ceylon cinnamon extract might improve hay fever symptoms, but says more reliable evidence is needed, and it is unclear whether cassia works as a mosquito repellent.

NIH lists stomach and bowel upset and allergic reactions as the most common problems with larger amounts or long use. Topical cinnamon oil or powder may cause skin irritation or contact dermatitis, so undiluted oil should not be applied to the skin. There are theoretical reasons to suspect that some components of cinnamon may interact with an anticancer medicine or with nicotine, and anyone taking medicine should ask a pharmacist before using cinnamon or other herbal products.

On pregnancy, NIH says studies of Ceylon and cassia suggest food-level amounts are safe, but that Ceylon cinnamon in larger amounts during pregnancy is considered unsafe, and little is known about larger amounts while breastfeeding. The NHS gives general advice to avoid herbal medicines in pregnancy and breastfeeding, and not to give them to children.

NIH specifically warns against consuming cinnamon powder as part of the internet dare known as the cinnamon challenge, noting that serious harm is possible. Dry cinnamon powder is irritating to the airways and does not dissolve, and swallowing a spoonful can cause choking, coughing and inhalation of powder into the lungs. Anyone who develops breathlessness, chest pain, wheeze or persistent cough after such a dare should seek urgent medical help, and parents should talk to teenagers about it.

Cinnamon as a spice is food. Capsules and extracts are usually food supplements, which should not claim to treat diabetes or any disease. If a seller presents a product as treating a condition, the MHRA may treat it as a medicine, since herbal or natural ingredients do not exclude medicinal status and it examines claims, ingredients and presentation, including websites and social media. A registered traditional herbal medicine carries a THR number and mark. The NHS recommends asking a pharmacist before taking a herbal product alongside prescribed medicines.