Comfrey: skin ointments, internal use and pyrrolizidine alkaloids

Comfrey is a good illustration of how the route of exposure changes the risk. Rubbed onto intact skin for a few days, a carefully prepared ointment is regarded by the European herbal committee as a plausible, traditionally supported option for minor injuries. Swallowed as a tea, a capsule or a "tonic", the same plant delivers pyrrolizidine alkaloids into the bloodstream and liver, where they can do harm.

The EU summary states plainly that comfrey root contains pyrrolizidine alkaloids with toxic effects on the liver when taken by mouth. It adds that no significant risk is expected when the specified medicines are used on the skin for short periods.

The Committee on Herbal Medicinal Products at the European Medicines Agency concluded that, on the basis of long-standing use, comfrey root medicines can relieve symptoms of minor sprains and bruises. The monograph covers only a preparation made by ethanol extraction, supplied in semi-solid forms such as creams or ointments, to be applied to the skin.

The committee set explicit boundaries. The medicine should be used only in adults and should not be used for longer than ten days. The amount of pyrrolizidine alkaloids has to be specified in each product, and patients should not be exposed to more than 0.35 microgram per day. It also noted four clinical studies using a different comfrey preparation which suggested reduced swelling and pain in sprains and bruises, but because that preparation's composition is not known, the data were not used. The conclusion therefore rests on documented traditional use.

Pyrrolizidine alkaloids are produced by many plants as chemical defences. After absorption they are converted in the liver into reactive compounds that can injure the cells lining the small liver veins and may cause serious liver disease. NIH describes the same family of compounds in butterbur as able to damage the liver and lungs and possibly cause cancer, and it warns that they may be absorbed through cuts and scrapes.

That is why comfrey products are treated differently from a plain herbal cream. A home-made poultice, or a preparation made from the whole plant, has no controlled alkaloid content, and a garden comfrey leaf is not a quantified dose.

• Do not put comfrey on open wounds, grazes or broken skin.

• Do not swallow comfrey tea, capsules or root powder.

• Avoid in pregnancy and breastfeeding; the NHS advises avoiding herbal medicines in both unless a clinician has said otherwise.

• Do not give comfrey products to children; the NHS does not recommend herbal medicines for them.

A comfrey ointment sold for sprains is a medicine by presentation, because it is intended to treat an injury. In the UK that means it should hold a traditional herbal registration or product licence rather than appear as a cosmetic or an unregistered remedy. The MHRA explains that herbal or natural ingredients do not stop a product being treated as a medicine, and that it considers claims and presentation, including websites and social media.

The MHRA also maintains a list of banned or restricted herbal ingredients for medicinal use drawn from the Human Medicines Regulations 2012, which sellers must check. Check a product carries a THR number and a mark on the pack; the NHS also advises against buying unlicensed herbal medicines online.

Most sprains settle with relative rest, support, gentle early movement and simple pain relief, which a pharmacist can advise on. Urgent assessment through A&E or a minor injuries unit is appropriate for an inability to put weight on a limb, severe swelling or deformity, numbness, a cold or pale foot or hand, or an injury after significant force. A bruise that appears without injury, grows quickly or comes with other bleeding needs medical review rather than a topical remedy.