People taking warfarin to prevent blood clots should stay away from chamomile products, Canadian doctors caution. They describe the case of a 70-year-old woman, while being treated with warfarin, who developed severe internal bleeding after drinking chamomile tea to soothe a sore throat and applying chamomile lotion to relieve chest congestion and reduce foot swelling.

This case makes clear the potential dangers of mixing herbal or other natural remedies with physician-prescribed drugs, report Drs. Robert Segal and Louise Pilote from McGill University Health Centre in Montreal in the Canadian Medical Association Journal.

“Warfarin,” Pilote told Reuters Health, “can interact with both prescribed medications and natural remedies.”

While warfarin is known to interact with garlic, onion, and ginger, this is believed to be the first documented case of a drug-herb interaction between warfarin and chamomile. In the U.S., warfarin is sold under the trade name Coumadin.

Warfarin is derived from coumarin - a naturally occurring chemical compound with anti-clotting properties found in many plants, including chamomile. It’s believed that the chamomile tea and lotion acted in concert with the warfarin in this case to cause bleeding.

Warfarin thins the blood, thereby reducing the chances of blood clots and stroke. It is widely used in patients with a variety of heart conditions. In the present case, the woman had received a mechanical heart valve implant and was taking warfarin to reduce the risk of blood clots.

Many people drink chamomile tea to soothe a range of ailments including toothache, sore throat and digestive problems. Because it acts as a mild sedative, many people who have trouble sleeping drink chamomile tea at night.

Chamomile creams are often used to soothe insect bites and treat skin conditions such as psoriasis, eczema and acne.

Pilote advised patients “to make their physicians and pharmacists aware of what prescribed and herbal products they consume so that potentially harmful interactions can be identified.”

SOURCE: Canadian Medical Association Journal, April 25, 2006.