BACKGROUND:The purple bell-shaped foxglove flowers are readily visible all over Ireland. The tall stems gave rise to one of the many Irish names for the plant: an lus mór (the great herb). Another name highlights foxglove's toxicity: méaracán daoine marbh (dead people's thimble). The word for thimble in Latin is digitalis, from which comes the plant's scientific name, Digitalis purpurea. This gave rise to the herbal remedy digitalis and eventually to the purification of several drugs called cardiac glycosides, of which digoxin is the best known.
Digitalis has long been recognised in folk medicine. In Ireland, foxglove was often used in connection with fairies. People under a fairy spell would be given foxglove to figure out if they could be released. If they got sick and recovered, the fairy spell was broken. If they died, or refused to drink the potion, that signified they were under the control of the fairies. Such lethal effects have continued to raise concerns about digitalis, even as it became an important therapeutic for heart failure.
FOXGLOVE HISTORY
William Withering, an 18th-century English physician with an interest in botany, learned from some older women about a mixture of more than 20 herbs used to treat “dropsy”. This term referred to the accumulation of fluid in the lower limbs, sometimes due to congestive heart failure. Withering figured that foxglove might be the active ingredient and spent nine years studying its effects in patients. He gave various doses from various plant parts collected at different times of the year and in 1785 published his findings with recommendations for the best way to use foxglove.
Withering’s work is often regarded as the birth of modern therapeutics. Over the following centuries, more was learned about digitalis to make it an important drug for heart failure. In 1841, French scientists identified one of the active ingredients, with others identified at various points until digoxin was isolated in 1930. With each purified ingredient, the digitalis mechanism of action became better understood.
However, beginning in the 1980s, other studies began to report higher death rates among certain cardiac patients given digoxin, and serious interactions with other, newer heart medications. While the use of digoxin has decreased, it remains an important drug for people with certain types of arrhythmias (when the heart beats at an inappropriate rate).
PROBLEMATIC ASPECTS
Foxglove remains a classic example of a herbal remedy leading to a useful drug. Because it affects heart rate and blood pressure, the amount taken is extremely important. The difference between a therapeutic dose and an overdose is very slim. Given the natural variation in the active ingredient in the plant, purified and standardised tablets allow doctors to know exactly how much is given to patients. For similar reasons, foxglove was viewed as too toxic to consume in many parts of Ireland. In some areas, though, it was recommended for external use or as a tonic in highly diluted teas.
Accidental poisoning is occasionally reported for foxglove. The initial symptoms are vomiting, blurred vision and a strong slow pulse, leading to dizziness and muscle weakness, and eventually convulsions and death.
In 2010, an outbreak of digitalis poisoning was reported in China. The problem began when people looking for comfrey leaves mistakenly picked foxglove. A tea made from foxglove led to several people being hospitalised. Fortunately, doctors recognised the symptoms of digitalis poisoning and administered antidotes. Several similar cases of foxglove being mistaken for comfrey have been reported in the medical literature from a number of European countries.
RECOMMENDATIONS
Foxglove is a significant herb in the history of pharmacy. Digitalis remains an important drug, but one that must be used selectively and carefully. Foxglove should never be used to self-medicate because of its toxicity and uncertainty about how much the plant contains. Children and pets should be kept away from foxglove, especially given the draw of the attractive flowers. All parts of the plant are poisonous.
Dónal O’Mathúna has a PhD in pharmacy, researching herbal remedies, and an MA in bioethics, and is a Senior Lecturer in the School of Nursing, Dublin City University.
