Foxglove, also called Digitalis purpurea, is a common biennial garden plant that contains digitoxin, digoxin, and other cardiac glycosides. These are chemicals that affect the heart. Digitalis is poisonous; it can be fatal even in small doses. It was the original source of the drug called digitalis.
Foxglove is a native of Europe. It was first known by the Anglo-Saxon name foxes glofa (the glove of the fox), because its flowers look like the fingers of a glove. This name is also thought to be related to a northern legend that bad fairies gave the blossoms to the fox to put on his toes, so that he could muffle his footfalls while he hunted for prey. The legend may account in part for some of the common names of digitalis: dead man's bells, fairy finger, fairy bells, fairy thimbles, fairy cap, ladies' thimble, lady-finger, rabbit's flower, throatwort, flapdock, flopdock, lion's mouth, and Scotch mercury.
Foxglove was first introduced to the United States as an ornamental garden plant. During the first year, foxglove produces only leaves. In its second season it produces a tall, leafy flowering stalk that grows 3–4 ft (0.9–1.2 m) tall. In early summer, many tubular, bell-shaped flowers bloom; they are about 2 in (5.08 cm) long and vary in color from white to lavender and purple.
Foxglove was originally used for congestive heart failure and atrial fibrillation (chaotic contractions across the atrium of the heart). Foxglove helps the muscles of the heart to contract, reduces the frequency of heartbeats, and lowers the amount of oxygen the heart needs to work. The cardiac glycosides in foxglove block an enzyme that regulates the heart's electrical activity. The dried leaves, ripe dried seeds, and fresh leaves of the one-year-old plant, or the leaves of the two-year old plant are the parts that were used in medicine.
In spite of its use in the past, foxglove has been largely replaced as a heart medicine by standardized pharmaceutical preparations because it is one of the most dangerous medicinal plants in the world. Foxglove is, in fact, a useful example of the importance of standardization in testing the efficacy and possible toxicity of present-day popular herbal medicines. Its sap, flowers, seeds, and leaves are all poisonous; the leaves, even when dried, contain the largest amount of cardiac glycosides. The upper leaves of the stem are more dangerous than the lower leaves. Foxglove is most toxic just before the seeds ripen. It tastes spicy hot or bitter and smells slightly bad.
In folk medicine, foxglove was first used in Ireland. Its use spread to Scotland, England, and then to central Europe. It was taken to treat abscesses, boils, headaches, paralysis, and stomach ulcers. It was also applied to the body to help wounds heal and to cure ulcers. It has not been proven to be an effective treatment for any of these ailments.
In 1775, William Withering, an English doctor, first discovered the accepted medicinal use of foxglove. He identified digitalis as a treatment for swelling or edema
associated with congestive heart failure. Withering published a paper in 1785 that is considered a classic in the medical literature. Foxglove was used to treat heart disease during the eighteenth and nineteenth centuries.
Foxglove is no longer used as a heart medicine because the therapeutic dose and the lethal dose are very close. Seasonal variations in the level of cardiac glycosides in the plant make the safe dose impossible to estimate except by an experienced physician and prescriber of the herb who monitors the patient on an hourly basis for signs of overdose. Few living doctors and herbalists can safely use digitalis as a plant extract. Specific standardized doses of pharmaceutical digoxin are used instead. Even so, patients receiving the drug must be closely monitored.
In present-day usage, foxglove is used as an ingredient in a class of heart drugs called digitalis. Digoxin (Lanoxin) is the most common drug made from digitalis. Digitalis is usually taken orally, as capsules, as an elixir, or as tablets. It can also be given in an injection.
Used improperly, foxglove is deadly; it can make the heart stop or cause a person to suffocate. Eating any part of the plant can be fatal. The therapeutic dose of foxglove is very close to the lethal dose. Foxglove should therefore not be used.
An overdose of foxglove interferes with the heart's normal electrical rhythms; it can make the heart beat too slowly or cause extra heartbeats. An overdose of foxglove may also cause diarrhea, headache, loss of appetite, and vomiting. More serious and potentially deadly reactions to an overdose affect the heart and the central nervous system. Foxglove can disrupt the heart's rhythm, including life-threatening ventricular tachycardia, or atrial tachycardia with atrioventricular block. In the central nervous system, foxglove can cause confusion, depression, drowsiness, hallucinations, psychoses, and visual disturbances.
Poisoning from foxglove occasionally occurs from the misuse of such herbal preparations as dried foxglove leaves used in a tea, or from overdoses of prescribed digitalis. It can also occur when foxglove is confused with comfrey, a plant used for tea that belongs to the borage family. The two herbs look very much alike.
Some patients who take pharmaceutical preparations of digitalis may experience such side effects as too much muscle tone in the stomach and intestines, diarrhea, headache, loss of appetite, and vomiting. These side effects are the same as some symptoms of a foxglove overdose. Digitalis preparations can have toxic side effects due to overdose or other conditions. The most serious are arrhythmias, abnormal heart rhythms that can be life-threatening.
The use of digitalis can increase the toxicity of other cardioactive drugs. Hypersensitivity to digitalis, dehydration, or the use of diuretics that cause people to lose fluids and salts may increase the risk of side effects from digoxin. The risk of cardiac arrhythmias is increased when people taking digitalis also take amphetamines or diet pills; medicine for asthma or other breathing problems; or medicine for colds, sinus problems, hay fever, or other allergies. Taking any of these drugs with digitalis also affects how much digitalis is in the body and how effective it will be.
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Lori De Milto
Rebecca J. Frey, PhD