Dropsy is an old word for an older problem. Afflicted, you begin to swell—first your ankles, then your legs. Walking gets difficult, even sitting becomes painful. Then the swelling gets so bad your skin splits open.

The commonest cause is heart failure. Think of the revolving door of an office building. As the door gets older it turns more slowly. Eventually people have to queue, spilling out onto the street. As your heart gets feebler, your blood queues up in the same way. Under that pressure, your veins leak fluid, which builds up between cells, pooling wherever gravity directs.

Because we have a double circulation system, there's a double problem. Unable to get into a weakly pumping right ventricle, ready to go to the lungs, deoxygenated blood backs up in your extremities, and fluid collects in your tissues. You swell from the feet upwards. And instead of being pumped out to the body from your left ventricle, oxygenated blood backs up, and fluid gets squeezed into your lungs, filling the spaces where air should be. You get breathless.

Doctors used to try to relieve dropsy, which today we call edema, by making holes in people’s bloated legs to let the fluid out. That could help, a little. Anything that reduced the amount of fluid in your body, even leeches and bloodletting, could make you feel better.

Welcome to the world of medicine that Dr. William Withering knew in the late eighteenth century, a scintillatingly brilliant time that crackled with new discoveries, political revolutions, and the excitement generated by the birth of the Enlightenment. Yet Withering lacked the charisma of his era. He was raised in a medical family in Shropshire, England, in 1741, and after four years' apprenticeship to a local physician, he went to Edinburgh to get his degree. There were the typical diversions: he golfed, published bad poetry, and learned to play the bagpipes. He loathed, in particular, the botany he was forced to study. Nevertheless, he completed his degree, then set off on a professional pilgrimage to hospitals around Europe.

Unfortunately, Withering's trip ended early. His traveling companion, a healthy young man of his own age, developed a skin infection in Paris. "An abscess grew upon his shoulder, a fever came on, the wound gangren'd and yesterday he died," wrote Withering, who lived in a time when death came suddenly and doctors were impotent.

. . .

In 1767 Withering began to practice. One of his first patients was seventeen-year-old Helena Cooke. She liked drawing flowers. While she lay sick in bed, Withering scoured the countryside to find her fresh subjects, apparently moved by her charms to overcome his antipathy toward botany. It turned out to be the start of two lifelong relationships: Withering married his patient and acquired a lasting fondness for plants.

In 1775 worldly opportunity called on the young couple, arriving in the overwhelming shape of Erasmus Darwin, pockmarked, rotten-toothed, and enormous. He had as great an appetite for food and free love as for science and verse. (Famous in his time for his fertile mind and his poetry about the sex lives of plants, he is best known these days for his grandson Charles.) Darwin recognized the young Withering’s intellect and helped him secure the position of town physician for Birmingham.

Combined with Darwin's friendship, the post brought membership in a remarkable club: the Lunar Society, named for its habit of meeting at full moons to make traveling home safer. The Lunaticks, as they called themselves, men like Matthew Boulton, Joseph Priestley, and Josiah Wedgwood, were the leading British scientists and entrepreneurs of the day.

While his friends manufactured Britain's industrial revolution, Withering lived conservatively. His son described him as methodical, known more for "steady sense and correct judgment than for the flights of fancy or the eccentricities of genius." His letters are fabulously dreary. But for all his lack of glitter, Withering was sharply observant. When his opportunity for major discovery came, he jumped:

"In the year 1775, my opinion was asked concerning a family receipt for the cure of the dropsy. I was told that it had long been kept a secret by an old woman in Shropshire, who had sometimes made cures after the more regular practitioners had failed. . . . This medicine was composed of twenty or more different herbs; but it was not very difficult for one conversant in these subjects to perceive that the active herb could be no other than the Foxglove."

Any of some twenty plants in the genus Digitalis, foxglove had been used as a medicine by hosts of people, from ancient Greeks to medieval Welsh. During the Middle Ages in Western Europe, it was believed to cure a whole range of diseases for which it is actually useless. In the mid-1600s, for example, the herbalist Nicholas Culpeper recommended foxglove for treating epilepsy. It doesn’t.

Foxglove also had obvious problems. In the sixteenth century, a botanically misguided Dutch chef fed his guests a foxglove omelet. Their consequent diarrhea and vomiting were a good demonstration of the plant’s milder side effects; too much of it, and you die. In the seventeenth century, forty years before Withering, a Frenchman investigated foxglove’s effects on a turkey. "He did not attempt to perch," ran his poignant report on the poultry, "he uttered plaintive cries." The bird stumbled drunkenly for a few days, refused to eat, and died.

At medical school I was taught the blunt truth that only inactive drugs are without side effects. Taking a pill is like thumping the side of a misbehaving television: you might bang the right piece into place, but you might make things worse. Withering knew this (minus the television analogy), worrying that "the lives of men" could be "hazarded" by his new drug. His understanding of medical science evoked the modernity of the Lunar Society. It was not enough, he realized, to compile only encouraging stories.

"It would have been an easy task to have given select cases, whose successful treatment would have spoken strongly in favour of the medicine, and perhaps been flattering to my own reputation. But Truth and Science would condemn the procedure. I have therefore mentioned every case in which I have prescribed the Foxglove, proper or improper, successful or otherwise."

Altogether Withering wrote up 163 cases of dropsy in which he used the plant, and the majority of those patients (no doubt the ones whose dropsy was caused by heart failure) got better. Swollen legs reduced in size, breathing was eased. Noting that patients produced copious amounts of urine as they began to improve, Withering suspected that digitalis extract worked as a diuretic, causing excess fluid to be lost as urine and so reducing the amount of work the heart needed to do. "It charms the shapeless monster into man," Erasmus Darwin wrote in his poem about it. Praise even came from the other side of the world: a physician from New Hampshire wrote congratulating Withering and asking for help in seeding the flower in America.

In 1790, with the Revolution raging in France, Withering diagnosed himself with tuberculosis. Over the next several years, consumption sucked his life away. A friend was struck by how weak he became: "The flower of English physicians is indeed withering." (Medicine has progressed over the centuries, but puns are as bad as ever.) In October 1799 he died. His memorial tablet was carved with a plant of the genus Witheringia on one side and, separated by a terrifically bad poem, a foxglove on the other.

. . .

Hearts are well evolved, but not miraculous. That means compromise and ultimate failure. Let's say that you hope to manufacture something with a lifetime guarantee, a watch, for example; you would engineer it to last a hundred-odd years, not a thousand. Evolution is equally careful about not wasting resources. Therefore no body part outlasts the rest, or not by much. Even our solid skeleton weakens and fails as we reach old age.

Modern medicine follows a similar aim, hoping to keep any organ from failing too far ahead of the rest, thus extending our healthy lives and compressing our decay into the fewest possible days. The heart, however, often proves difficult to save, as muscle cells don’t divide—at least, not abundantly enough to regenerate healthy tissue. If you exercise them, heart muscle cells might get bigger, but few or no new ones will be made. So as you age, they can die off gradually—particularly under the stress of high blood pressure, diabetes, or coronary artery disease—or, in the case of a heart attack, die off suddenly and potentially catastrophically. Either way, the heart never grows back.

Foxglove—or digitalis, as the pharmaceutical compounds derived from it are commonly called—works not by directly causing increased urination, as Withering thought, but by making the heart beat more strongly and efficiently. The trouble is, drugs that increase the strength of the heart's beats don’t make it grow new muscle; they only drive what’s left a little harder. And any coach will tell you that if you push any athlete too far, you risk collapse.

When an aging heart fails and begins to pump less blood, our bodies make a mistake. They notice the reduced flow, but presume we're bleeding or dehydrated. Evolutionarily, surviving blood loss and dehydration is more critical than extending old age. So, when the body detects a lack of blood flow, it assumes that a lack of blood is to blame. In response we get thirsty and our kidneys reduce urine output. By drinking more and peeing less, our bodies compensate for a loss of fluid that has not occurred—as a result, we end up with too much. Having to pump more fluid is exactly what a failing heart needs least. Withering correctly observed that digitalis made people feel better, yet could it have been killing them just the same? Neither Withering nor anyone long after him took a systematic look.

In 1997, nearly two centuries after Withering's death, a medical trial came back with the answer. Almost 7,000 people were randomly allocated to digitalis or a placebo for more than three years. The results: for every thirty-nine people taking digitalis for a year, one avoided a hospital admission. The effect on mortality? Zero. Digitalis is neither the lifesaver people had imagined nor the killer some suspected.

It has taken two centuries to go from hailing digitalis as a miracle cure to wondering if using it makes any sense at all. Some doctors feel that the harms are hardly worth the benefits. Others continue to prescribe it liberally, as though it really is a lifesaver. Yet for certain patients, digitalis remains modestly effective, making them feel and function better than any razzle-dazzle new synthetic drug. And that, even if they don't live longer, is no small thing.

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Druin Burch is a medical resident and a tutor at the University of Oxford. His first book, Digging Up the Dead (2007), profiles the pioneering surgeon, body snatcher, and revolutionary democrat Astley Cooper, who incidentally trained John Keats as a surgeon. Burch lives in a village in the Cotswalds with eight ducks and 500 of his countrymen.