TWO CENTURIES AGO drugs were thought to counteract illness by correcting certain dynamic imbalances in the body. The principal pathological imbalances were in the four liquid “humors”—blood, phlegm, yellow bile, and black bile—and in the tone, the strength, the irritability of the solid organs, especially the blood vessels and nerves.

These imbalances were most often attributed to the weather or to contagions transmitted in the air from sewage, stagnant water, or other patients; the microbes that actually were responsible would not be understood for another century. Other illnesses were blamed on factors ranging from alcohol to volcanic eruptions.

Whatever the supposed causes, treatments were designed to counteract the associated symptoms. Diarrhea, coughing, pain, weakness, convulsions, flushed skin, and skin rashes were common clues that both patients and doctors understood. The doctors also interpreted fast pulse rates as signs of fever: the short clinical thermometer had not been invented.

Treatments were usually unpleasant, but eighteenth-century patients accepted their violent effects as evidence that the medicines were doing something: the normal equilibriums of the strong laxatives and enemas may have kept patients on their chamber pots for hours each day, but the cathartic effect guaranteed that unbalanced humors were being flushed from the body. Neither patient nor doctor knew that catharsis itself does not contribute much toward restoring health. Nevertheless, if the patient recovered, it was easy to conclude that the drug had helped.

Drugs that cause vomiting—emetics—were thought to eliminate foul humors via the stomach and to strengthen the weakened patient by stimulating the stomach fibers so that their strength could be transmitted, via the nerves, to the rest of the body. Thus, emetics were important in the treatment of fevers, as were diaphoretics or febrifuges, drugs that made the patient “sweat out” a disease.

Two treatments were reserved for the sickest patients. One was a solution of a powdered Mediterranean beetle; applied to the skin, it raised a large, painful blister into which disturbed humors could escape. More important, the artificial irritation was presumed to neutralize the natural inflammation associated with most fevers and injuries. The other drastic treatment was to induce bleeding from an arm vein, about twelve ounces at a time, to remove chemically unbalanced blood and relax the tensions in the fibers of fevered blood vessels.

The application of cathartics, emetics, diaphoretics, blisters, and bleeding continued until the mainstay of fever treatment, the tonics, did their work. These bitter drugs were thought to increase the tone of weakened bodies. Similarly, red meat was added to weak patients’ diets as a stimulant, while a bland, watery diet was prescribed to calm feverish bodies—hence, “Feed a cold, starve a fever.”

One drug had several distinct effects. This was opium, usually given as the alcoholic solutions called laudanum and paregoric. Because the morphine and codeine in opium do produce sedation and relieve pain, coughing, and diarrhea, it