Why did doctors keep draining patients' blood for more than 2,000 years?

Because it fit the theory everyone believed, it seemed to work, and nobody was counting results. Bloodletting, the deliberate removal of blood to prevent or cure illness, was the most common medical practice performed by surgeons from antiquity until the late 19th century, a span of over 2,000 years. It was based on humorism, the ancient system in which blood and other bodily fluids had to stay in proper balance to maintain health. If a fever meant too much blood, taking some out was simply logical.

The method was also backed by the biggest name in medicine. Galen's ideas reinforced its popularity across the classical Mediterranean world. In his system, blood was made and then used up, so it did not circulate and could stagnate. He even worked out how much to remove based on the patient's age, the season, the weather and the place. Following Galen was the safe choice for a doctor, and over the centuries bloodletting became a standard treatment for almost every ailment.

Two things kept it going. First, doctors from the 17th century onward gained a great deal of anatomical knowledge, but the key to curing disease remained elusive, and the underlying belief was that it was better to give any treatment than nothing at all. Second, the psychological benefit to the patient, a placebo effect, may sometimes have outweighed the physical harm.

The practice faded only when someone counted. In the 1830s the French physician Pierre Louis used numbers to show that bleeding was ineffective for pneumonia and various fevers. Even then, some physicians resisted, and a textbook still recommended bloodletting in 1923. Today, a very small number of conditions are treated with a careful, modern form of blood removal, which is a different thing from the old cure-all.