Controversy and use into the 20th century

Bloodletting gradually declined in popularity over the course of the 19th century, becoming rather uncommon in most places, before its validity was thoroughly debated. In the medical community of Edinburgh, bloodletting was abandoned in practice before it was challenged in theory, a contradiction highlighted by physician-physiologist John Hughes Bennett. Authorities such as Austin Flint I, Hiram Corson, and William Osler became prominent supporters of bloodletting in the 1880s and onwards, disputing Bennett's premise that bloodletting had fallen into disuse because it did not work. These advocates framed bloodletting as an orthodox medical practice, to be used in spite of its general unpopularity. Some physicians considered bloodletting useful for a more limited range of purposes, such as to "clear out" infected or weakened blood or its ability to "cause hæmorrhages to cease"—as evidenced in a call for a "fair trial for blood-letting as a remedy" in 1871.

Some researchers used statistical methods for evaluating treatment effectiveness to discourage bloodletting. But at the same time, publications by Philip Pye-Smith and others defended bloodletting on scientific grounds.

Bloodletting persisted into the 20th century and was recommended in the 1923 edition of the textbook The Principles and Practice of Medicine. The textbook was originally written by Sir William Osler and continued to be published in new editions under new authors following Osler's death in 1919.