Anthropology

For instance, it probably worked well for people in hypertensive crisis. They wouldn't have known what was causing the headache or other symptoms, but they knew reducing circulating volume reduced the crisis. It left them markedly anemic for a while, unlike today's diuretics, but it worked.

It would also work for the various polycythemias.

It just did nothing much for everything else. It was what they did when they didn't know what else to do.

The problem, of course, was the church and its monopoly over health care, stressing the four humors theory and rejecting any study of anatomy. One thing the Black Death did for us is start to break that monopoly.