Berochot 33: Bloodletting a Bechor

The Gemara discusses what should be done when a bechor suffered from achazo dam, an overabundance of blood.  The recommended treatment was hakazat dam, bloodletting.  The rabbis offered four options:

1. Rabbi Meir permits bloodletting in a place where the animal will not receive an injury that would render it a ba'al mum, a permanently blemished animal
2. The Sages permit bloodletting even if it caused a blemish, but not in the place on the animal where it would be slaughtered
3. Rabbi Shimon permits the bloodletting even in the place where the animal would be slaughtered
4. Rabbi Yehuda prohibits bloodletting under any circumstances, even if the animal will die without treatment.

Steinsaltz explains bloodletting as a common preventative and treatment over thousands of years.  Apparently it was used for almost all ailments.  Steinsaltz also notes that today, we have learned that bloodletting is not effective when treating most diseases.