Blood-letting was a simple enough and rather brutal procedure. You went to the blood letter and he sliced into your vein. After a while, when the blood-letter had determined that you'd lost just the right amount of blood, the wound was bandaged, and off you went, looking forward to being cured of whatever had led you to the blood-letter in the the first place. The procedure was thought to be the way to cure any number of illnesses, including fever and asphyxia (Yoma 84a). It dates back at least to the 5th century BCE, and is mentioned in the writings of Erasistratus (300-260 BCE) who opposed the procedure, and Galen (c. 130-200 CE) who used it and taught that it was an important tool that could heal the sick.
Blood-letting is frequently mentioned in the Talmud. Most famously, in Shabbat 129a, there is an extensive discussion of some of the do's and dont's of blood letting:
Rab Judah said in Rab's name: One should always sell [even] the beams of his house and buy shoes for his feet. If one has let blood and has nothing to eat, let him sell the shoes from off his feet and provide the requirements of a meal therewith. What are the requirements of a meal? — Rab said: Meat; while Samuel said: Wine. Rab said meat: life for life. While Samuel said, Wine: red [wine] to replace red [blood]. ..For Samuel on the day he was bled a dish of pieces of meat was prepared; R. Johanan drank until the smell [of the wine] issued from his ears; R. Nahman drank until his milt swam [in wine]; R. Joseph drank until it [the smell] issued from the puncture of bleeding. Raba sought Wine of a [vine] that had had three [changes of] foliage.
…Rab and Samuel both Say: If one makes light of the meal after bleeding his food will be made light of by Heaven, for they say; He has no compassion for his own life, shall I have compassion upon him!
Rab and Samuel both say: He who is bled, let him, not sit where a wind can enfold [him], lest the cupper drained him [of blood] and reduced it to [just] a revi’it, and the wind come and drain him [still further], and thus he is in danger.
Samuel was accustomed to be bled in a house [whose wall consisted] of seven whole bricks, and a half brick [in thickness]. One day he bled and felt himself [weak]; he examined [the wall] and found a half-brick missing.
Rab and Samuel both say: He who is bled must [first] partake of something and then go out; for if he does not eat anything, if he meets a corpse his face will turn green; if he meets a homicide he will die; and if he meets swine, it [the meeting] is harmful in respect of something else.
Rab and Samuel both say: One who is bled should tarry awhile and then rise, for a Master said: In five cases one is nearer to death than to life. And these are they: When one eats and [immediately] rises, drinks and rises, sleeps and rises, lets blood and rises, and cohabits and rises.
Samuel said: The correct interval for blood-letting is every thirty days. Samuel also said: The correct time for bloodletting is on a Sunday Wednesday and Friday, but not on Monday or Thursday…
There is absolutely no place for this intervention today, other than for the rare illness called polycythemia vera. In this illness, the body makes too many red blood cells (hence its name, poly=many, kytos=cells, hamia=blood), and one way to keep the illness in check is to remove those excess blood cells at a regular intervals. But other than for this disease, blood-letting, (called today phlebotomy or venepuncture, which do sound a whole lot more palatable but describe the same procedure) is harmful. Do not try this at home.
Having made this very clear, let's introduce some nuance. Palliative blood-letting may be useless, but from this is does not follow that it is a good idea to restore the hematocrit (the concentration of red blood cells in the blood) to normal in every disease state. For example, virtually all patients on dialysis (due to chronic kidney disease) become anemic, but in these patients, trying to restore the hemoglobin concentration to a higher level (~13g/dL for those interested) seems to be associated with increased risk, when compared with those in whom the hemoglobin level was lower. And when tiny premature babies get anemic, there does not seem to be an advantage to keeping the hemoglobin in a higher range (though to be fair, more research needs to be done). But these two examples do not in any way lend support to the notion that blood-letting is anything other than a bad idea.
The procedure, which had been in use for at least 2,000 years, only stopped being part of standard medical practice in the late 19th century. Writing in 1875, one Englishman could not bring himself to believe that the era of blood-letting was really over. "Is the relinquishment of bleeding final?" he wrote,
or shall we see by and by, or will our successors see, a resumption of the practice? This, I take it, is a very difficult question to answer; and he would be a very bold man who, after looking carefully through the history of the past, would venture to assert that bleeding will not be profitably employed any more.
(In fact, blood letting was even suggested as a therapy during a severe influenza outbreak at a British Army camp in northern France in the winter of 1916-17. Amazing.) While we no-longer practice this all but useless intervention, the prayer associated with it is worth recalling. Maimonides ruled (Berakhot 10:21) that before undergoing blood-letting, the patient pray the procedure be effective,and this ruling is found as part of normative Jewish practice, recorded in the שולחן ערוך אורח חיים רל ס׳ק ד:
הנכנס להקיז דם אומר "יהי רצון מלפניך ה' אלהי שיהא עסק זה לי לרפואה כי רופא חנם אתה". ולאחר שהקיז אומר "ברוך רופא חולים
Before undergoing blood letting say: May it be your will Lord my God, that this procedure will heal me, for you are an unconditional healer. And when it is finished he says: Blessed are you God, healer of the sick.
The procedures have changed, but the prayers have stayed the same.