There were lots of different kinds of cures in Galenic medicine, not just bleeding. Here were some other therapies in the medieval physician's repertoire: induced vomiting, enemas, sweating, application of poultices, even bathing. Those were just the purgatives. There were also medical manuals that would tell you what sorts of foods you should eat to counteract imbalanced humors. For example, if you had a sanguine temperament you weren't supposed to eat salt. Galen actually did advise people with 'hotter' temperaments to drink water rather than wine. I think we tend to focus on bleeding, even though it was only one among many humoral therapies, because it's sensational, clearly harmful in many instances, and it enables us to feel superior to people who lived in the past. It also makes for better TV than a medieval doctor who induced vomiting...

Hi there. I was asked to help on this question, so here's my take on it.

The comments below have some good tidbits, particularly the part where ScottSowerby (sorry - bad at formatting on here, but wanted to give credit) says that bleeding was not the only prescription. That is certainly true, and blood letting has, for a long time, been thought of as the staple treatment of medieval physicians. I think it has something to do with the mental image of blood letting and the ongoing tendency for the masses to think of the medieval period as the barbaric "Dark Ages." There were, however, plenty of other Galenic therapies that physicians would and could have used. They could have prescribed mulled wine stewed with herbs, certain food combinations (depending on the malady), or may have suggested burning spices such as juniper or rosemary. Blood letting was not always the go-to solution, and even when it was, very minimal blood was shed. A typical measurement was a half eggshell full, so really, it was not so drastic.

You ask why they didn't simply "water down" the blood, too, and I think it's important to remember that we have the advantage of seeing these things from a modern scientific view. Of course we know that blood contains water, but to medieval physicians and barbers (another point to add - barber surgeons, not physicians, were the ones bleeding patients, and these surgeons were not trained physicians at least until the late 15th century when some medical programs began to include surgery), the two were separate liquids altogether. So adding water would not solve the perceived issue of too much blood.

Unfortunately, there were no "magic tables" that told physicians how to solve these issues. They instead had to balance humors based on what symptoms the patient exhibited using what knowledge they had. Hopefully this is helpful for you!