As a growing number of doctors now recognize, blood is the most dangerous drug in their arsenal. Why? It’s much more than a drug: transfusing it amounts to an organ transplant (albeit a liquid organ), yet it’s administered with none of the precautions associated with other organ transplants. Its use could sometimes be described as casual. As one doctor told me about transfusing single units of blood, “No one has ever needed one unit of blood. If you need only one unit, you don’t need blood.” Now more and more surgeons are recognizing that outcomes are better without blood, even in extremely complex cases of surgery and battlefield trauma.