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We are witnessing great promise in medicine today. The vocabulary dazzles: “precision medicine,” “stem cells,” “moon shot,” and other terms that hint at exciting new possibilities. But it is also a time of great vulnerability for patients.

I’m particularly worried about stem cells or, more precisely, stem cell therapies. In theory, stem cells could help treat — or even cure — conditions ranging from cancer to heart failure. As a cardiologist, I’m especially interested in their application to the latter condition. Even though the hype far exceeds the evidence, patients are already turning to so-called stem cell clinicsthat claim to reverse heart failure.

So-called “pluripotential” stem cells have the ability to develop into different cell types. They can be derived from various sources, including a patient’s own blood, fat tissue, and bone marrow, or from a donor, as with umbilical cord blood. Stem cells can be mobilized and directed to target sites by infusing them into a vein or artery or even directly injecting them into a particular organ or tissue.

For several years, researchers have explored how...