Weighing up the evidence used by direct-to-consumer stem cell businesses

Hundreds of businesses across the United States offer direct-to-consumer stem-cell-based interventions that have not been approved by the Food and Drug Administration. Here, we characterize the types of evidence used on the websites of 59 stem cell businesses in the Southwest United States to market their services. We identify over a dozen forms of evidence, noting that businesses are less likely to rely on "gold-standard"scientific evidence, like randomized clinical trials, and instead draw substantially on forms of evidence that we identify as being "ambiguous."Ambiguous evidence has some scientific or medical basis, but its interpretation is highly contextdependent. These findings highlight the interpretive responsibility placed on prospective patients. We identify actions for regulators and professional societies to assist with evaluating evidence, but caution that focusing on the (in)validity of particular evidence types is unlikely to eliminate demand for stem-cell-based treatments in this complex marketplace.