The ingredient in question is aristolochic acid, a compound found in leafy, flowery vines called Aristolochia, or birthwort. For centuries, birthwort has been used in traditional medicine in China (and ancient Greece before that) to treat arthritis and ease childbirth, among other conditions. (The flower is shaped like a uterus.) Today aristolochic acid—pronounced "a-ris-to-LOW-kick”—is found in supplements for weight loss, menstrual symptoms, and rheumatism. It’s widely used in Asia, where it’s added to medicinal wine, ointments, and diet pills. One study found that between 1997 and 2003, fully one-third of Taiwanese were prescribed birthwort supplements by a Chinese medicine practitioner.

Warnings about the herb first emerged in the early 1990s, when a scandal involving dozens of women in Belgium who had inadvertently taken it for weight loss surfaced. As reported in The Lancet in 1993, several of the patients developed severe kidney failure.