When Savannah Crosby started posting videos on TikTok about using berberine, a dietary supplement, for weight loss, she had about 500 followers. About two months later, Crosby now has more than 21,000. “I have a lot of joy and gratitude over the fact that sharing my experience has been able to help other women with the same struggles that I have” in trying to lose weight, said Crosby, a 34-year-old who lives in San Antonio and works for a property management company. Crosby’s popularity is part of a larger trend. People have nicknamed the supplement, which can be purchased online...