As millions flock to GLP-1s, doctors warn the drugs can cause rapid and significant muscle loss. Chanel Robinson achieved exactly what the gold rush of blockbuster weight-loss drugs promised: She lost nearly 100 pounds, lowered her cholesterol to normal levels and reined in her polycystic ovary syndrome. Yet, nearly three years into her journey on Mounjaro, the 30-year-old from Atlanta, Ga., is discovering the hidden costs of the slimmed-down life.