It's Forecast to Be Bestselling Drug Ever. Now, a Big Caveat

The full results from Eli Lilly's study of its newly approved weight-loss drug Zepbound serve as a splash of cold water in the face of those who believed a few dozen weekly injections would cause pounds to melt off and stay off. The FDA approved Zepbound, with the active ingredient tirzepatide, as a weight loss drug in November after initial results from an 88-week clinical trial showed patients lost 20% of their body weight. Some Wall Street analysts have since predicted Zepbound could become the best-selling drug of all time. After all, 70% of Americans are overweight or obese, per CNN. However, data from the phase 3 trial published Monday in JAMA shows patients who took Zepbound for 36 weeks regained roughly half the weight lost after going off the treatment for a year, CNBC reports.

Half of 670 overweight or obese patients continued taking Zepbound, which mimics hormones to give the feeling of fullness, for the remaining 52 weeks of the study, while the other half unknowingly took a placebo. Those who continued with Zepbound lost an additional 6.7% of body weight on average, per CNBC. Those who stopped taking the drug regained 14.8% of their weight, though they averaged 9.9% less weight than at the start of the study. "If you look at the magnitude of the weight gain, they gain back about half the weight they had originally lost," lead study author Dr. Louis Aronne, an obesity medicine specialist at Weill Cornell Medicine, tells CNN. Just 17% of those who stopped Zepbound maintained at least 80% of their original weight loss, compared to 90% of those who continued with the drug.

"One out of six, this would say, is able to maintain the weight loss without medication," Aronne tells CNN, which reports improvements in blood pressure, blood sugar, and cholesterol also seemed to evaporate over the year among the placebo group. Eli Lilly's Dr. Jeff Emmick sees the upside of the study. It shows "continued therapy can help people living with obesity maintain their weight loss," he says, adding that "patients, providers, and the public do not always understand obesity is a chronic disease that often requires ongoing treatment." But health insurers fear the high costs of long-term coverage of weight loss drugs like tirzepatide and semaglutide, per CNBC. They cost about $1,000 per month and come with side effects, including pancreatitis and bowel obstructions in rare cases, per CNN. (More tirzepatide stories.)