As more doctors and patients turn to the latest weight-loss drugs, researchers are trying to figure out which drug is right for which patient—and at what point in their weight-loss journey.Key to making those decisions is how effective the drugs are and which side effects people might experience while taking them.

People taking Eli Lilly's obesity drug, Zepbound, lost nearly 50% more weight than those using rival Novo Nordisk's Wegovy in the first head-to-head study of the blockbuster medications. Clinical trial participants who took tirzepatide,

The first head-to-head trial of two blockbuster weight-loss drugs has shown Mounjaro is more effective than rival Wegovy. Both drugs led to substantial weight loss, but Mounjaro's 20% weight reduction, after 72 weeks of treatment, exceeded the 14% from Wegovy, according to the trial's findings.