Eli Lilly & Co.’s obesity drug Zepbound helped people trim about two inches more off their waists than Novo Nordisk A/S’s Wegovy in the first head-to-head study of the rival medicines.

Patients lost an average of 18.4 centimeters around their waists on Zepbound and 13 centimeters on Wegovy. Belly fat is linked to a higher risk of diseases such as heart attacks, strokes and ...