Some new medications in development are also trying to solve for a persistent side effect of existing drugs: Patients who lose fat also tend to lose muscle. This can be particularly dangerous for older adults because it makes them more likely to fall and can worsen osteoporosis.

Patients who took an experimental drug from Eli Lilly & Co. together with Novo Nordisk A/S’s Wegovy maintained muscle while losing weight, offering a potential solution to one of the key problems that’s emerged with popular obesity shots.