New drug aims to stop muscle-loss side effect of obesity injections

GLP-1 obesity injections have driven the biggest shift in weight-loss medicine for a generation, but a second concern is now occupying clinicians: the lean tissue patients shed alongside fat. The BBC's roundup of fresh clinical data tracks how drugmakers are racing molecules through the late stages of development to address what specialists call a meaningful side effect.

The two leading GLP-1 therapies, Novo Nordisk's semaglutide (Wegovy) and Eli Lilly's tirzepatide (Mounjaro/Zepbound), curb appetite and modulate the insulin response. Trials reviewed by the BBC suggest about a third of the weight lost on these drugs comes from lean tissue, much of it from skeletal muscle. Clinicians describe that loss as a real risk for older patients, where balance, bone health and metabolic rate are at stake.

New phase 2b data cited by the BBC indicate that Eli Lilly's bimagrumab, given alone or combined with tirzepatide, preserves fat loss while substantially blunting lean-tissue loss. Bimagrumab is an antibody that targets the activin type II receptor pathway, dampening a brake on muscle growth.

Professor Tricia Tan, an endocrinologist at Imperial College London, told the BBC that preventing lean tissue loss would redefine what success looks like in obesity therapy. She emphasised that muscle preservation matters most in patients over 60, where it bears on falls, mobility and independence in daily activities.

Veridian Therapeutics and Regeneron are reported to have early-stage candidates working on related pathways. Other activin type II receptor antibodies are being investigated for both lean preservation and effects on resting metabolic rate. Companies have signalled an intention to enter phase 3 trials in early 2027.

A notable design feature in the trials is that control arms use GLP-1 monotherapy rather than diet alone, so the contribution of bimagrumab-class agents is measured as added value on top of the current standard. Researchers stress that long-term bone-mineral-density outcomes from muscle preservation will need to be tracked beyond the initial efficacy window.

On safety, antibodies that target the myostatin pathway have historically been associated with mild oedema and transaminase elevations. Lilly's statements describe the phase 2b safety profile as consistent with earlier studies and discontinuation rates as low. How regulators interpret the data will be central to phase 3 design.

The combination raises a parallel question of pricing and reimbursement. GLP-1 drugs are already at the centre of cost-effectiveness debates in the United States and Europe; an additional antibody will need to clear its own health-economics review. Lilly has argued the muscle-preservation benefit could offset costs by reducing falls and hospital admissions in older patients.

From the patient side, the new data reinforce rather than replace existing advice. Clinicians continue to recommend at least two resistance-exercise sessions a week for GLP-1 users and a daily protein intake of around 1.2 grams per kilogram of body weight. The new drugs are framed as an addition to those routines, not a substitute.

The next chapter of obesity treatment, the BBC notes, is shifting from how much weight patients lose to the composition of that loss. Combination regimens may become standard care in the coming years, with phase 3 outcomes and real-world data set to deliver the final verdict. This is not medical advice.