Bimagrumab Guide
The Muscle-Preserving Antibody Paired with GLP-1 Therapy
Bimagrumab is a monoclonal antibody that increases muscle mass while reducing fat — a direct answer to the biggest downside of GLP-1 weight loss: lean-mass loss. In the Phase 2 BELIEVE trial, bimagrumab plus semaglutide drove 22% weight loss with 92% coming from fat. Here's how it works and where it stands.
What Is Bimagrumab?
Bimagrumab is a human monoclonal antibody that blocks activin type II receptors (ActRII) on muscle. Blocking these receptors releases muscle from a growth brake — increasing lean mass — while independently reducing fat mass. It was originally developed for muscle-wasting conditions and is now being studied for obesity, owned by Eli Lilly through its Versanis Bio subsidiary.
Its appeal in the GLP-1 era is specific: GLP-1 and dual-agonist drugs are highly effective at reducing weight, but a meaningful share of that loss — up to roughly 40% in some analyses — can come from lean muscle. Bimagrumab targets the opposite tissue. Pairing it with a GLP-1 aims to deliver fat loss while protecting, or even adding, muscle.
Bimagrumab is administered as a periodic (roughly monthly) infusion or injection, not a daily or weekly self-dose. It is investigational for obesity and not FDA-approved. Note that Lilly discontinued a separate diabetes study, but the obesity combination data published in 2026 is what has driven interest.
BELIEVE Trial Results (Phase 2)
How Bimagrumab Works
What Bimagrumab Could Change
Better Body Composition
Losing fat while keeping muscle improves strength, metabolic rate, and long-term weight maintenance — not just the number on the scale.
Healthier Aging
Muscle loss is especially risky for older adults on GLP-1s. A muscle-sparing add-on could make weight loss safer for that group.
Maintenance Phase
Muscle preservation may help defend against weight regain after stopping or tapering a GLP-1, when lean mass is most vulnerable.
Combination Future
Bimagrumab points to a "GLP-1 + muscle agent" stack as the next step beyond single-drug weight loss.
Not a Standalone
Bimagrumab is designed to complement a GLP-1, not replace it — the fat loss still comes largely from the incretin partner.
Still Investigational
Phase 2 data is promising but obesity approval is not here yet. Larger, longer trials are needed before clinical use.
Status and Safety
Bimagrumab is investigational for obesity and not available by prescription for weight loss. The strongest evidence to date is the Phase 2 BELIEVE trial published in Nature Medicine in 2026, which combined bimagrumab with semaglutide across multiple dose arms.
Reported side effects in bimagrumab studies have included mild muscle cramps and diarrhea, and modest changes in some metabolic markers; long-term safety in a broad obesity population is still being established. Because it is delivered as a periodic infusion or injection and targets muscle biology, it is a fundamentally different tool than a GLP-1 — and not something to source outside a trial.
If you are concerned about muscle loss on a GLP-1 today, the practical levers remain adequate protein intake and resistance training — and tracking your weight, strength, and body measurements over time so loss of lean mass is caught early.
Guide FAQs
Bimagrumab is a monoclonal antibody that blocks activin type II receptors on muscle, increasing lean mass while reducing fat. It is being studied for obesity (owned by Eli Lilly via Versanis) as a muscle-preserving partner for GLP-1 drugs.
In the Phase 2 BELIEVE trial, bimagrumab plus semaglutide produced 22% weight loss with 92% from fat and only 2.9% lean-mass loss — far better muscle preservation than semaglutide alone.
No. Bimagrumab is investigational for obesity and not available by prescription for weight loss. The leading evidence is Phase 2; larger trials are needed before clinical use.
It is administered as a periodic (roughly monthly) infusion or injection, not a daily or weekly self-dose. It is intended to complement a GLP-1, not replace it.
Until muscle-sparing drugs are approved, the proven levers are adequate protein and resistance training. Track your weight, strength, and measurements in Shotlee so lean-mass loss is caught early.
Track Body Composition in Shotlee
Free tracking for weight, measurements, protein, and GLP-1 doses — so you can watch fat loss without losing muscle.