Bimagrumab: Activin Receptor Antibody for Weight Loss
Bimagrumab explained: an activin type II receptor antibody studied with semaglutide to preserve muscle during weight loss. Not a peptide. Not medical advice.
Anonymous user report. Not medical advice. Peptides discussed are not FDA-approved for the uses described. Editorial process →
Short answer
Bimagrumab is a monoclonal antibody (not a peptide) that blocks activin type II receptors, the pathway myostatin and activins use to limit muscle growth. Eli Lilly is studying it with semaglutide or tirzepatide to shift weight loss toward fat and away from lean mass. Phase 2 data reported more fat loss and better lean-mass preservation than GLP-1 alone, with side effects including muscle spasms, diarrhoea and acne. It is investigational and given by infusion in trials. Peptide Confessions does not publish dosing, timing, injection or sourcing guidance.
Phase 2 results (BELIEVE)
507 adults with obesity, published in Nature Medicine (2026). Week-72 figures. Last reviewed October 15, 2026.
| Group | Weight change | Body composition |
|---|---|---|
| Bimagrumab alone (higher dose) | 10.8% | All weight lost was fat; lean mass rose about 2.5% |
| Semaglutide alone (Wegovy strength) | 15.7% | About 72% of weight lost was fat; lean mass fell about 7.4% |
| Combination (higher doses) | 22.1% | About 92% of weight lost was fat; lean mass fell about 2.9% |
Why muscle preservation matters
On GLP-1 drugs, a meaningful share of weight lost can be lean mass, which matters for strength and metabolism, especially in older adults. Bimagrumab blocks signals that normally limit muscle growth, so it is being tested to keep muscle while fat falls. See Ozempic muscle loss. Lilly is also testing it with tirzepatide.
Side effects and status
Common bimagrumab side effects in trials were muscle spasms, diarrhoea and acne. It is given by intravenous infusion every few months in trials, is not approved anywhere, and is not available outside studies. Research-market "myostatin blockers" such as follistatin are different, unproven products.
What users report here
Anonymous accounts are collected on the Semaglutide reports. They are personal stories, often with other compounds in the mix, not evidence of effect.
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Community Q&A
- How much weight did people lose on bimagrumab with semaglutide?
- 22.1% on average at 72 weeks in phase 2, vs 15.7% with semaglutide alone, with more of the loss coming from fat.
- Does bimagrumab preserve muscle?
- In phase 2, lean mass fell about 2.9% with the combination vs about 7.4% with semaglutide alone.
- Is bimagrumab a peptide?
- No. It is a monoclonal antibody.
- Is bimagrumab approved?
- No. It is in clinical trials, mainly combined with GLP-1 drugs.
- Does this page give dosing advice?
- Peptide Confessions does not publish dosing, timing, injection or sourcing guidance.
Sources
Anonymous first-hand reports plus sourced context — educational only, not medical advice. Editorial policy & corrections.