Bimagrumab
BYM338
An antibody that blocks the myostatin pathway and adds muscle while removing fat. Its real future is probably as the thing you take alongside a GLP-1.
Human trials
Investigational. Randomised phase 2 trials in obesity, type 2 diabetes and sarcopenia have been published, and a combination trial with semaglutide reported in 2026. Not approved for any indication.
What it is
Bimagrumab is a monoclonal antibody against the activin type II receptors. Myostatin and activins bind those receptors to restrain muscle growth, which is why animals and people with myostatin loss-of-function mutations are extraordinarily muscular. Blocking the receptor removes the brake.
It is included here because it is the only compound in this pathway with real randomised human data, and because the peptide community's interest in myostatin inhibition consistently points at things with none.
What the evidence shows
Bimagrumab does two things simultaneously, which is unusual. In randomised trials in adults with obesity and type 2 diabetes it increased lean mass by several percent while reducing fat mass substantially, with improvements in insulin sensitivity. A meta-analysis of the randomised evidence confirms the direction on both body composition endpoints.
Muscle spasms are the signature adverse effect, and raised creatine kinase and liver enzymes appear consistently. Cardiac safety of chronic myostatin-activin pathway inhibition has been specifically examined in healthy older adults, which is the kind of study that gets run when a mechanism raises a reasonable question.
Why it matters now
Because of what GLP-1 drugs do to lean mass.
Weight loss of 20% or more from semaglutide, tirzepatide or CagriSema is not all fat. A meaningful fraction is lean tissue, and at the magnitudes now being achieved that has become a genuine clinical concern rather than a talking point.
A phase 2 trial published in Nature Medicine in 2026 tested bimagrumab with semaglutide, alone and in combination, in obesity. That is the pairing the mechanism was waiting for: one drug removes fat, the other protects the muscle you would otherwise lose along with it.
Practical notes
- It is an antibody given by infusion, not a peptide you inject. Nothing sold through peptide suppliers is bimagrumab, and the delivery alone rules out self-administration.
- This is the honest reference point for myostatin inhibition. Follistatin and the various research-grade myostatin products have no human efficacy data. Bimagrumab has randomised trials, and it is still not approved.
- Lean mass preservation is the indication to watch. The standalone obesity case is decent. The combination case is the one that would change practice.
Side effects and warning signs
Commonly reported
- Muscle spasms, the most characteristic effect of blocking this pathway
- Acne
- Diarrhoea
- Raised creatine kinase and liver enzymes
- Injection or infusion reactions
Stop and get medical help
- The activin pathway is not confined to muscle, and long-term consequences of blocking it are uncharacterised
- Cardiac safety has been specifically studied in older adults, which tells you it was a live question rather than a settled one
Sources
From the community
Discussion for this compound concentrates in r/Semaglutide and r/Peptides.
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