Bimagrumab
Bimagrumab (Anti-ActRII Monoclonal Antibody)
Also called BYM338, bimagrumab, activin receptor antibody
An investigational antibody studied for fat loss with muscle gain
Bimagrumab is the odd one out in weight-loss research: instead of reducing appetite, it blocks a signal that limits muscle growth. In its phase 2 trial people lost about a fifth of their body fat while gaining lean mass. It is an antibody given by intravenous infusion, and it is investigational.
Most weight drugs reduce appetite. Bimagrumab does something different: it blocks a signal that stops muscle growing past a point. In a phase 2 trial, participants lost about a fifth of their body fat while gaining lean mass, the opposite of what usually happens with weight loss. It is an antibody given by intravenous infusion in trials, and it is not approved.
- Half-life
- Weeks, as with most antibodies
- Route studied
- Intravenous infusion (trials)
- Status
- Investigational; not approved
Human trials published, with more underway
What Bimagrumab has been studied for
The reasons people look it up, roughly in order of how often, and what research exists for each. Interest in a use is not evidence that it works; the evidence is stated under each one.
- Fat loss with lean mass preserved
- In a phase 2 trial in adults with obesity and type 2 diabetes (JAMA Network Open, 2021), fat mass fell around 20% while lean mass rose, the opposite of what usually happens with weight loss.
- Combined with GLP-1 drugs (in trials)
- Being studied with semaglutide, on the idea that one reduces weight while the other preserves muscle. Not yet established in a completed trial.
Mechanism of Action
How Bimagrumab works
This one is not an appetite drug at all.
- The brake it releases. The body makes signals whose job is to stop muscle growing indefinitely. Bimagrumab blocks the receptor those signals use, so the message never arrives.
- Why fat falls too. Muscle is expensive tissue, and in the phase 2 trial fat came off while lean mass went up.
- Why it is studied with GLP-1 drugs. Weight loss on those drugs takes some muscle with the fat. The idea under study is that a GLP-1 drug reduces weight while bimagrumab preserves muscle. Not yet established.
- The format. It is an antibody given by intravenous infusion in trials.
Bimagrumab side effects: what studies reported
Because it does not act on appetite, the stomach issues that dominate GLP-1 drugs were largely replaced by muscle-related ones in the trial.
Most commonly reported
- Muscle cramps and spasms
- Diarrhoea
- Acne
Worth keeping an eye on
- Blood markers of muscle turnover going up, expected given what it does
- Infusion reactions
Source: Phase 2 randomised trial, JAMA Network Open, 2021. This is a summary of published findings, not a complete safety profile and not medical advice. Questions about Bimagrumab belong with a licensed clinician who knows your history.
Your Genetics & Bimagrumab
The genes involved in how Bimagrumab works
Bimagrumab works through the receptors these genes build, and the genes vary from person to person. Researchers have linked some of that variation to differences in how people respond. A DNA report tells you which versions you carry.
ACTN3 · ••
The R577X variant determines whether alpha-actinin-3 is made in fast-twitch fibres, and is associated with sprint and power performance in athlete studies. It has not been studied with bimagrumab.
Myostatin is one of the growth brakes that signal through the receptor bimagrumab blocks. The K153R variant has been studied for association with muscle traits, with mixed results. Not studied with bimagrumab.
ACVR2B builds one of the receptors bimagrumab blocks. Variants have been studied for association with muscle and metabolic traits. No study has tested whether they change response to bimagrumab.
The dots after each gene are where your own result goes. Your report fills them in with the two letters you carry at that position.
Which of these variants do you carry?
Your report reads the DNA file you already have against this research.
Often discussed alongside
Compounds that come up next to Bimagrumab in research and discussion. Each has its own page. None of this is a suggestion to combine them.
Semaglutide
studied together in the BELIEVE trialStrong EvidenceSemaglutide (GLP-1 Receptor Agonist)
A weekly injection sold as Ozempic for diabetes and Wegovy for weight loss. It is a rebuilt copy of a hormone your gut releases after eating, the one that tells your brain you are full, engineered to last a week instead of minutes. Its SELECT trial also showed fewer heart attacks and strokes in people with heart disease and obesity, which became an approved use for Wegovy.
4
Gene variants
200+
Studies
Access
Access and legal status
Bimagrumab: Investigational; not approved. Decisions about any prescription medicine belong to a prescriber. We don't link to any seller, and nothing on this page is a recommendation to obtain or use it.
Your DNA and Bimagrumab: see which of the variants studied alongside it you carry, and what that research does and does not show. Read my DNA, $99 →
Educational information only, not medical advice. Most peptides are not FDA-approved. Do not start, stop or change any medication or peptide because of this page; talk to a licensed clinician.
Worth knowing about Bimagrumab
Interactions and situations where there is a specific reason to be careful. Where the honest answer is that nobody has studied something, it says so and says what that leaves open.
- Investigational, given by infusion in trials
- It is an antibody given intravenously under trial supervision. Anything sold online under the name is not what was tested.
- Not approved for weight loss
- It has been through Phase 2 for body composition and was originally developed for muscle-wasting conditions. There is no approved use for fat loss.
Last updated
Frequently Asked Questions
What is bimagrumab used for?
Bimagrumab is an investigational antibody that blocks the activin type II receptor. In a phase 2 trial it reduced fat mass while lean mass rose. It is being studied alongside semaglutide, and it is not approved for any use.
How is it different from a GLP-1 drug?
GLP-1 and GIP drugs reduce appetite and overall body weight, including some muscle. Bimagrumab acts on the muscle-regulation pathway instead; in its phase 2 trial fat fell while lean mass rose.
Is bimagrumab approved?
No. As of 2026 bimagrumab is investigational, with phase 2 data including the BELIEVE trial. It is an intravenous biologic, not an available prescription.
Watch
What clinicians and researchers say about Bimagrumab
Your DNA and Bimagrumab
What the research says about your variants.
The report
Which Bimagrumab variants do you carry?
Your report reads the DNA file you already have against the published research on 31 peptides, marker by marker, and says what that research does and does not show.
Read my DNA, $99