Bimagrumab
Bimagrumab (Anti-ActRII Monoclonal Antibody)
Also called BYM338, bimagrumab, activin receptor antibody
The muscle-preserving add-on
Bimagrumab is the odd one out in weight loss: instead of making you eat less, it blocks a signal that limits muscle growth. In its main trial people lost about a fifth of their body fat while gaining muscle. It is an antibody given by drip, not a peptide you inject at home, and it is not approved.
Every other drug on this list makes you eat less. Bimagrumab does something different: it takes the brakes off muscle. Your body runs a signal that stops muscle growing past a point, and this blocks it. The result in trials was unusual, people lost about a fifth of their body fat while actually gaining muscle, which is the opposite of what normally happens when you lose weight. It is given by drip in a clinic, not injected at home, and it is not approved.
- Half-life
- Weeks, as with most antibodies
- Route
- Intravenous infusion in a clinic, roughly monthly
- Typical dose
- 10 mg/kg every four weeks in the trial
- Availability
- Research-only
Human trials published, with more underway
What people use Bimagrumab for
Reported uses, roughly in order of how often they are the reason someone goes looking. Whether it works for any of them is a separate question, answered under each one.
- Losing fat without losing muscle
- The whole argument for it. Ordinary weight loss takes muscle with the fat. In the JAMA trial fat mass fell around 20% while lean mass went up, which is the opposite of what usually happens.
- Alongside GLP-1 drugs
- The obvious pairing, and the one being explored: let the GLP-1 take the weight off and let this protect the muscle underneath. Interesting on paper, not yet proven in a finished trial.
Mechanism of Action
How Bimagrumab works
This one is not an appetite drug at all.
- The brake it releases. Your body makes signals whose job is to stop muscle growing indefinitely. Bimagrumab sits on the receptor those signals use, so the message never arrives and muscle is free to build.
- Why fat falls too. Muscle is expensive tissue. More of it raises what you burn at rest, and in the trials fat came off while lean mass went up, which almost nothing else manages.
- Why people pair it with Ozempic-type drugs. Ordinary weight loss takes muscle with the fat, often a quarter to a third of what you lose. The obvious idea is to let a GLP-1 take the weight off while this protects what is underneath. Sensible on paper, not yet finished in a trial.
- The catch. It is an antibody on a drip, roughly monthly, in a clinic. This is not something you do at home.
Bimagrumab dosage calculator
Opens at 10 mg, from the doses reported for Bimagrumab. Change any box to match your own vial. It works out where to pull the plunger to.
How much is in the vial?
Printed on the label, in milligrams
How much water did you add?
Bacteriostatic water, in millilitres
What dose are you taking?
The dose you already intend to use
Which syringe?
Barrel size, printed on the wrapper
Draw to
Too much
That dose is 2 mL, more than a 0.5 mL syringe holds. Use a larger syringe, or add less water.
- Strength once mixed
- 5 mg/mL
- Doses in the vial
- 1
10 mg in 2 mL makes 5 mg/mL. A 10 mg dose is 2 mL of that, which is more than the barrel holds.
This is arithmetic, not advice. It converts a dose you already have into a mark on a syringe. It does not tell you what dose to take, and Bimagrumab is not prescribed by us. Dosing belongs with a qualified clinician.
Bimagrumab side effects: what people reported
Different drug, different complaints. Because it is not working on appetite, the stomach issues that dominate GLP-1 drugs are largely replaced by muscle-related ones.
What most people notice
- Muscle cramps and spasms
- Diarrhoea
- Acne
Worth keeping an eye on
- Blood markers of muscle turnover going up, which is expected given what it does
- Infusion-site reactions, since this is given by drip rather than a pen
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. Bimagrumab should only be considered with a qualified 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 defines fast-twitch (power) versus endurance muscle. An RR (power/fast-twitch) phenotype is the kind of lean mass most worth protecting during weight loss — strengthening the rationale for a muscle-preserving adjunct like bimagrumab.
Myostatin is the growth brake bimagrumab releases. Variants altering myostatin tone set how much room there is for muscle gain when the activin/ActRII signal is blocked.
ACVR2B is the receptor bimagrumab blocks. Variation in receptor expression can influence the magnitude of the muscle-sparing and muscle-building response.
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 variants do you carry?
Upload your DNA data, or get tested through our partner, to find out.
Commonly combined with
What Bimagrumab is most often paired with, and why:
Tirzepatide
Lean-Mass StackStrong EvidenceTirzepatide (GIP/GLP-1 Dual Receptor Agonist)
A weekly injection for weight loss and type 2 diabetes, sold as Zepbound and Mounjaro. It copies two of the gut hormones that tell your brain you have eaten enough, where most drugs in this class copy one. That second signal is why it beat semaglutide when the two were compared directly: about 20% of body weight against 13.7% over 72 weeks.
Semaglutide
Body Composition StackStrong 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. It is also the only weight-loss drug shown to cut heart attacks and strokes rather than just weight.
MariTide
Metabolic StackEmerging EvidenceMariTide (Maridebart Cafraglutide)
MariTide is Amgen's answer to a simple annoyance: nobody enjoys injecting themselves every week. It is built to last a month. To manage that, it is not really a peptide at all but an antibody with two small peptides attached, which is a much bigger, slower molecule that your body takes weeks to clear. In its Phase 2 trial people lost around 20% of their body weight on twelve injections a year. It is still in testing.
4
Gene variants
12+
Studies
Sourcing & access
Where to buy Bimagrumab
Bimagrumab is sold as a research compound, not a licensed medicine, so quality and legal status vary widely by country and vendor. If you're sourcing it, treat independent purity testing as non-negotiable.
- Insist on a recent third-party Certificate of Analysis (COA) for the exact batch — labs like Janoshik verify purity and identity.
- Check the legal status where you live before ordering — it differs from country to country.
- Reconstitute correctly with bacteriostatic water and start at a conservative dose.
Before you buy: see whether your DNA actually responds to Bimagrumab, and at what dose. Analyze my DNA — $99 →
Educational information only, not medical advice. Some outbound links are affiliate links (disclosed, at no extra cost to you). Most peptides are not FDA-approved — consult a qualified professional and check your local laws before purchasing.
Worth knowing before you consider 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.
- Not something you can self-administer
- It is an antibody given intravenously in a clinical setting. Anything sold as bimagrumab for home use 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 to reduce fat while preserving or increasing muscle. Its most logical use is alongside GLP-1/GIP weight-loss drugs, which otherwise cost a large fraction of lost weight as muscle.
How is it different from a GLP-1 drug?
GLP-1/GIP drugs reduce appetite and overall body weight — including muscle. Bimagrumab works on the muscle-regulation pathway instead, so it loses fat while protecting (or building) lean mass. They're complementary, not competing.
Is bimagrumab approved?
No. As of 2026 bimagrumab is investigational, supported by Phase 2b (BELIEVE) data. It is a clinic-administered IV biologic, not a self-administered peptide, and not an available prescription.
Learn more about Bimagrumab
Your next move
Two ways forward with Bimagrumab.
Not sure it's for you?
How does Bimagrumab relate to your genes?
Your report scores Bimagrumab against your receptor, pharmacogene and pathway variants and shows the genetic markers relevant to it, with commonly cited dosing information to review with a clinician.
Analyze my DNA — $99