Bimagrumab

What bimagrumab is

Bimagrumab is a fully human monoclonal antibody, originally developed by Novartis and today used in obesity research with funding from Eli Lilly. In the studies it was given as an intravenous infusion at intervals of several weeks. Unlike most substances on this site, it is not a gray-market peptide but a clinical development candidate that so far has only been administered in studies.

The target is a signaling axis every strength athlete knows: myostatin. This messenger and its relatives from the activin family slow muscle growth. They act via activin type II receptors. Bimagrumab occupies these receptors and thus blocks not only myostatin but several braking signals at once.

How it is supposed to work

In muscle, the blockade removes the growth brake, and the fibers gain mass. In fat tissue the same signaling axis acts differently: without the activin signal, more fat is mobilized and broken down. The authors of the large 2026 study describe that bimagrumab apparently does not act via appetite but directly in fat and muscle.

This makes bimagrumab the counterpart to incretin drugs such as semaglutide. These reduce appetite, and part of the weight loss comes at the expense of lean mass. The idea behind the combination: semaglutide provides fewer calories, bimagrumab directs the loss into fat and protects the muscles.

The authors of the combination study summarize how big the problem is: people who lose weight through calorie reduction, whether by diet, medication or surgery, lose mostly fat, but about 25 to 40 percent of the weight loss is lean mass, including skeletal muscle. In the authors’ assessment, this loss can reduce the metabolic benefit of weight loss and impair physical performance in people who already have little muscle mass. This is exactly where the hope for bimagrumab comes in.

What it is used for

Earlier studies were conducted in a rare inflammatory muscle disease, in age-related muscle loss and after hip fractures. There the hope was that more muscle mass would lead to more strength and mobility. Today the focus is on overweight and type 2 diabetes, alone and in combination with semaglutide.

In the biohacking scene, bimagrumab is seen as a great hope for weight loss without muscle loss and as an anti-aging tool against muscle wasting in old age. In practice it is not accessible: an antibody in clinical development cannot be ordered online like a peptide, and the infusion belongs in the hands of a physician.

What the studies show

Combination with semaglutide, 507 participants, 2026

In a double-blind phase 2 study, 507 adults with obesity were divided into 9 groups: placebo, bimagrumab, semaglutide or both. After 48 weeks they lost 3.3 kilograms with placebo, 9.3 with the higher bimagrumab dose alone, 14.2 with the higher semaglutide dose and 17.8 kilograms with the high-dose combination. Under semaglutide alone, lean mass fell by 4.7 to 6.9 percent, in the combinations by only 0.8 to 2.3 percent. Of the weight lost under the high combination, 92.3 percent was fat, under semaglutide alone 71.1 percent.

Muscle wasting in old age, 180 participants, 2020

Rooks and colleagues gave people aged 70 and over with sarcopenia bimagrumab or placebo for 6 months, both groups with good nutrition and home exercise. Lean mass rose considerably more under bimagrumab. The primary endpoint, a test of walking, standing up and balance, however improved similarly in both groups, 1.34 versus 1.03 points; the difference was not significant.

After hip fracture, 250 participants, 2021

After hip surgery, people aged 60 and over received bimagrumab or placebo for 24 weeks. In the two higher dose groups, lean mass increased by 1.9 and 2.8 kilograms, under placebo by 0.2 kilograms. Walking speed and physical function, by contrast, recovered equally in all groups. The RESILIENT study with 251 patients with inclusion body myositis showed the same pattern: no difference in walking distance after 52 weeks.

More mass is not more function

The most important limitation concerns exactly what muscle preservation is really about. In three studies in older or ill people, muscle mass grew, but the participants did not walk faster, did not stand up more easily and did not get further than under placebo. Whether the additional mass translates into strength and everyday function is open. Bimagrumab has not been studied in young healthy people or athletes.

There are also methodological limits. All published efficacy studies are phase 2 studies; no pivotal approval study was found. Lean mass in a body scan includes skeletal muscle and internal organs, so actual muscle mass is only measured indirectly. The longest controlled data cover 48 weeks, with an open-label extension to 72 weeks. The meta-analysis also found an increase in LDL cholesterol of 0.47 millimoles per liter, whose significance for the heart and blood vessels is unknown.

Status, approval and legal

Bimagrumab is not approved and is in clinical development. It is only available within studies, so no usage or dosage information is given. In sport the situation is clear: the World Anti-Doping Agency’s 2026 Prohibited List explicitly names bimagrumab under S4.3 as an example of antibodies against the activin receptor IIB. The ban applies at all times, in competition and in training.

Safety

The typical side effects are muscle cramps, diarrhea and acne. In the RESILIENT study, depending on the dose, up to 68 percent had muscle cramps, under placebo 21 percent, and up to 52 percent diarrhea, under placebo 18 percent. In the large combination study, 14.0 to 21.4 percent under bimagrumab alone stopped treatment because of side effects, under placebo 3.6 percent. The meta-analysis calculates a relative risk of 5.75 for treatment discontinuation, 10.44 for cramps and 4.91 for diarrhea. The heart was unremarkable in the study designed for that purpose. Long-term data are lacking, as are data on pregnancy and breastfeeding.

Sources

• Heymsfield SB et al., JAMA Network Open 2021 — type 2 diabetes, 75 participants, 48 weeks
• Heymsfield SB et al., Nature Medicine 2026 — bimagrumab and semaglutide, 507 participants
• Rooks D et al., JAMA Network Open 2020 — sarcopenia, 180 participants
• Hofbauer LC et al., Lancet Healthy Longevity 2021 — after hip fracture, 250 participants
• Hanna MG et al., Lancet Neurology 2019 — RESILIENT, inclusion body myositis
• Rooks D et al., Journal of Clinical Endocrinology and Metabolism 2026 — cardiac safety in older adults
• Shao C et al., Diabetes, Obesity and Metabolism 2026 — meta-analysis, 4 studies
• WADA Prohibited List 2026, officially published in the Austrian Federal Law Gazette III No. 219/2025