Weight-loss injections are everywhere at the moment. Medications in the GLP-1 family, such as semaglutide (sold as Ozempic and Wegovy) and tirzepatide (Mounjaro), can produce weight loss that was previously hard to achieve without surgery. In clinical trials, average weight loss has ranged from roughly 5% to 18% of body weight, alongside benefits for blood glucose, heart and kidney health in the right patients.

That is genuinely important progress. But as a musculoskeletal GP, I want patients to understand a part of the story that often gets skipped in the advertising: not all of the weight you lose is fat. Some of it is muscle and bone. This matters for how strong you feel, how well you move, and your long-term risk of falls and fractures. The good news is that there is a lot you can do about it.

Where does the weight actually come from?

When you lose weight in any way — diet, surgery or medication — you lose a mixture of fat mass and fat-free mass. Fat-free mass includes your skeletal muscle. This has always been true; it is not unique to the injections.

What has changed is the speed and size of the weight loss these medications can produce. Because they work partly by reducing appetite and food intake, some people eat much less overall, including less protein, which is the main building block your body uses to maintain muscle.

In a small 2026 real-world study of people starting semaglutide or tirzepatide without any structured nutrition or exercise support, roughly a quarter of the weight lost was estimated to come from skeletal muscle, and intake of protein and several micronutrients fell. Because that study was small and short, the exact numbers should be read as a signal rather than a precise figure — but the direction is consistent with what specialists have been reporting.

Australian context. In Australia these medications are approved by the TGA, but access and cost vary. Ozempic is subsidised on the PBS only for type 2 diabetes, not for weight loss. Wegovy and Mounjaro for weight management are generally paid for privately, and supply has been intermittent. If you are considering one, your GP can talk you through eligibility, current availability and cost rather than relying on online suppliers.

Why muscle and bone loss matters

Muscle is not just about appearance. It drives your strength, balance, metabolism and independence as you age. Losing muscle while losing fat can leave some people lighter but weaker — a pattern sometimes described as “sarcopenic obesity” when low muscle and excess fat occur together.

Bone is affected too. A 2026 New England Journal of Medicine review noted that, while the side effects of GLP-1 medications are mostly gastrointestinal, they can also include loss of muscle and bone mass, and that the long-term functional consequences of this are still being studied. For someone who is older, already has low bone density, or has a history of falls, protecting bone and muscle during rapid weight loss is not a minor detail.

None of this means the medications are unsafe or that weight loss is a bad idea. Carrying excess weight has its own well-established risks for the joints, heart and metabolism. The point is to lose weight in a way that keeps you strong, not just smaller.

How to protect your muscles and bones

The reassuring part is that the strategies that preserve muscle and bone are the same ones we already recommend for healthy ageing. A 2025 joint advisory from several obesity and nutrition societies, and a 2025 review on weight loss and muscle, both point to the same core approach.

Resistance training is the priority. Progressive strength work — using weights, bands, machines or bodyweight — is the single most effective way to signal your body to hold onto muscle while you lose fat. Two to three sessions a week covering the major muscle groups is a sensible starting target for most people. If you are new to it or have joint pain, start light and build gradually.

Eat enough protein. Adequate protein intake is repeatedly linked with better preservation of muscle during weight loss. In the small 2026 study above, people who ate more protein held onto more muscle. Because the medications suppress appetite, hitting your protein target takes deliberate effort — spreading protein across meals and prioritising it over less nutritious foods helps.

Keep moving overall, and mind your other nutrients. Regular walking and general activity support your heart and joints. Because eating less can reduce intake of vitamins and minerals — including those important for bone, such as calcium and vitamin D — it is worth reviewing your overall diet, and in some cases your bloods, with your GP.

Australian context. A useful framing that aligns with RACGP and Choosing Wisely thinking is that medication is a tool, not a stand-alone treatment. The best results come from pairing it with strength training, adequate protein and the everyday habits that keep you well. Your GP can help you build that around the medication, rather than leaving it to chance.

What the research is still working out

This is a fast-moving field, and some questions are genuinely unresolved. We do not yet know exactly how much the muscle and bone changes translate into real-world weakness or fracture risk for different groups of people. We also know that many people regain weight after stopping these medications, which raises questions about what happens to body composition over the longer term.

There is also active work on the next generation of these drugs, with researchers specifically trying to design medications that target fat while preserving muscle. That is emerging science, and it is too early to draw conclusions from it — but it shows the muscle question is being taken seriously.

The bottom line

GLP-1 weight-loss medications can be a valuable option for the right person, under proper medical supervision. But losing weight quickly comes with a trade-off worth managing: some of the loss is muscle and bone, not just fat. Building in resistance training and adequate protein from the start gives you the best chance of coming out of the process stronger and more resilient, not simply lighter.

If you are taking one of these medications, or thinking about it, the sensible step is a conversation with your GP about your goals, your muscle and bone health, and a plan to protect them along the way.

This article is general information, not personal medical advice. It does not replace an individual assessment. Decisions about weight-loss medication, and about exercise if you have pain or other health conditions, should be made with your own GP or treating doctor. If you have unexplained weight loss, weakness, or new symptoms, please see your GP.