The Method Weight loss
Will you lose muscle on semaglutide or tirzepatide?
Some of it, yes. The drug makes the deficit easier to hold. Lifting and protein decide how much muscle it costs.
StrongGood evidence, with a caveat worth knowing.
Updated . The Vanguard coaching team.
What most people miss
These drugs work by making a deficit easier to hold, and a deficit takes some lean tissue along with the fat. In the STEP 1 trial's body-scan group, about 39% of the fat and lean mass lost on semaglutide was lean.1 In SURMOUNT-1's, about a quarter of the weight lost on tirzepatide was lean, the same share as on placebo: the drug moved more weight, not a worse mix.2
The two trials measured differently, so these numbers do not rank the drugs. And a scan counts water and organs as lean, so the muscle share is smaller than it looks. It is still real.
What to do about it is no longer guesswork. A 2025 advisory from four medical and nutrition societies pairs these drugs with strength training at least three times a week and enough protein, and says protein without the lifting is likely not enough.3
So the medication and the training do different jobs. Whether to start, stay on or stop a medication is a decision for you and your prescriber. How much muscle you keep along the way is where the training comes in.3
The research
The papers behind the answer
Each one checked against the paper itself. The grade describes the strength of the evidence, never anyone's results.
Wilding, Batterham, Calanna et al., 2021 · N Engl J Med
Randomised trial, STEP 1 · body-scan group n = 140
On semaglutide, fat mass fell 19.3% and lean mass 9.7%. Lean mass was about 39% of the fat and lean lost. A scan counts water and organs as lean, so the muscle share is smaller.
Read the paper by Wilding, Batterham, Calanna et al., 2021 (opens in a new tab)Look, Dunn, Kushner et al., 2025 · Diabetes Obes Metab
Randomised trial, SURMOUNT-1 · body-scan group n = 160 · 72 weeks
On tirzepatide, body weight fell 21.3%, fat mass 33.9% and lean mass 10.9%. About 75% of the weight lost was fat and 25% lean, on tirzepatide and on placebo alike.
Read the paper by Look, Dunn, Kushner et al., 2025 (opens in a new tab)Mozaffarian, Agarwal, Aggarwal et al., 2025 · Am J Clin Nutr
Joint advisory · four medical and nutrition societies
Covers GLP-1 drugs and their combinations. Strength training at least three times a week alongside the drug, with enough protein. Protein without the lifting is likely not enough.
Read the paper by Mozaffarian, Agarwal, Aggarwal et al., 2025 (opens in a new tab)
Research describes groups of people. Your coach will tell you how it applies to you.
From the floor
Tell your coach if you start one of these drugs, and the lifting becomes the priority of every session, logged set by set. Your prescriber should know you train.
The Vanguard coaching team
GLP-1 consultations are available from our naturopathic doctor, in the same building and booked separately from training.
GLP-1 consultations at the clinic