Fatigue is a listed adverse reaction in the US prescribing information for both semaglutide and tirzepatide, with numbers attached: 11% versus 5% on placebo, and 5–7% versus 3%. Muscle cramps are not listed in either label — a full-text search of both documents for "muscle cramp", "myalgia" and "cramp" returns no matches at all. Patients report cramps anyway, and the difference between a symptom the label recognises and one it does not is worth understanding rather than glossing over. This article covers both, and what happens to training in a calorie deficit.
Listed adverse reaction: an event that occurred often enough in the pivotal trials, and distinguishably enough from placebo, to be printed in the label's adverse-reaction table. Absence from that table means the trials did not identify it — not that it never occurs.
Is fatigue a side effect of GLP-1 medication?
It is, and both labels quantify it.
Fatigue was reported by 11% of semaglutide-treated patients against 5% on placebo, with the label footnoting that the figure "Includes fatigue and asthenia" — weakness as well as tiredness. For tirzepatide the figure is 5–7% depending on dose, against 3% on placebo (prescribing information).
These are US-label figures for the injectable products; Singapore's registered product information is a separate document. What they establish is that tiredness is a recognised, measured feature of treatment rather than something people imagine.
Why cramps are not in the label
Because they were not identified at a rate distinguishable from placebo in the trials that produced the label.
That is a narrower statement than it sounds. It does not mean cramps never happen on these medicines. It means the pivotal trials did not separate them from background rates, so no figure exists to print (prescribing information). A symptom can be real, common in clinic, and absent from a label.
Two routes to muscle symptoms during weight loss do have evidence behind them, and neither is a pharmacological action of the drug.
Restriction itself. In a randomised trial of a protein-supplemented very-low-calorie diet — 106 adults, no medication involved — 11% of participants on the restrictive arm reported myalgia, against 0% on the less restrictive comparator (Cho 2024). The authors described the events as mild and transient. Two caveats travel with that finding: the symptom measured was muscle aching rather than cramping, and severe prescribed restriction is a different physiological situation from appetite suppression, so this does not transfer directly to GLP-1-driven weight loss. It does establish that eating far too little can make muscles hurt with no drug anywhere in the picture.
Fluid and electrolyte loss through the gut. Vomiting occurred in 24% of semaglutide-treated adults against 6% on placebo, and diarrhoea in 30% against 16% (prescribing information). The US Medication Guides state plainly that "Diarrhea, nausea, and vomiting may cause a loss of fluids (dehydration) which may cause kidney problems" (Medication Guides). A 2025 review documents electrolyte disturbances requiring monitoring after rapid-weight-loss interventions (bariatric surgery specifically), and separately notes a plausible pathway by which GLP-1 receptor agonists could influence potassium handling (Crintea 2025). In that review the two observations sit separately; it does not join them into a causal chain ending in cramps or fatigue.
So the mechanism most people reach for — low electrolytes — is plausible and partly documented, but it has not been demonstrated as the explanation. What is documented is the dehydration risk, and that is a reason to take persistent vomiting or diarrhoea to a doctor rather than to the electrolyte aisle.
What happens to training performance in a deficit?
It does not automatically fall. In the studies below that measured function, it improved.
In a six-month trial of 160 previously sedentary obese older adults, mean age around 70, every exercise group lost about 9% of body weight — and the group doing both aerobic and resistance training improved on a physical performance test by 21%, significantly more than either single modality (Villareal 2017). Losing weight and getting more capable happened together.
In a shorter and harsher study, 40 untrained overweight young men trained six days a week for four weeks on roughly a 40% energy deficit. The group taking 2.4 g of protein per kg of body weight per day gained 1.2 kg of lean mass and lost more fat than the group at 1.2 g/kg (Longland 2016). That is a supervised protocol in a narrow population, not a template — but it shows a severe deficit does not by itself dictate what happens to muscle.
On medication, the longest measurement available is a single-arm cohort of 106 completers with a BMI of 40 or above. Lean-mass loss plateaued after seven months while fat loss continued, and handgrip strength rose 4.5 kg by month 12 (Alissou 2026). With no placebo arm that cannot show the effect belongs to the drug, and grip strength is a strength measure rather than a measure of muscle mass. It is still a functional number moving in the right direction over a year of treatment.
What those three have in common is a training stimulus and enough protein. Most guidance during active weight loss lands between 1.2 and 1.6 g of protein per kg of body weight per day (Leidy 2015), which is the harder half of the job when appetite has gone — covered separately in hitting your protein target when you have no appetite. The training half is smaller than people expect; see the minimum resistance training that works.
At GetLean, our philosophy is that the medication is the catalyst and what you keep is the result. Tiredness matters here mainly because it is the thing most likely to end the training, and the training is the part that decides what you keep.
When tiredness is not just tiredness
Some fatigue is a symptom of something that needs attention rather than a side effect to push through.
Vomiting or diarrhoea that does not settle is the clearest example, because of the dehydration and kidney risk the Medication Guides describe (Medication Guides). The same documents list dizziness, shakiness, sweating, weakness and confusion among low-blood-sugar symptoms, which overlap with ordinary tiredness and are worth distinguishing with a doctor rather than alone. We cover the full list in red flags: when to stop and call a doctor.
Individual results vary, and clinical-trial figures describe the populations studied. Never change or stop a prescribed medication on your own, and check with a doctor before starting resistance training with any existing joint, cardiac or metabolic condition.
Common questions
Is fatigue a side effect of GLP-1 medication?
Yes, and it is quantified. The US labels report fatigue in 11% of semaglutide-treated adults versus 5% on placebo, and 5-7% by dose on tirzepatide versus 3% (prescribing information).
Are muscle cramps a listed side effect of GLP-1 medication?
No. A full-text search of both current US labels for muscle cramp, myalgia and cramp returns no matches anywhere (prescribing information). That means cramps were not identified at a rate distinguishable from placebo in the pivotal trials - not that they never happen.
What causes muscle aches during weight loss?
Restriction itself can do it. In a randomised trial of a very-low-calorie diet with no drug involved, 11% of participants reported muscle pain against 0% on the less restrictive arm (Cho 2024).
Will my gym performance drop on GLP-1 medication?
It does not automatically. In dieting older adults losing about 9% of body weight, combined training improved physical performance by 21% (Villareal 2017), and in a single-arm cohort on semaglutide, grip strength rose 4.5 kg over 12 months (Alissou 2026). Individual results vary.
When should tiredness on GLP-1 be checked by a doctor?
When it comes with vomiting or diarrhoea that does not settle. The US Medication Guides state that fluid loss from these symptoms may cause kidney problems and instruct patients to report them right away (Medication Guides).