Here's the short version. Retatrutide is an investigational weight-loss drug that acts on three hormone receptors: GIP, GLP-1 and glucagon. The glucagon part is where the "energy" idea comes from. But the energy in question is calories your body burns, not how lively you feel.
"Energy" in the research means calories burned
In obese mice, adding glucagon-receptor activity increased energy expenditure on top of the appetite-lowering effect of GIP and GLP-1 [1]. Put simply, the mice burned more calories as well as eating less.
That paper came from the drugmaker's own scientists, and the energy-expenditure finding was in mice. Whether people burn meaningfully more calories on retatrutide is still being studied. Either way, burning more calories isn't the same thing as feeling more energetic.
What the trials reported about how people felt
The cited trial abstracts do not establish a benefit for subjective energy or vitality. Their most commonly reported adverse effects were gastrointestinal.
- Phase 2 (338 adults, 48 weeks): the most common side effects were gastrointestinal, rose with dose and were mostly mild to moderate. A lower starting dose partly reduced them [2].
- TRIUMPH-1 (2,339 adults with obesity, 80 weeks): the most common side effects were gastrointestinal [3].
- TRIUMPH-2 (1,152 adults with obesity and type 2 diabetes, 80 weeks): diarrhea affected 27% to 34% of people on retatrutide versus 13% on placebo, and nausea 14% to 28% versus 8% [4].
The cited abstracts do not establish a numerical fatigue risk. Absence from an abstract is not absence from the full trial report, and it is not evidence that an adverse effect cannot occur.
| Claim | What the evidence shows | Source |
|---|---|---|
| It boosts energy (how you feel) | No subjective energy benefit established by the cited trials | Trial abstracts [2] [3] [4] |
| It raises energy expenditure | Yes, in mice, through the glucagon receptor | Coskun 2022 [1] |
| It causes fatigue | Not quantified in the trial abstracts; reported anecdotally online | Trial abstracts |
| Most common side effects | Gastrointestinal, rising with dose | Jastreboff 2023 [2]; Bellido 2026 [4] |
| Heart rate | Rose with dose, peaked at 24 weeks, then eased | Jastreboff 2023 [2] |
All four trial papers were funded by Eli Lilly, which is developing retatrutide.
Why some people say they feel tired
The usual explanations are eating much less, stomach side effects and not drinking enough. These are reasonable guesses, not measured findings.
There is one useful comparison. Tirzepatide, an approved drug that shares two of retatrutide's three targets, lists fatigue among its common side effects: 5% to 7% of people on it versus 3% on placebo in its weight-loss trials [6]. That doesn't tell you retatrutide's rate. It does show that tiredness is a recorded effect in this drug family. More on that is in does Zepbound give you energy.
Commercial retatrutide products are unapproved and cannot be assumed to match the trial drug. Trial participation and authorized expanded access are different from buying a research-use vial online; an online anecdote without a verified product cannot establish the trial drug’s adverse-effect rate.
Other effects people notice: heart rate and skin sensations
In the phase 2 trial, heart rate rose in a dose-dependent way, peaked at 24 weeks and declined after that [2]. A higher resting heart rate is something to raise with a clinician, not a sign of extra energy.
Dysesthesia, meaning unusual skin sensations such as burning or tingling, has also been seen. In TRIUMPH-2 it affected 4% to 7% of people on retatrutide versus 1% on placebo [4]. A 2026 drug-safety analysis found the same signal across GLP-1 drugs, more often at higher doses [5].
Where retatrutide stands
Retatrutide isn't FDA-approved for any use. The FDA says it "cannot be used in compounding under federal law" and hasn't been found safe and effective for any condition [7]. It isn't on the FDA's list of new drugs approved in 2026 [8].
For what retatrutide does to muscle and body composition, see does retatrutide build muscle. For an approved drug in the same family, see tirzepatide and building muscle on tirzepatide.
If energy is your goal
Low energy has many causes, from sleep and iron to thyroid and mood. Those are worth checking with a clinician before reaching for any drug. Our energy shortlist covers the options people ask about most, with the evidence for each.