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Tiredness comes up constantly in reta talk. Here is what the trials recorded. And the four ordinary things that also explain it.
- Not a top trial finding
- Eating less explains a lot
- Fluids and salts matter
- Not approved anywhere
Tiredness is not one of the headline findings in the retatrutide trials. Gut effects are. But tiredness has plenty of ordinary causes here. Eating far less, feeling sick, being dried out, sleeping badly, or blood sugar moving around will all do it. Retatrutide is not approved anywhere and nothing here is medical advice.
Does retatrutide make you tired? The quick answer
Some people in the trials did report feeling tired. It was not one of the common problems.
In the 2023 Phase 2 obesity trial, 22 of 337 people on retatrutide reported fatigue. That is 7%. On placebo it was 3 of 70. That is 4%.
So a bit more than placebo, and not by much. The rates moved around between dose groups.
Retatrutide is still being tested. It is not approved by Health Canada or the FDA. Nothing in the published research can tell you how any one person will react.
This page summarises what has been published. It is not medical advice. And it is not a suggestion to use a research product on yourself. Tiredness that is severe, that will not go away, or that is getting worse needs a doctor.
How common is tiredness in the research?
There is one study everyone is quoting. A Phase 2 obesity trial published in the New England Journal of Medicine, with 338 people on retatrutide and 70 on placebo.
Here is what it reported for fatigue.
| Study group | Participants reporting fatigue | Rate |
|---|---|---|
| Placebo | 3 of 70 | 4% |
| Retatrutide 1 mg | 3 of 69 | 4% |
| Retatrutide 4 mg, 2 mg initial dose | 4 of 33 | 12% |
| Retatrutide 4 mg, 4 mg initial dose | 2 of 35 | 6% |
| Retatrutide 8 mg, 2 mg initial dose | 1 of 35 | 3% |
| Retatrutide 8 mg, 4 mg initial dose | 3 of 35 | 9% |
| Retatrutide 12 mg | 6 of 62 | 10% |
| All retatrutide groups | 22 of 337 | 7% |
Two things to take from that table, and they pull in opposite directions.
Fatigue happened. But the table does not show that retatrutide caused every one of those reports. People in trials get tired for all the ordinary reasons too. That is why 4% of the placebo group reported it as well.
And look at the group sizes. Some of them are 33 or 35 people. At that size, one extra person changes the percentage by three points. So do not read the jumps between dose groups as a pattern. There is not enough there to be a pattern.
What is retatrutide?
It is an investigational once-a-week molecule built to switch on three hormone receptors at the same time.
- GIP receptors – glucose-dependent insulinotropic polypeptide
- GLP-1 receptors – glucagon-like peptide-1
- Glucagon receptors
Most drugs in this family hit one of those, or two. Retatrutide goes for all three. And researchers are still working out what that does to weight, appetite, blood sugar and energy.
Still. Testing is not finished. That means nobody has the full safety picture or any idea about the long term. There is more background on the research in our library if you want it.
Why might retatrutide make you tired?
The trial recorded that fatigue happened. It did not work out why. Here are the plausible reasons, and more than one can be true at once.
You are eating a lot less
Retatrutide cut appetite in the studies. That is the point of it.
Cut your food intake sharply and you will feel flat. That happens to anyone, on any diet, with or without a drug involved. So some of the tiredness may have nothing to do with the compound itself and everything to do with eating less.
Your gut is upset
Stomach trouble was by far the most common problem in the trial. Feeling sick, throwing up and diarrhea all cut down what you eat and drink.
Less food and less fluid means weakness, dizziness and dehydration. Tiredness is downstream of all of that.
The dose is going up
Most of the gut trouble in the trial happened while doses were being raised. And there was more of it in the higher-dose groups.
That finding is about gut symptoms though. It does not prove fatigue follows the same timeline for everyone. That is a guess people make, not something the study showed.
It might not be the drug at all
Tiredness is the least specific symptom there is. Bad sleep causes it. So does stress, an infection, low iron, a thyroid problem, blood sugar moving around, or another medication entirely.
Which is why blaming the newest thing you took is usually the wrong move. Look at the whole picture first.
Is retatrutide fatigue permanent?
Nothing in the Phase 2 study says it is.
Nothing in it says how long it lasts either. The trial recorded that fatigue was reported. It did not follow the symptom through to work out when it stopped.
So anybody telling you it wears off after two weeks is making that up. The drug is still being tested. And this is one of the many things nobody has checked yet.
When to see a doctor
Get tiredness looked at if it is severe, if it will not shift, if it is getting worse, or if it is stopping you doing normal things.
Go sooner if it comes with any of these: fainting, confusion, chest pain, trouble breathing, throwing up that will not stop, bad stomach pain, or signs you are dehydrated.
Do not use this page to work out what is wrong with you, to change a medication, or to decide what to do with an investigational compound.
Common questions
Does retatrutide make you tired?
It can. But it was not one of the common effects. 7% of the retatrutide groups in the Phase 2 obesity trial reported fatigue against 4% on placebo. That is an association, not proof the drug caused each case.
Does retatrutide make you sleepy?
Nobody measured that. Tired and sleepy are not the same thing. And the trial’s table recorded fatigue only. Daytime sleepiness was never counted separately.
Is fatigue a common retatrutide side effect?
Not compared with the gut problems. That dominated. Fatigue came in at 7% across the retatrutide groups in the 2023 Phase 2 obesity trial, against 4% on placebo.
Does a higher dose cause more fatigue?
Some higher-dose groups reported more of it. But not consistently. look at the table and the numbers zigzag. The groups were small enough that a couple of people swing the percentage. So the trial does not support that claim.
Can dehydration contribute to tiredness?
Yes. And it is one of the more likely explanations here. Feeling sick, throwing up, diarrhea or just drinking less all lead there. And dehydration makes you tired and dizzy. Anyone who cannot keep fluids down needs medical help.
Does retatrutide cause fatigue?
It is not a leading trial finding. Tiredness reported by users usually tracks back to eating far less, dehydration or poor sleep.
How long does the tiredness last?
No published figure covers it. Reports describe it easing as the dose settles, but that is not trial data.
What helps with low energy?
Enough protein, enough fluid and enough salts are the usual answers. Persistent tiredness needs a doctor, not a forum.
Is tiredness a sign something is wrong?
It can be. Tiredness with dizziness, a fast heart, or symptoms that keep getting worse should be checked.
Is retatrutide approved?
No. It is still in testing and no regulator has cleared it.
The short version
- 7% of people on retatrutide reported fatigue in the Phase 2 obesity trial. 4% on placebo did too.
- It was nowhere near the most common problem. The gut symptoms were.
- The trial did not work out why. And eating far less would explain a lot of it on its own.
- The dose groups were small, so the differences between them do not mean much.
- Nobody knows how long it lasts, because nobody followed it.
- Retatrutide is still investigational. Tiredness that is severe or persistent needs a doctor, not a forum.
References
- Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity – A Phase 2 Trial. New England Journal of Medicine. 2023;389:514-526. doi:10.1056/NEJMoa2301972.
- Jastreboff AM, et al. PubMed record for the Phase 2 obesity trial. PMID: 37366315.
- Rosenstock J, Frias J, Jastreboff AM, et al. Retatrutide for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled Phase 2 trial. The Lancet. 2023;402:529-544. PMID: 37385280.
Evidence review updated July 27, 2026. This page will be revised as peer-reviewed clinical evidence develops.
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View the retatrutide listing Read the lab reportsResearch and buying information only. Not medical advice. Not dosing advice. Not a recommendation for human use. Retatrutide from Red Leaf Research Labs is lab research material and is not approved by Health Canada for injection, ingestion, or any human or animal use. Prices and shipping terms change; check the live product page. Reviewed September 5, 2026 by Baba Kahn, founder of Red Leaf Research Labs.