Does Retatrutide Make You Tired? Fatigue and Low Energy

Retatrutide makes some people tired, but fatigue is one of its less common side effects. In the 48-week Phase 2 obesity trial, 19 of the 267 people on retatrutide reported fatigue (7.1%, about 1 in 14), against 3 of 70 on placebo (4.3%). So the answer to "does retatrutide make you tired" is sometimes, and only a little more often than a dummy shot. None of the six Phase 3 readouts Lilly has released lists fatigue among its common side effects. When it does happen, the trial data point to side effects you can check for yourself.

Fatigue gets far less attention than nausea in Lilly's announcements, so most numbers below come from ClinicalTrials.gov and FDA labels of the two closest approved drugs. Where we added numbers up ourselves, we say so.

How often does retatrutide make you tired in trials?

The two Phase 2 trials are the only retatrutide trials with fatigue counts on ClinicalTrials.gov. Their results list every non-serious adverse event that reached 5% in at least one group, and fatigue made both lists. Here's every group, including the dose each one started on.

There's no clean dose pattern. The highest rate in the obesity trial was on 4 mg, not 12 mg, and in groups of 23 to 35 people a single extra report moves the rate by 3 to 4 points.

One hint repeats at 8 mg. In both trials, the 8 mg group that started at 4 mg had more fatigue than the one that started at 2 mg: 3 people against 1 each time. The NEJM paper on the obesity trial found that starting at 2 mg rather than 4 mg partly reduced stomach side effects, which fits. The 4 mg groups don't follow it, though: in the obesity trial, the group that started at 2 mg had more fatigue (4 against 2). Counts this small can't settle it.

Phase 3 hasn't filled the gap. Lilly has released results from six Phase 3 trials, the latest being TRANSCEND-T2D-2 against Ozempic 2 mg on October 28, 2026, and none of the releases lists fatigue among the most common side effects. Those lists name only a handful of events, led by nausea and diarrhea, so fatigue may still sit further down the full tables. The abstracts of the TRIUMPH-1 paper in NEJM and the TRIUMPH-2 paper in The Lancet give no fatigue rate either, and we couldn't read their full tables. As of October 2026, no Phase 3 trial has posted results on ClinicalTrials.gov.

Is retatrutide fatigue worse than on Zepbound or Wegovy?

The FDA labels for tirzepatide (Zepbound) and semaglutide (Wegovy) give the nearest comparison. That's class evidence from approved drugs, not retatrutide data, and each trial ran for a different length in different people.

That last column tilts things. The labels pool several related terms into one fatigue row, while the retatrutide tables count "fatigue" by itself. Weakness, lethargy and malaise never reached the 5% reporting line in any Phase 2 retatrutide group, so their counts aren't public.

Our read: retatrutide's fatigue rate sits in the same range as the approved drugs, on a narrower definition, so "about the same" is the fair summary until Phase 3 fatigue counts are public.

Why does reta make you tired?

No trial has worked out why people on retatrutide feel tired. What the trials do show is a group of common side effects that can each drain energy, and the stomach ones bunch up around dose increases.

You're eating a lot less

In the Phase 2 obesity trial, 29.0% of people on 12 mg reported decreased appetite (18 of 62), against 8.6% on placebo. In TRIUMPH-3 it was 13.5% on 9 mg and 14.5% on 12 mg, against 3.0%. People on 12 mg in TRIUMPH-1 lost 28.3% of their body weight on average over 80 weeks (Lilly's efficacy estimand). That's a lot of fuel no longer coming in, and fast weight loss can take some muscle with it; see retatrutide muscle loss.

Fluid loss from vomiting and diarrhea

In TRIUMPH-1, 25.3% of people on 12 mg vomited at some point and 32.0% had diarrhea, against 4.8% and 13.5% on placebo. The NHS lists thirst, headache, light-headedness and dark, strong-smelling pee as signs of dehydration, names being sick or having diarrhea among the causes, and counts feeling unusually tired as a reason for an urgent appointment. The Zepbound label adds that dehydration after these stomach effects has led to kidney injury in some people on tirzepatide (class evidence). If nausea is the root of it, start with retatrutide nausea.

Light-headed when you stand

Dizziness was reported by 8.1% of people on 12 mg in the Phase 2 obesity trial (5 of 62), against 2.9% on placebo. Low blood pressure (hypotension) showed up in 3 of 35 people in the 8 mg group that started at 4 mg, and in no one on placebo. In the TRIUMPH-2 Lancet paper, 6% of people on 12 mg had hypotension (18 people), against less than 1% on placebo. Blood pressure drops as weight comes off: in TRIUMPH-1, systolic pressure fell by up to 12.3 mmHg (efficacy estimand). That drop is part of the benefit, and it can still leave you feeling washed out when you stand up fast.

Low blood sugar, if you take insulin or a sulfonylurea

In TRANSCEND-T2D-1, where nobody took other diabetes medicines, no one had severe low blood sugar. The risk rises when a drug like this is added to insulin or a sulfonylurea. On the Zepbound label, 10.3% of people also taking a sulfonylurea had glucose under 54 mg/dL, against 2.1% without one (class evidence). The NIH lists feeling tired among the signs of low blood sugar, along with shakiness, hunger, dizziness and irritability.

Bad nights and a flat mood

Waking at 3 am with reflux or a racing pulse leaves you tired the next day; see retatrutide insomnia. A mood where nothing feels rewarding can also pass for fatigue; see retatrutide anhedonia.

Here's how to match what you feel to the likeliest driver.

Does reta give you energy?

Some people say it does. A JAMA Network Open analysis of 148,640 Reddit posts and comments about retatrutide found fatigue (15.2%) and increased energy (12.4%) among the most common self-reported terms, across 7,823 users. Those are self-reports about products nobody verified, not trial data.

The science behind the energy claim is thinner than it sounds. In mice, the glucagon part of retatrutide raised how many calories were burned, but that hasn't been shown in people. A Lilly study measuring calorie intake and 24-hour energy use in 85 people with obesity (NCT06313528) finished in August 2025 and, as of October 2026, has posted no results. Calories burned isn't the same as feeling awake, either. No retatrutide trial has published an energy or vitality score.

There's a longer-term case for more energy, and it runs through weight loss. In the TRIUMPH-1 sleep apnea group (243 people), breathing interruptions fell by 34.3 events an hour on 9 mg and 31.7 on 12 mg, against 9.9 on placebo (NEJM, hybrid-treatment estimand). The NIH lists daytime sleepiness and tiredness among sleep apnea symptoms. That link is indirect: neither the paper's abstract nor Lilly's releases report whether anyone felt less tired. For the numbers on how much weight comes off, see retatrutide weight loss results.

When does the tiredness start, and how long does it last?

Nobody has published that for retatrutide. The Phase 2 tables count anyone who reported fatigue at any point, with no timing attached.

The side effects most likely behind it do have a pattern. In Lilly's Phase 3 trials the dose went up every 4 weeks, and in TRANSCEND-T2D-1 nausea, diarrhea and vomiting happened mainly during those dose increases.

If your tiredness bunches up in the week or two after each step, that points to the stomach effects and eating less. If it's steady wherever you are in the dose schedule, look harder at sleep, blood sugar and your other medicines. Retatrutide's half-life is about 6 days, according to the Phase 1b trial, and Lilly Medical Information gives a peak 12 to 72 hours after a dose, so it stays in your body through the week between shots. See the half-life page for the curve, and the best day and time for your shot if your worst days follow the injection. For how long the stomach effects themselves tend to last, see how long retatrutide side effects last.

What helps when you're tired on reta

1. Keep a 2-week log of your energy, meals, fluids, sleep, the day since your last shot and your current dose.
2. Eat something at regular times even when you're not hungry, and ask your doctor or a dietitian how much protein you need while you're losing weight quickly.
3. Drink steadily through the day, and more on days you vomit or have diarrhea.
4. Get up slowly from lying or sitting if you feel light-headed.
5. If you use insulin or a sulfonylurea, check your glucose when you feel tired and shaky. The Zepbound label tells prescribers to consider lowering those doses (class evidence).
6. Ask your prescriber about staying on your current dose longer before the next step. When a step isn't tolerated, the Wegovy label says to consider delaying the next one by 4 weeks (class evidence).
7. If the tiredness lasts past a few weeks, ask whether a blood test makes sense. The NHS advises seeing a doctor in that case, and no retatrutide trial has published a fatigue work-up to tell you what to expect.

One cause is specific to retatrutide bought online: the vial may not hold what the label says. No approved version exists yet, and Lilly's October 2026 FAQ says retatrutide "has not been reviewed or approved by the FDA or any regulatory agency in the world, and no one can legally sell it for human use". Australia's TGA tested a vial sold as retatrutide after a serious reaction and found no retatrutide in it, just semaglutide at about 8 times the level in approved products. If you're looking at retatrutide for sale online, none of the fatigue numbers on this page apply to a vial whose contents nobody has checked.

When is tiredness a warning sign?

Tiredness with low mood is worth raising too. The FDA advises people on GLP-1 medicines to tell their doctor about new or worsening depression, suicidal thoughts or unusual mood changes; in the US you can call or text 988.

When Lilly or a journal publishes a full Phase 3 adverse event table with fatigue counts, we'll add those rates here. For the wider picture on side effects and risk, see is retatrutide safe and the side effects overview.