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GIP/GLP-1/glucagon triple receptor agonist (investigational) · safety

Retatrutide side effects

Also called Reta or Reta peptide.

The retatrutide side effects reported most often are nausea, diarrhea and vomiting. The full list is below, with the cited trial report cited at the bottom of the page.

Rarer, but worth being able to spot: increased heart rate.

These effects tend to cluster at the start and after each dose increase, and to settle down while the dose stays the same.

Retatrutide is an experimental weekly injection being tested for weight loss. It acts on three hormone signals linked to appetite and metabolism. People buy unapproved versions under the nickname Reta in hopes of losing weight.

A published phase 2 trial found large average weight loss. Phase 3 trials are underway, but FDA has not approved retatrutide.

Further down: the full list with frequencies · when to get medical help · questions

Medically reviewed by Jennifer Montecillo, MD · non-practicing medical reviewerReviewed 2026-08-16open the primary sources

At a glance

Common reactions listed

5

Serious reactions listed

1

Boxed warning

No label exists

Strongest source

Human trial

The counts show how many distinct reactions the cited trial report names. The percentages show how often a reaction appeared in a study group, not the chance that it will happen to you.

Common reactions

The full list, as reported across the available trial evidence. There is no approved label to read frequency bands off, so these are the trials’ own figures.

◆ Common reactionstrial-reported
ReactionFrequency
NauseaReported in trials (dose-dependent)
DiarrheaReported in trials (dose-dependent)
VomitingReported in trials (dose-dependent)
ConstipationReported in trials
Decreased appetiteReported in trials

Serious reactions to watch for

These are rarer, and they are the ones clinicians monitor for. Seek medical care if you have symptoms.

◆ Serious — monitorwarning/precaution
ReactionFrequency
Increased heart rateReported in trials — monitor

Human trialEvery reaction above comes from a cited trial. These are study findings, not instructions on what to take.

When to seek medical care

Each one below is a serious reaction from the table above, written as the signs you would actually notice. Spotting them is the job of this section; working out what they mean is a clinician’s.

Increased heart rate

Urgent

Signs: A resting pulse that stays faster than usual, palpitations, getting out of breath doing ordinary things, or feeling lightheaded.

Report a lasting change to a clinician, who can put it next to the rest of the picture. Chest pain, fainting, or breathlessness while resting is an emergency call, not a clinic call.

EmergencySome symptoms should not wait for an appointment. Call emergency services (911 in the US) if they are severe, getting worse fast, or involve trouble breathing, chest pain, fainting or confusion.

2Read deeperTiming, evidence limits, and product context

When these effects tend to occur

GLP-1 labels and trial reports place stomach and gut effects most often at the start and after dose increases. A later increase can bring that early pattern back.

The same sources describe most of these effects as temporary — showing up after a step and easing off while the dose stays put. That is the pattern across whole trial populations. It is not a promise about you, and the sources do not commit to how long the easing takes.

The studied escalation

This is why approved schedules do not start at the full dose. The opening dose introduces the drug rather than treating the condition, and each step lasts for a minimum time before the next one is considered. The schedule is designed around how well people can handle each step as much as the dose itself. The evidence here is a 48-week phase 2 dose-finding trial in 338 people, with arms at 1 mg · 4 mg · 8 mg · 12 mg. Reactions were recorded against that build-up, not against a faster route to the same dose.

See the retatrutide trial dosing program →

Making side effects easier to discuss

None of this is a recommendation, and none of it is treatment. These are the side-effect topics clinicians and drug labels tend to raise. They are written down here so your conversation with a prescriber starts further along than it otherwise would.

Meal size and pace
Portion size and how fast you eat are usually the first things raised when nausea tracks with meals. They come up because they are things you can describe precisely at your next appointment, which makes them useful information either way.
Fluids
Fluid intake comes up in almost every side-effect conversation. The reason is already on this page: where kidney injury appears in the serious list, dehydration is the route named. Vomiting and diarrhea are what lose fluid. That is why clinicians want to hear about them early rather than have you put up with them quietly.
Noticing patterns rather than guessing
Clinicians often suggest keeping track of what came before a bad day — the meal, the timing relative to the injection, how recently the dose changed. A specific account is worth more at an appointment than a vague one, and it is the part only the person taking the compound can supply.
How fast the dose moves
How quickly the dose goes up is a clinician’s call and belongs in that conversation, not on this page. Approved schedules set a minimum time at each step and treat the next step as conditional on the current one going well. Staying longer at a step is a recognized option, and the prescriber is the one who uses it.
Constipation as its own topic
Constipation usually gets discussed separately from nausea, because what helps is different and because anything reached for over the counter interacts with the rest of a medicine list. That makes it a question for a clinician or pharmacist rather than a search box.
Reporting rather than enduring
A side effect is information, not a test you failed. Clinicians want to hear about reactions early. They can hold a step, change the timing or look for another cause. The right choice depends on your symptoms, history and other medicines.

Starting, holding, changing, pausing or stopping a dose is a decision for the person who prescribes it.

Common questions

How long do retatrutide side effects last, and do they go away?

The 48-week trial report does not establish a set number of days for any reaction. It records whether an event occurred during the study, not a reliable personal countdown for when it will stop.

What are the most common retatrutide side effects?

Nausea, diarrhea, vomiting and constipation — the table above has the rest, each with its frequency band and source.

Does retatrutide have a boxed warning?

No, because a boxed warning is part of an FDA-approved label and retatrutide has no approved label. That reflects what regulatory review has and has not covered, not a finding that serious risks were looked for and ruled out.

Can the retatrutide dose be lowered if side effects are hard to tolerate?

That decision belongs to the prescribing clinician. No approved schedule exists for this compound, so there is no labeled framework for adjusting a dose. That leaves the prescriber as the only place the decision can sit.

Were hair loss, fatigue or constipation listed for retatrutide?

In the 48-week report, constipation was listed; hair loss and fatigue were not. That is a statement about the paper’s recorded terms, not proof that an unlisted symptom cannot occur.

What should someone do about a serious retatrutide reaction?

Contact a clinician promptly, and emergency services if symptoms are severe or getting worse quickly. The section above describes what those reactions actually look like, so they can be spotted early rather than explained away.

Do the frequencies on this page mean the chance it happens to one person?

No. These figures describe groups in a study. A band shows how often a reaction appeared under that trial’s conditions. It cannot predict what will happen to one person.

Related

The schedule these reactions were recorded against, the comparisons, and the sourcing rules behind every grade on this page.

How strong the evidence is for retatrutide

The strongest source for retatrutide is a published human trial.

Retatrutide dosing evidence, without an approved schedule

1 published retatrutide dose value, each linked to the original source.

Retatrutide concentration and dose-draw tool

The concentration in mg/mL for every retatrutide vial and water pairing, printed in full.

Where retatrutide stands in retatrutide vs tirzepatide

Dosing, source strength, half-life and reactions for retatrutide vs tirzepatide, cited row by row past retatrutide.

Every calculator on this site, retatrutide and the rest

Every intermediate value is printed, for retatrutide or any other vial.

The constipation question separate from retatrutide

Which sources name constipation, under what wording and at what frequency, beyond retatrutide.

Diarrhea weighed class-wide, including retatrutide

Named diarrhea entries, grouped figures and absences kept distinct, apart from retatrutide.

Nausea across every page, not just retatrutide

12 of 36 pages carry nausea, counted class-wide rather than within retatrutide.

1primary source, each with the day we read it — open the documents used on this page