Retatrutide: three receptors, phase III, and nothing on a pharmacy shelf

A single peptide acting at three receptors, with phase II weight-loss results above anything approved. It is not approved anywhere, and that fact is the most important thing on this page rather than a footnote to it.

The glucagon receptor is the addition

Semaglutide acts at one incretin receptor; tirzepatide acts at two. Retatrutide adds a third target that is not an incretin receptor at all: the glucagon receptor. Established clinical use Glucagon is usually thought of as the hormone that raises blood glucose, which makes agonising its receptor in a metabolic drug sound backwards. The rationale is that glucagon also increases hepatic fat oxidation and raises energy expenditure, and that the GLP-1 component offsets the glycaemic effect — so the combination is intended to add a calorie-burning arm to appetite suppression.

That is a coherent design and it is also where the open safety questions sit. A molecule that raises energy expenditure and acts on hepatic glucose handling has more places to go wrong than one that only suppresses appetite, and heart rate, hepatic effects and glycaemic control in people with diabetes are all things a phase III programme exists to characterise.

What phase II reported

Established clinical use The phase II trial in adults with obesity, published in 2023, reported mean weight reduction of about 24% at 48 weeks at the highest amount tested, against roughly 2% on placebo. That is the largest figure reported for any agent in this class and it is what drives the interest. It is also a phase II result: a smaller population, a shorter duration and a narrower safety picture than a phase III programme produces.

Gastrointestinal effects dominated, as with the rest of the class, and were amount- and titration-related. The app models the half-life at about six days and flags it as an estimate, which is appropriate for a compound whose full pharmacokinetic characterisation is not published; weekly dosing follows from that figure.

Off-label or community practice The muscle-mass question is more pressing here than elsewhere in the class for the simple reason that the weight loss is larger. Loss of lean mass scales with the size of the deficit, and nothing about this mechanism protects it. Resistance training and adequate protein are the standard response and are inference from body-composition principles rather than a trial of the combination.

Nothing available is the trial compound

This is the part of the page that matters most. Retatrutide is not approved anywhere and is not commercially available. Anything obtainable is a research-supply preparation made by someone with no obligation to match the trial product's identity, purity or concentration, and no regulator has looked at it. The dosing rows above are the amounts used in trials, reproduced so that the numbers people cite can be seen in context — they are not a protocol, and they were administered under trial monitoring that does not exist outside a trial.

The specific risk with a compound at this stage is that the safety profile is still being written. Phase III exists precisely because phase II is not large enough or long enough to find uncommon harms, and using something at that stage means accepting the part of the risk the programme has not yet measured — without the monitoring a trial would provide.

Anyone considering this should be talking to a clinician who can weigh it against approved alternatives that have completed the process, and anyone using it who develops persistent vomiting, dehydration or severe abdominal pain should be seeking care rather than adjusting a schedule.

The curve, the accumulation ratio and the peak-to-trough figures have a page of their own: Retatrutide half-life and steady state.

The dosing rows the app records

Animal-only or theoretical Reproduced from the app’s reference so you can see what it holds, not as a recommendation. Which row applies to a particular person, if any, is a clinical decision this page does not make — and rows describing supraphysiological or post-cycle use are filtered out before this table is built, so what you see here is a subset.

What the app monitors alongside it

The panel below is the app’s own monitoring note for Retatrutide, verbatim. The analytes in it that have a page here are linked; those pages cover what each one measures, which assay produced it and how to read a trend.

• Hematocrit on testosterone therapy
• HbA1c and fasting glucose

Drawbacks and risks the app records

From the app's own entry, and the last item is the operative one: the compound is not commercially available, so nothing on the market is the thing the trials studied.

• Not yet FDA approved — Phase III as of 2024
• All GLP-1 class side effects apply plus glucagon effects
• Muscle loss without testosterone and resistance training
• Nausea and GI effects potentially more pronounced than tirzepatide
• Long-term safety data still accumulating
• Not yet commercially available — research/clinical trial access only

Interaction rules that name Retatrutide

From the app’s interaction data, filtered so no rule naming a compound this site does not publish appears. Not exhaustive, and not a safety clearance: a combination that is not listed is one nobody has documented here, which is not the same as one that is fine. The combination checker has the rest.

Questions people actually ask

Is it better than tirzepatide?

Its phase II weight-loss figure is larger than tirzepatide's phase III figures, and comparing across trials with different populations, durations and designs is weak evidence. No head-to-head trial has reported. "Larger number in a smaller earlier trial" is what the evidence supports.

When will it be approved?

Phase III trials are ongoing. Timelines in the app's own regulatory string are projections rather than decisions, and this page will not add a guess to them.

How long does it stay in the system?

The app models about six days and flags the figure as an estimate. At that half-life a weekly schedule accumulates for roughly a month before it plateaus, and clearance after stopping takes a similar time. The half-life page carries the estimate caveat on every number.

What monitoring would a trial do?

The app's panel reflects it: glycaemic markers, lipids, liver enzymes, and body composition alongside weight. A trial would add scheduled clinical review, adverse-event capture and stopping rules — which is the part that cannot be reproduced outside one.

Where the load-bearing numbers come from

Named rather than linked. Publisher URLs move, and a citation that resolves to a 404 two years from now is worse than one you can search for by name — every entry below is findable from the title and year alone.