Tirzepatide (LY3298176) and retatrutide (LY3437943) come from the same developer and the same design lineage. Tirzepatide activates GLP-1 and GIP receptors. Retatrutide activates both of those plus the glucagon receptor. Because they overlap on two of three targets, comparing them isolates one variable in a way that comparing either against semaglutide cannot.
Mechanism: The Glucagon Arm Is the Only Difference
Tirzepatide is a dual agonist. The GLP-1 arm drives glucose-dependent insulin secretion, suppresses glucagon release after meals, slows gastric emptying, and acts on hypothalamic appetite centers. The GIP arm adds a second insulinotropic signal and appears to improve the tolerability of GLP-1 activity, which is part of why tirzepatide reaches higher effective exposure than a GLP-1 mono-agonist without proportionally worse nausea.
Retatrutide keeps both of those arms and adds glucagon receptor agonism. That third arm works on the opposite side of the energy balance equation: glucagon receptor activity increases hepatic glucose output and raises resting energy expenditure. Tirzepatide reduces how much you eat. Retatrutide reduces how much you eat and increases how much you burn.
It sounds contradictory to give a compound that both lowers and raises blood glucose. In practice the GLP-1 and GIP arms dominate glycemic control while the glucagon arm contributes thermogenesis, though the balance can shift early in treatment before the incretin arms are fully established.
Dose Ladders: Different Shapes, Similar Ceilings
Both are dosed once weekly by subcutaneous injection, and both top out in the low double digits of milligrams. The shape of the ladder differs: tirzepatide climbs in fixed 2.5 mg steps across six rungs, while retatrutide roughly doubles at each of five rungs.
- Tirzepatide: 2.5 mg start, then 5, 7.5, 10, 12.5, and 15 mg once weekly, holding each step at least 4 weeks
- Retatrutide: 1 mg start, then 2, 4, 8, and 12 mg once weekly, holding each step at least 4 weeks
- Tirzepatide dose range spans 6-fold from start to ceiling (2.5 to 15 mg)
- Retatrutide dose range spans 12-fold from start to ceiling (1 to 12 mg)
- Half-life: roughly 5 days for tirzepatide, roughly 6 days for retatrutide, both comfortably supporting weekly injection
The 12-fold retatrutide span is the practical headache. A concentration that makes a 1 mg starting dose readable will make a 12 mg dose exceed one syringe, so you will change concentration partway through a cycle. Tirzepatide titrates within a narrower band, so a single concentration can often carry the whole ladder.
Reconstitution: Pick Concentration for the Dose You Are On
Tirzepatide ships in 10, 15, 20, and 30 mg vials. Retatrutide is most commonly sold in 10 mg vials. Both are lyophilized powders reconstituted with bacteriostatic water, and both dissolve readily with gentle swirling.
For tirzepatide, a 30 mg vial with 1.5 mL of BAC water gives 20 mg/mL, which puts every rung of the ladder on a clean mark: 2.5 mg is 12.5 units, 5 mg is 25 units, 10 mg is 50 units, and 15 mg is 75 units. If you are only in the first two steps, a 10 mg vial with 1 mL gives 10 mg/mL, where 2.5 mg is 25 units and 5 mg is 50 units.
For retatrutide, a 10 mg vial with 2 mL gives 5 mg/mL, which suits the bottom of the ladder: 1 mg is 20 units, 2 mg is 40 units, 4 mg is 80 units. At 8 mg that same concentration demands 1.6 mL, past the capacity of a 1 mL syringe, so the standard move is to switch to 10 mg/mL (10 mg vial with 1 mL BAC water) once you pass 4 mg. Even then, 12 mg lands at 1.2 mL and needs either a larger syringe or a split injection.
- Tirzepatide 30 mg vial + 1.5 mL BAC water = 20 mg/mL (2.5 mg = 12.5 units, 15 mg = 75 units)
- Tirzepatide 10 mg vial + 1 mL BAC water = 10 mg/mL (2.5 mg = 25 units, 5 mg = 50 units)
- Retatrutide 10 mg vial + 2 mL BAC water = 5 mg/mL (1 mg = 20 units, 4 mg = 80 units)
- Retatrutide 10 mg vial + 1 mL BAC water = 10 mg/mL (8 mg = 80 units, 12 mg = 120 units, over one syringe)
Confirm the unit mark in the PeptiTools reconstitution calculator each time you open a new vial or change concentration mid-cycle. Enter vial size in mg, BAC water volume in mL, and target dose, and it returns the exact draw with a live syringe diagram.
Titration Side by Side
Both protocols exist for the same reason: gastrointestinal side effects are dose-dependent and concentrated in the first weeks at any new dose. Neither ladder rewards climbing fast.
- Tirzepatide: 2.5 mg for weeks 1-4, then increase by 2.5 mg every 4 weeks as tolerated, up to 15 mg from week 21 onward. The 2.5 mg dose is an initiation dose, not a maintenance dose.
- Retatrutide: 1 mg for weeks 1-4, 2 mg for weeks 5-8, 4 mg for weeks 9-12, 8 mg for weeks 13-16, then 12 mg from week 17 onward.
- Either compound: if nausea, vomiting, or diarrhea is intolerable at a step, hold there an extra 4 weeks rather than advancing. Dropping back one rung is also reasonable.
- Retatrutide only: check fasting glucose and resting heart rate during the first 8 weeks. The glucagon arm can transiently raise both, and that effect is absent with tirzepatide.
- Either compound: keep the injection on the same weekday. If you miss a dose, both tolerate a shift of a few days given their multi-day half-lives, but do not double up.
What the Trial Data Actually Shows
SURMOUNT-1 tested tirzepatide in adults with obesity or overweight without diabetes and reported mean body weight reductions of about 15.0% at 5 mg, 19.5% at 10 mg, and 20.9% at 15 mg over 72 weeks. Tirzepatide also has head-to-head data against semaglutide in type 2 diabetes (SURPASS-2), where every tirzepatide dose beat 1 mg semaglutide on both HbA1c and weight.
The Phase 2 retatrutide obesity trial reported 17.5% at 4 mg, 22.8% at 8 mg, and 24.2% at 12 mg over 48 weeks. The 24.2% figure came in 24 weeks earlier than tirzepatide reached 20.9%, and the retatrutide curve had not clearly plateaued at 48 weeks.
Read that gap carefully. These were separate trials with different durations, enrollment criteria, and analysis estimands, and there has never been a randomized head-to-head of tirzepatide against retatrutide. The comparison suggests the glucagon arm adds real effect, and the mechanism gives a plausible reason why, but it does not measure the size of that addition.
Regulatory Status and What Is Actually in the Vial
Tirzepatide is FDA-approved as Mounjaro for type 2 diabetes and Zepbound for chronic weight management, with published prescribing information, verified manufacturing, and a growing post-marketing safety record. Retatrutide has no approved indication in any jurisdiction. It is in Phase 3 trials and is sold outside that context only as a research chemical, where purity, actual milligram content, and sterility are unverified by any regulator.
That difference outweighs the percentage point gap in trial weight loss. With approved tirzepatide you know the dose in the vial. With research retatrutide you are stacking uncertainty about vial content on top of uncertainty about the long-term safety of chronic glucagon receptor agonism, which has no multi-year human dataset.
Side Effect Profiles: Same Core, One Addition
Nausea, vomiting, diarrhea, constipation, and reduced appetite dominate both profiles, are dose-dependent, and fade at a stable dose. Both can cause injection site reactions, and both carry the incretin class concerns around gallbladder events and pancreatitis, plus the rodent thyroid C-cell tumor signal that puts a contraindication on personal or family history of medullary thyroid carcinoma or MEN 2.
Retatrutide adds two things tirzepatide does not have: transient elevation in fasting glucose from hepatic glucose output, and a modest heart rate increase seen in some Phase 2 participants. Neither is a reason to rule the compound out, but both are reasons to monitor if you have diabetes, prediabetes, or any cardiac history.
Storage and Handling: Identical
Both are stable as lyophilized powder at room temperature before mixing, and both keep for up to 28 days refrigerated at 2-8°C after reconstitution. Do not freeze either once mixed, do not shake, and discard any vial that turns cloudy, discolors, or develops particulates. See the peptide storage guide for handling detail and the injection sites guide for rotation.
Which Comparison Answers Your Question
If you want to know what the glucagon arm buys, this is the right comparison, because everything else is held constant. If you want to know what two generations of development bought over a GLP-1 mono-agonist, compare retatrutide against semaglutide instead. And if you are deciding between what is approved and prescribable today, semaglutide vs tirzepatide is the practical one.