Evidence-Based Comparison

Retatrutide vs Tirzepatide (Zepbound / Mounjaro): Full Comparison

How retatrutide and tirzepatide differ in mechanism, dosing, clinical trial weight-loss results, side effects, and practical decision points.

Note: Retatrutide is still in clinical development; tirzepatide is approved in multiple markets (as Zepbound for chronic weight management in the US and as Mounjaro for type 2 diabetes in the UK/EU). Products sold for research purposes only. Read our research disclaimer

Last updated: February 28, 2026

Key Takeaways

Tirzepatide is a dual agonist: GLP-1 + GIP.

NEJM SURMOUNT-1

Retatrutide is a triple agonist: GLP-1 + GIP + glucagon receptor.

NEJM Phase 2

Retatrutide reported up to 24.2% mean weight reduction at 48 weeks (12 mg).

NEJM Phase 2

Tirzepatide reported up to 22.5% mean weight reduction at 72 weeks (15 mg) in SURMOUNT-1.

NEJM SURMOUNT-1

Both commonly cause gastrointestinal side effects (nausea, vomiting, diarrhea), especially during dose escalation.

NEJM Phase 2

Main Difference in Plain English

Retatrutide adds a third pathway. Tirzepatide targets appetite and glucose control through GLP-1 and GIP. Retatrutide targets the same two receptors plus the glucagon receptor, which researchers associate with higher energy expenditure (calories burned).

That extra receptor is the core reason these drugs can behave differently in trials.

Source: NEJM Phase 2 Trial

How They Work

Dual Agonist

Tirzepatide

GLP-1 + GIP

Tirzepatide activates two gut-hormone receptors that influence appetite and insulin response:

>GLP-1 slows gastric emptying and reduces appetite.
>GIP improves insulin secretion in response to meals and may support additional weight loss effects.

Source: FDA

Triple Agonist

Retatrutide

GLP-1 + GIP + Glucagon

Retatrutide activates GLP-1 and GIP and the glucagon receptor.

The glucagon pathway can increase energy expenditure and shift substrate use (for example, fat oxidation), which may contribute to greater average weight reduction in early studies.

Source: NEJM Phase 2

Clinical Results Compared

Retatrutide (48-week Phase 2)

24.2%

mean weight reduction (highest dose)

In adults with obesity, the Phase 2 trial reported dose-dependent weight reductions at 48 weeks, with the highest dose showing 24.2% mean weight reduction.

NEJM Phase 2 Trial

Tirzepatide (72-week SURMOUNT-1)

22.5%

mean weight reduction (15 mg dose)

In SURMOUNT-1, tirzepatide produced 16.0% to 22.5% weight loss depending on dose (5 mg, 10 mg, 15 mg) over 72 weeks, with the highest dose reaching 22.5%.

NEJM SURMOUNT-1

Cross-trial context: These figures come from different trials with different durations (48 vs 72 weeks), populations, and protocols. Use them to understand ranges, not to claim one drug "wins" in every case.

Dosing Schedules Compared

Retatrutide Dosing

2 mgonce weekly for 4 weeks

4 mgonce weekly for 4 weeks

8 mgonce weekly (maintenance)

12 mgonce weekly (highest recommended)

Full Dosing Guide

Tirzepatide Dosing

Starts low and increases every 4 weeks to reduce GI side effects.

2.5 mgonce weekly (starting)

5 mgonce weekly (after 4 weeks)

+2.5 mg stepsevery 4+ weeks

15 mgonce weekly (label max)

Zepbound Prescribing Information (FDA)

Side Effects and Tolerability

What They Share

Both drugs commonly produce gastrointestinal adverse effects, especially during titration:

Nausea

Vomiting

Diarrhea

Constipation

Reduced appetite

How They Tend to Differ

Retatrutide

Phase 2 paper reports gastrointestinal events that were dose-related and often mild to moderate, with mitigation from lower starting doses and titration.

Source: NEJM

Tirzepatide

Labels emphasize gradual escalation to lower gastrointestinal risk and define maintenance doses (5/10/15 mg).

Source: FDA Label

Common question: "Will side effects be worse at higher doses?" Yes -- trial data and labels consistently show a dose/titration relationship for GI symptoms. That is why both protocols step up over weeks, not days.

Which One Is "Stronger" for Weight Loss?

Trial ranges suggest both can reach ~20%+ average weight reduction at higher doses, with retatrutide showing a higher top-line mean in a 48-week Phase 2 study and tirzepatide showing up to 22.5% over 72 weeks in SURMOUNT-1.

The mechanistic reason retatrutide can trend higher is the added glucagon receptor pathway (energy expenditure). However, the fair conclusion requires head-to-head studies.

NEJM Phase 2 NEJM SURMOUNT-1

Quick Comparison Table

Category	Retatrutide	Tirzepatide (Zepbound / Mounjaro)
Targets	GLP-1 + GIP + glucagon receptor	GLP-1 + GIP
Trial duration (headline)	48 weeks	72 weeks
Top mean weight reduction	24.2% (highest dose)	22.5% (15 mg)
Typical titration	2 mg > 4 mg > 8 mg > 12 mg	2.5 mg > 5 mg > +2.5 mg steps to 10/15 mg
Max dose	12 mg (recommended)	15 mg (label max)
Common adverse effects	GI effects, dose-related	GI effects; gradual escalation advised

How to Choose Between Them

People usually compare these options based on three practical factors:

Mechanism preference

Choose tirzepatide for a dual incretin approach; consider retatrutide if the triple pathway (including glucagon receptor) aligns with goals around energy expenditure.