Retatrutide Dosage in Clinical Trials: What Was Tested and Why

Retatrutide doses in Lilly's trials: 2 mg start, raised every 4 weeks to 4, 9 or 12 mg weekly. Weight loss and side effects by dose. Not a dosing guide.

Written by Olha Orlyk. Updated October 22, 2026.

In its Phase 3 trials, retatrutide was given as a once-weekly injection at three maintenance doses: 4 mg, 9 mg and 12 mg. Everyone started at 2 mg, and the dose was raised every four weeks until it reached the assigned level, so the 12 mg group needed 16 weeks to get there. The higher the dose, the more weight people lost, and the more often they had nausea and vomiting.

This page explains the doses used in Lilly's trials and why they were chosen. It isn't a dosing guide. Retatrutide is not approved, there is no official dose for the public, and the products sold online as "research" retatrutide are not the trial drug.

Retatrutide Doses in Phase 3 (TRIUMPH)

How doses were escalated in TRIUMPH-1

Assigned dose

Escalation steps (every 4 weeks)

Weeks to reach full dose

4 mg

2 mg → 4 mg

9 mg

2 → 4 → 6 → 9 mg

12 mg

2 → 4 → 6 → 9 → 12 mg

Starting low and stepping up slowly is the standard approach for this drug family. Semaglutide and tirzepatide are started the same way. The goal is to let the stomach adapt and keep nausea and vomiting manageable.

How Much Weight Was Lost at Each Dose

TRIUMPH-1, 80 weeks, adults with obesity and no diabetes

Dose

Average weight loss (efficacy estimand)

Stricter published analysis

Lost at least 30%

4 mg

19.0%

17.6%

15.3%

9 mg

25.9%

23.7%

37.9%

12 mg

28.3%

25.0%

45.3%

Placebo

2.2%

3.9%

0.5%

The jump from 4 mg to 9 mg added much more weight loss than the jump from 9 mg to 12 mg. That pattern is common with drugs like this: benefits flatten at the top while side effects keep rising. It's why Lilly tested three doses rather than only the highest. Full details in retatrutide weight loss results.

Side Effects Rise With the Dose

Selected side effects by dose in TRIUMPH-1

Side effect

4 mg

9 mg

12 mg

Placebo

Nausea

28.6%

38.4%

42.4%

14.8%

Vomiting

10.6%

22.8%

25.3%

4.8%

Dysesthesia (skin sensitivity)

5.1%

12.3%

12.5%

0.9%

Stopped due to side effects

4.1%

6.9%

11.3%

4.9%

Vomiting more than doubled between 4 mg and 9 mg, and nearly three times as many people stopped treatment on 12 mg as on 4 mg. That trade-off is the main reason the eventual label, if approved, is likely to offer more than one maintenance dose. More on our side-effects page.

What the Phase 2 Trial Taught About Dosing

The 2023 Phase 2 study tested 1 mg, 4 mg, 8 mg and 12 mg over 48 weeks, with some groups starting at 2 mg and others jumping straight to 4 mg. Average weight loss was about 8.7% on 1 mg, 17.1% on 4 mg, 22.8% on 8 mg and 24.2% on 12 mg, versus 2.1% on placebo. Starting at 2 mg rather than 4 mg reduced stomach side effects, which is why Phase 3 begins at 2 mg for everyone.

Why Once a Week?

Retatrutide is a peptide with a fatty-acid chain that makes it bind to albumin in the blood. That slows its clearance so that it lasts about six days, long enough for a weekly injection, the same schedule as Ozempic, Wegovy and Zepbound.

Doses in Other Trials

1TRIUMPH-2 (type 2 diabetes): 4 mg, 9 mg and 12 mg for 80 weeks; up to 20.8% weight loss.
2TRIUMPH-3 (obesity with heart disease): 9 mg and 12 mg for 80 weeks; up to 22.6%.
3TRIUMPH-4 (knee osteoarthritis): up to 12 mg for 68 weeks; up to 28.7%.
4TRIUMPH-1 extension: people with a starting BMI of 35 or more went up to their maximum tolerated dose, 9 or 12 mg, for 104 weeks in total.

Why You Shouldn't Copy Trial Doses

Trial doses describe a specific, quality-controlled product given under medical supervision, with regular checks and a clear plan for side effects. None of that applies to vials sold online as retatrutide, "reta" or "GLP-3." Their actual content can differ from the label, and dosing errors with injectable weight-loss drugs have sent people to hospital. FDA says retatrutide can't legally be compounded. If someone has taken an unverified product and feels unwell, see our overdose page.

Bottom Line

In Lilly's trials, retatrutide was started at 2 mg and raised every four weeks to 4, 9 or 12 mg once weekly. Weight loss grew with the dose, from about 19% to 28% over 80 weeks, but so did nausea, vomiting and dropouts. These are research doses for an unapproved drug, not instructions. For where approval stands, see our approval watch, and for what the drug is, what is retatrutide.