"Switching from tirzepatide to retatrutide" is searched as though the answer were a conversion. There is none, and the reason is specific: retatrutide has no label, and Eli Lilly's retatrutide trials were built to keep out anyone who had recently taken a weight-loss drug. So the one population the search is about is the one population the trial record deliberately excluded. This page sets out what the record does contain — the trials' starting doses, the two drugs' half-lives, the one Lilly programme written for people coming off maximum approved therapy — and converts the steps to syringe units. It describes no switch.
The dose figures are on the retatrutide dosage page and the tirzepatide dosage page; the compounds are covered in Medibact's Retatrutide guide and Tirzepatide guide.
Who the retatrutide trials let in
The exclusion that matters is written the same way in each phase 3 record read on 2026-09-25: participants must not "have taken weight loss drugs, including over-the-counter medications, within 90 days prior to screening".
| Trial | Registry | Enrolled | Prior weight-loss drug rule |
|---|---|---|---|
| TRIUMPH-1 (obesity, 80 weeks) | NCT05929066 | 2,335 | Excluded within 90 days |
| TRIUMPH-2 (type 2 diabetes with obesity) | NCT05929079 | 1,152 | Excluded within 90 days |
| TRIUMPH-3 (obesity with cardiovascular disease) | NCT05882045 | 1,946 | Excluded within 90 days |
| Head-to-head against tirzepatide | NCT06662383 | 800 | Excluded within 90 days |
| Phase 3b: maintenance, dose-escalation schemes, placebo-controlled | NCT06859268, NCT07357415, NCT07232719 | 643, 600, 250 | Excluded within 90 days |
| Phase 2 obesity (48 weeks) | NCT04881760 | 338 | "must not be using medications that promote weight loss" |
At a 5-day half-life, 90 days is eighteen half-lives: a participant entering those trials had, by arithmetic, no measurable tirzepatide left. The headline retatrutide results — the -24.2% at 48 weeks in phase 2, the -28.3% at 80 weeks in TRIUMPH-1 — come from people starting from zero, not from people arriving on a GLP-1 medicine.
A PubMed search on 2026-09-25 for retatrutide and tirzepatide with switch, transition or convert in the title or abstract returned three records, all reviews. No case series, no pharmacokinetic study of the overlap, no equivalence model was found. For semaglutide to tirzepatide at least a 15-person series and a manufacturer-authored equivalence model exist, set out on switching from semaglutide to tirzepatide; for this move, nothing comparable exists.
The head-to-head trial, still running
NCT06662383 randomises adults with obesity to retatrutide or tirzepatide for about 89 weeks. It started on 1 November 2024, has 800 participants, and gives its primary completion as November 2026 (estimated). No results were posted on 2026-09-25. Like SURMOUNT-5 for tirzepatide against semaglutide, it compares people who start one drug with people who start the other; nobody in it switches.
The one programme written for people on maximum therapy
In June 2026 Lilly registered single-patient expanded access to retatrutide (NCT07629401, first posted 2026-06-05). Its inclusion criteria are the closest the record comes to the switching population: adults with a BMI of 35 or more, two or more serious or life-threatening obesity-related complications, obesity that persists "despite adherence to and tolerance of treatment with highest available dose of chronic weight management therapy", and no reachable trial. Tirzepatide at 15 mg is one such highest available dose. The record lists no starting dose, no interval and no protocol; expanded access is requested by a treating physician case by case.
The overlap arithmetic
The two molecules clear on similar clocks. ZEPBOUND's label gives tirzepatide about 5 to 6 days (MOUNJARO's, about 5), and the phase 1b retatrutide trial reported about 6 days (Urva et al., The Lancet 2022, PMID 36354040).
| Days after the last tirzepatide dose | Tirzepatide remaining, 5-day half-life | At 6 days |
|---|---|---|
| 7 | 37.9% | 44.5% |
| 14 | 14.4% | 19.8% |
| 21 | 5.4% | 8.8% |
| 28 | 2.1% | 3.9% |
Any first retatrutide dose inside that window adds to tirzepatide still circulating, and both act on the GLP-1 and GIP receptors; retatrutide adds the glucagon receptor. That is arithmetic from the two stated half-lives. No study has measured the combined level, and the half-life figures for both drugs are worked further on tirzepatide's half-life page and retatrutide's half-life page.
The retatrutide steps as units
Units on a U-100 syringe = mg ÷ mg/mL × 100. The steps below are the ones Lilly's releases describe for the TRIUMPH trials, which started at 2 mg and rose every four weeks.
| Retatrutide | 5 mg/mL | 10 mg/mL | 20 mg/mL |
|---|---|---|---|
| 2 mg | 40 units | 20 units | 10 units |
| 4 mg | 80 units | 40 units | 20 units |
| 6 mg | 120 units | 60 units | 30 units |
| 9 mg | 180 units | 90 units | 45 units |
| 12 mg | 240 units | 120 units | 60 units |
The units error this move invites is carrying a number across vials. Tirzepatide's approved single-dose pens and vials put every strength in 0.5 mL, 50 units; a research tirzepatide vial at 10 mg/mL puts 5 mg in 50 units; the same 50 units from a retatrutide vial at 10 mg/mL is 5 mg of a different molecule with a different dose ladder. The mg to units page sets out the formula and the tenfold misreadings, and the calculator's retatrutide setting works any vial.
What is not established
No label, trial, case series or pharmacokinetic study covers moving from tirzepatide to retatrutide; the trials' 90-day rule means no published retatrutide result applies directly to someone arriving on tirzepatide; no dose equivalence between the two exists in the record. The head-to-head trial has not reported. Nothing on this page is a starting dose, an interval or a plan; retatrutide is unapproved, and those decisions belong with a physician.
Sources and dates
Opened 2026-09-25: ClinicalTrials.gov v2 records for NCT05929066, NCT05929079, NCT05882045, NCT06662383, NCT06859268, NCT07357415, NCT07232719, NCT04881760 (eligibility criteria) and NCT07629401 (expanded access, inclusion criteria); the PubMed search described above via NCBI E-utilities; the ZEPBOUND and MOUNJARO labels via openFDA (manufacturer copies, effective 2026-08-28 and 2026-08-27), sections 2 and 12.3; the Urva et al. abstract (The Lancet 2022, PMID 36354040). Lilly's TRIUMPH figures and escalation as cited on this site's retatrutide dosage page (releases of 2026-05-21 and 2026-07-23). Remaining shares and units are arithmetic by this site. Corrections go to the contact page.