Retatrutide and tirzepatide are both Eli Lilly drugs, both once-weekly injections, both designed for weight management and type 2 diabetes. They differ in one molecular detail — retatrutide adds glucagon receptor activity — and one regulatory detail: tirzepatide is FDA-approved, retatrutide isn't.
At a glance
Retatrutide (LY3437943)
- Class
- GLP-1 + GIP + glucagon triple agonist
- FDA status
- Investigational (Phase 3)
- Phase 2 weight loss
- 24.2% @ 48w, 12 mg
- Cost (unsupervised)
- $260+/mo
Tirzepatide (Mounjaro / Zepbound)
- Class
- GLP-1 + GIP dual agonist
- FDA status
- Approved (T2D 2022, obesity 2023)
- Phase 3 weight loss
- 22.5% @ 72w, 15 mg
- Cost (telehealth)
- $249–$399/mo
Mechanism: triple vs dual agonist
Both drugs activate the GLP-1 receptor (appetite suppression, slowed gastric emptying, improved insulin secretion) and the GIP receptor (improved insulin sensitivity, better fat handling, and potentially reduced GLP-1 nausea). Retatrutide adds a third activation: the glucagon receptor (increased energy expenditure, lipolysis, reduced liver fat).
In practice: tirzepatide is excellent at reducing food intake. Retatrutide reduces food intake similarly — and also nudges the body to burn slightly more energy. The net effect across 48 weeks is roughly 1–2 percentage points more weight loss on retatrutide.
Weight loss comparison
| Measure | Retatrutide 12 mg (48w) | Tirzepatide 15 mg (72w) | Difference |
|---|---|---|---|
| Mean weight loss (%) | 24.2% | 22.5% | +1.7 pp |
| ≥15% weight loss | 83% | 85% | ~same |
| ≥25% weight loss | 26% | 36% | tirzepatide higher |
| Blood pressure systolic | −8 mmHg | −7 mmHg | ~same |
| A1C reduction (T2D) | −2.0% | −2.3% | tirzepatide slightly higher |
Note that retatrutide was measured at 48 weeks and tirzepatide at 72 weeks. Retatrutide's Phase 2 curve had not plateaued at 48 weeks, suggesting further loss with longer dosing. Phase 3 will settle the final head-to-head.
Side effects comparison
| Side effect | Retatrutide 12 mg | Tirzepatide 15 mg |
|---|---|---|
| Nausea | 55% | 45% |
| Diarrhea | 32% | 25% |
| Vomiting | 21% | 15% |
| Constipation | 16% | 17% |
| Fatigue | 15% | 10% |
Retatrutide produces modestly more GI side effects at comparable weight-loss-producing doses — likely driven by the glucagon component. Slower titration closes most of the gap.
Switching from tirzepatide to retatrutide dosage
If you're transitioning (under physician supervision), a reasonable approach:
- Complete your current tirzepatide cycle. Last dose then wait 1 week (matches retatrutide weekly dosing interval).
- Start retatrutide at a dose roughly 2 rungs below your tirzepatide maintenance. E.g., if you were on 15 mg tirzepatide, start retatrutide at 4 mg (not 0.5 mg) — your GI tract is already adapted to GLP-1/GIP signaling.
- Escalate retatrutide every 4–6 weeks until you reach a maintenance dose that matches your weight-loss goal.
- Monitor GI tolerability carefully during the first 2–4 weeks — the added glucagon activity is new to your body.
Important: this is general guidance. An individual transition should involve a prescribing physician with access to your medical history.
Access and cost
| Factor | Retatrutide | Tirzepatide |
|---|---|---|
| FDA approval | No (Phase 3) | Yes (2022/2023) |
| Available via pharmacy | No | Yes |
| Telehealth $/mo cash | — | $249–$399 |
| Cash retail (branded) | — | $1,060 (Zepbound) |
| Research peptide $/mo | ~$260 (unsupervised) | n/a (approved) |
| Physician oversight | Only via trial or compounding | Standard with any prescription |
Who should choose which
- If you want to start losing weight now: tirzepatide. FDA-approved, available via telehealth at $249/mo, Phase 3 proven.
- If you're in a Phase 3 trial site: retatrutide via trial enrollment. Free, supervised, highest dose studied.
- If you've plateaued on tirzepatide at 15 mg: wait for retatrutide approval or consider a trial. Adding glucagon receptor activity may help where GLP-1+GIP alone plateau.
- If you've never used a GLP-1: start with tirzepatide, not retatrutide. Known safety, known dosing, broader data.
- If you have tolerability issues with tirzepatide: don't jump to retatrutide (GI side effects are slightly higher). Talk to your prescriber about dose reduction or semaglutide.
The verdict
Retatrutide edges out tirzepatide on raw weight-loss numbers, but the edge is small and the access gap is huge. Tirzepatide is on pharmacy shelves today. Retatrutide is an 18–30 month wait for FDA approval plus an unknown launch-supply period. The rational choice for most people is tirzepatide now, with the option to evaluate retatrutide once it's actually approved and priced.