Retatrutide is the first triple GLP-1/GIP/glucagon receptor agonist to reach large-scale clinical trials, with a plasma half-life of ~6 days enabling once-weekly dosing. Phase 2 data showed 24.2% mean body weight loss at 48 weeks — a record for any obesity pharmacotherapy at time of publication.[1]
Human PK Study · Phase 2 NEJM 2023 ·
Jastreboff et al. PMID 37366315Phase 3 Ongoing
Plasma Half-Life (SC)
~6 days (144 h)
Source: Jastreboff AM et al. N Engl J Med. 2023;389(6):514–526. PMID 37366315
Drug Class
Triple GLP-1R / GIPR / GCGR agonist
Also Known As
LY3437943
Tmax (SC)
1–3 days
Protein Binding
>99% (albumin)
Bioavailability (SC)
Not published (class ~89%)
Steady State
~4–5 weeks (weekly dosing)
5 Half-Lives (Full Clearance)
~30 days
Clearance Mechanism
Proteolytic degradation, minor renal
FDA Status
Phase 3 — NOT approved (May 2026)
Phase 2 Peak Weight Loss
24.2% at 48 weeks (17.5 mg/wk)
Data Quality
Human PK Study (Phase 2)
Investigational Drug Notice: Retatrutide (LY3437943) is not FDA approved as of May 2026. It is available only in Phase 3 clinical trial settings. Do not use without physician supervision. All data on this page are from published Phase 2 trial results and should not be construed as prescribing information.
Mechanism of Action
1. GLP-1 Receptor Agonism — The Foundation
Retatrutide's GLP-1R component functions identically to semaglutide and tirzepatide: it drives glucose-dependent insulin secretion from pancreatic beta cells, delays gastric emptying to reduce post-meal glucose excursions, and activates hypothalamic circuits that suppress appetite and reduce caloric intake. The structural basis for retatrutide's extended half-life is a C18 fatty di-acid side chain — the same albumin-binding architecture used in semaglutide — which creates a depot effect after subcutaneous injection and slows renal clearance, achieving a plasma half-life of approximately 6 days (144 hours) based on Phase 2 human PK data.[1] This extended half-life supports once-weekly dosing without the need for depot formulation technologies.
2. GIP Receptor Co-Agonism — Metabolic Synergy
Retatrutide's GIPR agonism is described as "full agonism" — similar in magnitude to tirzepatide, which established the dual GLP-1R/GIPR class. GIP receptor activation potentiates glucose-stimulated insulin secretion through a cAMP-dependent pathway distinct from GLP-1R signaling, providing additive insulinotropic effects that do not appear to substantially increase hypoglycemia risk. At the adipocyte level, GIPR activation may enhance intracellular lipid mobilization and fatty acid oxidation, contributing to the superior weight loss seen with dual and triple agonists compared to GLP-1 monotherapy. The combination of GLP-1R and GIPR agonism in retatrutide appears to produce synergistic weight loss beyond what either mechanism achieves alone, as seen in preclinical models and reflected in the Phase 2 efficacy results.[1]
3. Glucagon Receptor Agonism — The Differentiating Mechanism
The glucagon receptor (GCGR) component is what sets retatrutide apart from semaglutide and tirzepatide. In the liver, glucagon receptor activation increases hepatic energy expenditure by upregulating fatty acid beta-oxidation, reduces de novo lipogenesis, and promotes export of triglycerides — collectively reducing hepatic steatosis and improving markers of metabolic dysfunction-associated steatohepatitis (MASH). At the whole-body level, GCGR agonism increases basal metabolic rate beyond what is achieved by caloric restriction alone, creating a caloric deficit that adds to the appetite-suppressive effects of GLP-1R agonism. A theoretical concern is that glucagon normally raises blood glucose; however, in the triple-agonist context the GLP-1R component dominates glucose control, suppressing the hyperglycemic counterregulatory effect. Phase 2 data confirmed that fasting glucose was well-controlled at all retatrutide doses, validating this mechanistic rationale.[1]
Model your retatrutide pharmacokinetics
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Understanding retatrutide's pharmacokinetics requires thinking in three nested timeframes that each have distinct clinical relevance:
Layer 1 — Plasma PK
~6-day half-life
Plasma concentration declines by 50% every 6 days. Governs trough levels, dosing interval, and interaction windows. Established from Phase 2 PK sampling (PMID 37366315).
Layer 2 — Full Clearance
~30 days (5 × t½)
Clinically meaningful concentrations persist for ~30 days after last dose. Relevant for washout before surgery, pregnancy, or drug interactions.
Layer 3 — Downstream Metabolic Effects
Days–weeks after clearance
Appetite hormones (GLP-1, ghrelin, PYY), hepatic lipid pathways, and glycemic setpoints normalize slowly after drug clearance. Weight regain typically begins within weeks of discontinuation, not days — a downstream hormonal lag distinct from the pharmacokinetic clearance curve.
Clearance Timeline After Last Dose
The following table assumes a single terminal dose. Each row represents one additional half-life (~6 days) of clearance. At 5 half-lives (~30 days), plasma concentration is below 3% of peak — considered clinically negligible.
Days After Last Dose
Half-Lives Elapsed
Approximate % Remaining
Clinical Note
6 days
1
~50%
Next scheduled dose window; full GI effects persist
12 days
2
~25%
Partial activity; appetite suppression still present
18 days
3
~12.5%
Reduced efficacy; GI side effects mostly resolved
24 days
4
~6%
Clinically sub-therapeutic; weight regain may begin
30 days
5
~3%
Effectively cleared — standard washout period
Administration Routes
Route
Available
Human Data
Notes
SC injection (weekly)
Yes
Phase 2 RCT
Established route; all Phase 3 trials use SC weekly
IV
Investigational
Limited
Used in early PK studies only; not a clinical route
IM
No
None
No published data; not under investigation
Oral
No
None
No oral formulation in development; SC remains the only route
In-Class Pharmacokinetic Comparison
Retatrutide sits within the GLP-1 agonist class but occupies a distinct mechanistic niche due to GCGR co-agonism. The table below compares pharmacokinetically similar compounds:
The landmark Phase 2 trial by Jastreboff et al. enrolled 338 adults with obesity (BMI ≥27 kg/m²) without diabetes and randomized them to retatrutide or placebo for 48 weeks.[1] Key findings by dose group at Week 48:
Dose (SC weekly)
Mean % Weight Loss
% Achieving ≥10% Loss
% Achieving ≥15% Loss
Placebo
~2.1%
16%
5%
1 mg
~8.7%
45%
27%
4 mg
~17.3%
82%
64%
8 mg
~22.8%
92%
83%
12 mg (max tested)
~24.2%
—
~90%
Note: The 17.5 mg dose group (added in a protocol amendment) also achieved ~24.2% weight loss. Figures approximate from published data (PMID 37366315). Phase 3 uses tiered dose escalation schedules; final efficacy data pending.
Data Quality
Parameter
Source
Evidence Level
Half-life, Tmax, PK parameters
Jastreboff AM et al. NEJM 2023 (PMID 37366315)
Human PK Study — Phase 2 RCT
Protein binding / albumin mechanism
Structural analogy to semaglutide (C18 fatty acid); class inference
Mechanistic extrapolation
FDA status / Phase 3
ClinicalTrials.gov; Eli Lilly public disclosures
Regulatory / sponsor communication
Safety Notes
BOXED WARNING (GLP-1 drug class): GLP-1 receptor agonists including retatrutide carry a class boxed warning for thyroid C-cell tumors based on rodent carcinogenicity studies. Causality in humans has not been established. Retatrutide should not be used in patients with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN2).
The most common adverse events observed in Phase 2 were gastrointestinal in nature — nausea (up to 42%), vomiting, and diarrhea — consistent with the GLP-1 drug class. These events were dose-dependent and typically peaked during dose escalation, then attenuated with continued use. At higher doses (≥8 mg), GI burden appeared more pronounced than historically reported for semaglutide 2.4 mg, though cross-trial comparisons should be interpreted cautiously.[1]
A modest, dose-dependent elevation in resting pulse rate was noted across retatrutide groups in Phase 2, consistent with effects observed with other GLP-1 and dual agonists. The clinical significance of this finding at the Phase 3 scale and in patients with pre-existing cardiovascular disease is under evaluation. Retatrutide is investigational — do not use outside clinical trial settings and consult a physician before considering any GLP-1 class agent.
Halflife Labs Medical Review Team
Last reviewed: May 2026 · Evidence level: Human PK Study (Phase 2) · Primary citation: Jastreboff AM et al. N Engl J Med. 2023;389(6):514–526. PMID 37366315
This page summarizes published pharmacokinetic and clinical data for informational purposes only. It does not constitute medical advice or a recommendation to use retatrutide, which is not FDA approved.
Frequently Asked Questions
What is the half-life of retatrutide?
Retatrutide has a plasma half-life of approximately 6 days (144 hours) via subcutaneous injection, based on Phase 2 human pharmacokinetic data from Jastreboff et al. published in the New England Journal of Medicine in 2023 (PMID 37366315). This half-life enables once-weekly dosing and is achieved through a C18 fatty acid side chain that promotes albumin binding — the same strategy used in semaglutide.
How does retatrutide differ from tirzepatide?
Retatrutide adds glucagon receptor (GCGR) agonism to the dual GLP-1R/GIPR mechanism shared with tirzepatide. This additional receptor target increases hepatic energy expenditure and promotes fatty acid oxidation beyond what dual agonism achieves. In Phase 2 trials, retatrutide at 12–17.5 mg/week produced approximately 24.2% mean body weight loss at 48 weeks, compared to approximately 21% for tirzepatide 15 mg at 72 weeks — though cross-trial comparisons carry significant caveats due to different populations, follow-up durations, and study designs.
Is retatrutide FDA approved?
No. As of May 2026, retatrutide (LY3437943) is not FDA approved for any indication. It is being evaluated in Phase 3 clinical trials for obesity, type 2 diabetes, and MASH (metabolic dysfunction-associated steatohepatitis). It remains an investigational drug available only within clinical trial settings.
How much weight loss does retatrutide cause?
In the Phase 2 randomized controlled trial (Jastreboff et al., NEJM 2023, PMID 37366315), participants receiving retatrutide 17.5 mg/week achieved a mean body weight reduction of 24.2% at 48 weeks — the highest weight loss reported for any pharmacological agent in a Phase 2 obesity trial at the time of publication. The 12 mg/week group also achieved approximately 24.2%. Lower doses produced progressively less weight loss: approximately 17.3% at 4 mg and 22.8% at 8 mg.
Why does retatrutide agonize the glucagon receptor?
Glucagon receptor (GCGR) activation increases hepatic energy expenditure by promoting fatty acid beta-oxidation, creating an additional caloric deficit beyond the appetite suppression and gastric emptying delay from GLP-1R agonism. Although glucagon normally raises blood glucose, the GLP-1R component of retatrutide dominates glycemic control and suppresses this counterregulatory hyperglycemic effect — making GCGR agonism metabolically "safe" within the triple-agonist framework. Phase 2 data confirmed that fasting glucose was well-controlled across all retatrutide doses.[1]
When will retatrutide reach steady state?
With once-weekly subcutaneous dosing, retatrutide reaches pharmacokinetic steady state in approximately 4–5 weeks — corresponding to five times the ~6-day half-life (~30 days). At steady state, peak and trough plasma concentrations stabilize, and clinical effects (weight loss, glycemic control) are expected to be maximal and consistent week-to-week. Dose escalation is typically performed gradually over this period to minimize GI adverse events.
What are the side effects of retatrutide?
In Phase 2, the most common adverse events were GLP-1 class GI effects: nausea (up to ~42% at highest doses), vomiting, diarrhea, and constipation. These were dose-dependent and most prominent during dose escalation. A modest pulse rate elevation was observed, consistent with dual and triple agonists generally. The GLP-1 drug class carries a boxed warning for thyroid C-cell tumors based on rodent data; retatrutide is contraindicated in patients with MEN2 or personal/family history of medullary thyroid carcinoma. Retatrutide is investigational — consult a physician.[1]
Does retatrutide treat fatty liver disease?
The glucagon receptor component of retatrutide promotes hepatic fatty acid oxidation, reduces de novo lipogenesis, and may improve MASH (metabolic dysfunction-associated steatohepatitis). Phase 2 data showed significant reductions in liver fat content assessed by MRI-PDFF. A dedicated MASH indication is under active investigation in Phase 3 trials separate from the obesity program. No MASH-specific approval has been granted as of May 2026.
Track retatrutide clearance in real time
The Halflife app plots your complete plasma concentration curve — see exactly when your dose peaks, when it troughs, and how long until clinical washout. Free iOS app, all data stored on-device.