Retatrutide
LY3437943
Retatrutide (LY3437943) is an investigational once-weekly injectable peptide that simultaneously activates three gut/metabolic hormone receptors — GIP, GLP-1, and glucagon (a "triple agonist"). It is being developed by Eli Lilly for obesity, type 2 diabetes, and fatty liver disease, and in Phase 2 trials produced very large weight loss (up to ~24% of body weight at 48 weeks at the 12 mg dose) and marked reductions in blood sugar and liver fat. As of this writing it is not an approved drug: it has completed only Phase 2 human trials, so long-term safety and cardiovascular-outcome data do not yet exist. The main risks reported so far are dose-related gastrointestinal effects (nausea, vomiting, diarrhea, constipation) that can cause dehydration, a dose-dependent rise in heart rate, and the general risks of any GLP-1-class agent (dehydration-related acute kidney injury, gallbladder disease, pancreatitis, and, because it slows stomach emptying, a documented anesthesia/aspiration risk around surgery). Because it is only available through unregulated channels outside of clinical trials, purity, dosing accuracy, and sterility are unverifiable, compounding every risk below. Anyone using or considering it should be under the care of a physician with regular bloodwork.
Mechanism of action
Pharmacokinetics
Approximately 6 days in humans, per Phase 1/2 data; pharmacokinetics are dose-proportional.
Long-acting: a single dose reduced body weight measurably out to ~day 43 in Phase 1, and the ~6-day half-life supports once-weekly subcutaneous dosing.
Subcutaneous injection, once weekly.
As a fatty-diacid-conjugated peptide it is expected to be cleared by proteolytic (peptide) catabolism into amino acids/small peptides rather than by hepatic CYP metabolism, consistent with the GLP-1 analog class; dedicated human renal/hepatic-impairment clearance studies were not identified in the primary literature retrieved. This PK profile is documented for monitoring and washout reasoning (e.g., that meaningful drug exposure persists for weeks after the last dose, which is relevant to peri-operative gastric-emptying/aspiration risk and to timing clinician discussions), not for any test-evasion purpose.
For monitoring and washout planning, not drug-test evasion.
Physiological & performance effects
- Substantial dose-dependent body-weight reduction in adults with obesity: least-squares mean −17.5% (12 mg) at 24 weeks and up to −24.2% at 48 weeks vs −2.1% placebo (Phase 2).
- Large reductions in HbA1c in type 2 diabetes (up to ~−2.0% at 24 weeks) with dose-dependent weight loss up to ~−16.9% at 36 weeks (Phase 2).
- Marked reduction in liver fat in participants with metabolic dysfunction-associated steatotic liver disease (MASLD): relative liver-fat reduction up to ~−82% at 24 weeks and ~−86% at 48 weeks, with resolution of steatosis (<5% liver fat) in >85% of the two highest-dose groups.
- Reported improvements in blood pressure, fasting lipids (including triglycerides), and markers of insulin sensitivity in the obesity program.
- Dose-related increase in beta-hydroxybutyrate (a ketone/fatty-acid-oxidation marker), not associated with ketoacidosis in trials.
- These are trial-reported physiological effects, not endorsements; efficacy figures are the doses/ranges as studied in Phase 2 and are always paired with the adverse effects and monitoring below.
Adverse effects by system
Dose-dependent increase in heart rate, which peaked around week 24 and declined thereafter in the Phase 2 obesity trial. Blood pressure and lipids generally improved. No long-term cardiovascular-outcome trial data exist yet, so effects on major adverse cardiac events are unknown.
No hepatotoxicity signal was observed through 48 weeks in the obesity trial or the MASLD substudy; ALT/AST did not worsen and liver fat fell substantially. Patients with advanced fibrosis or cirrhosis were not studied, so safety in those groups is unknown.
Retatrutide is not an androgen or anabolic-androgenic steroid and has no described mechanism of direct hypothalamic-pituitary-testicular (HPTA) suppression; no HPTA-suppression data were identified. It does affect glucose/insulin regulation and carries the class-typical low intrinsic risk of hypoglycemia (higher if combined with insulin or sulfonylureas). Marked weight loss can indirectly change sex-hormone levels, but this is not drug-specific androgen suppression.
No human reproductive or fertility data were identified. As with GLP-1-class agents, use in pregnancy is not recommended given lack of safety data and the metabolic demands of weight loss; effective contraception and clinician discussion are appropriate. This is a data-absence, not evidence of safety.
No specific psychiatric signal was highlighted in the retatrutide Phase 2 reports. The GLP-1 drug class is under ongoing regulatory monitoring for mood changes and suicidal ideation as a general precaution; retatrutide-specific data are lacking, so this is an area of uncertainty rather than a documented effect.
No direct nephrotoxicity was reported, but the class carries a well-recognized risk of dehydration from vomiting/diarrhea leading to pre-renal acute kidney injury; adequate hydration and dose caution are warranted. Dedicated renal-impairment data for retatrutide were not identified.
No clinically significant hematologic effects were identified in the primary trial literature; this reflects absence of reported data rather than proven absence of any effect.
Injection-site reactions can occur with subcutaneous peptide injectables as a class; no distinctive dermatologic toxicity was emphasized in the retatrutide trials. Sterility of product obtained outside clinical trials cannot be assured, adding infection/abscess risk not attributable to the drug itself.
HPTA suppression & recovery
Suppression: None described / not applicable by known mechanism (no data)
Retatrutide is not an androgen and has no established mechanism of HPTA suppression, so SERM-based post-cycle recovery is not a relevant framework here and none is described in the literature. If HPTA or fertility concerns arise (for example, in the context of large, rapid weight loss or concurrent use of other agents), evaluation and any management should be directed by an endocrinologist rather than self-managed; only single-SERM approaches would ever be appropriate to discuss with that clinician, and dual-SERM protocols are out of scope.
Monitoring
Cadence: Establish a clinician relationship before use; check heart rate/blood pressure and symptoms during dose escalation and periodically thereafter; labs at baseline and roughly every 3 months or as a clinician directs, with prompt testing if warning signs appear.
- Severe or persistent vomiting/diarrhea with signs of dehydration (dizziness, reduced urination) — risk of acute kidney injury
- Severe, persistent abdominal pain radiating to the back (possible pancreatitis) — seek urgent care
- Right-upper-quadrant pain, fever, jaundice (possible gallbladder disease)
- Sustained resting tachycardia or palpitations
- Signs of hypoglycemia when combined with insulin/sulfonylureas (shakiness, sweating, confusion)
- Any planned surgery/procedure — disclose use in advance due to aspiration risk
- New or worsening mood changes or thoughts of self-harm — seek help
Contraindications
- Not an approved therapy — investigational; use outside a supervised clinical trial means unverifiable identity, dose, purity, and sterility.
- Personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia type 2 (standard GLP-1-class boxed contraindication based on rodent C-cell tumor findings; retatrutide-specific human data lacking).
- History of pancreatitis — caution/avoid given class-associated acute pancreatitis risk.
- Pregnancy, attempting pregnancy, or breastfeeding — no safety data and weight loss is undesirable in pregnancy.
- Severe gastrointestinal disease including gastroparesis (drug further slows gastric emptying).
- Type 1 diabetes / diabetic ketoacidosis risk — not a substitute for insulin.
- Concurrent insulin or sulfonylureas without medical supervision (additive hypoglycemia risk).
- Scheduled surgery/anesthesia without disclosing use — retained gastric contents raise aspiration risk; the peri-operative plan must be managed by the anesthesia/surgical team.
Interaction profile
- ModerateWith a thermogenic stimulant: Additive cardiovascular strain
- MajorWith insulin: Metabolic / glucose
- ModerateWith another GLP-1 / incretin agonist: Metabolic / glucose
- ModerateWith metformin: Metabolic / glucose
- ModerateWith growth hormone: Metabolic / glucose
- ContraindicatedWith DNP: Additive cardiovascular strain
Check a specific combination in the interaction checker.
Reducing harm & when to stop
- This is an investigational drug with no approval and no long-term safety or cardiovascular-outcome data; the safest course is not to use it, and to pursue approved, physician-supervised options instead.
- Do not use without a clinician and baseline bloodwork; product obtained outside clinical trials cannot be verified for identity, concentration, purity, or sterility, which compounds every listed risk.
- GI side effects (nausea, vomiting, diarrhea) are common during dose escalation — maintain hydration and seek care for severe or persistent symptoms, which can cause dehydration and acute kidney injury.
- Stop and seek urgent medical care for severe abdominal pain radiating to the back (possible pancreatitis) or right-upper-quadrant pain with fever/jaundice (gallbladder disease).
- Because the drug slows stomach emptying and persists for weeks, disclose use well before any surgery or procedure requiring sedation/anesthesia due to aspiration risk.
- Monitor resting heart rate and blood pressure; report sustained tachycardia or palpitations.
- Do not combine with insulin or sulfonylureas without medical supervision because of additive hypoglycemia risk.
- Avoid in pregnancy, breastfeeding, or when trying to conceive; discuss contraception with a clinician.
- HPTA/fertility questions, or any interaction with other substances, should be evaluated by an endocrinologist — not self-managed.
Citations (6)
Every clinical claim above is tied to a primary source. Overall evidence grade A — graded to the best available evidence for its core claims.
- 01
Retatrutide (LY3437943) is a single-peptide triple agonist of the GIP, GLP-1, and glucagon receptors; glucagon activity adds energy expenditure to appetite/intake reduction; PK supports once-weekly dosing and weight reduction persisted to ~day 43 after a single Phase 1 dose.
PreclinicalLY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist... From discovery to clinical proof of concept.PMID 35985340 ↗
- 02
Retatrutide is ~0.3x as potent at the glucagon receptor and ~0.4x at the GLP-1 receptor versus endogenous ligands and ~8.9x more potent at the GIP receptor; pharmacokinetics are dose-proportional with a half-life of approximately 6 days enabling weekly subcutaneous administration.
PreclinicalLY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist... From discovery to clinical proof of concept.PMID 35985340 ↗
- 03
In adults with obesity, once-weekly retatrutide produced dose-dependent weight loss (LSM −17.5% at 24 weeks and up to −24.2% at 48 weeks for 12 mg vs −2.1% placebo); gastrointestinal adverse events were the most common, dose-related, mostly mild-to-moderate, and partly mitigated by a lower starting dose; heart-rate increases were dose-dependent and peaked at 24 weeks then declined.
RCTTriple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial.PMID 37366315 ↗
- 04
In type 2 diabetes, retatrutide reduced HbA1c by up to ~2.0% at 24 weeks and body weight by up to ~16.9% at 36 weeks; mild-to-moderate GI events (nausea, diarrhea, vomiting, constipation) were the main adverse events, with no severe hypoglycemia and no deaths.
RCTRetatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: phase 2 trial.PMID 37385280 ↗
- 05
In participants with MASLD, retatrutide reduced relative liver fat by up to ~82% at 24 weeks and ~86% at 48 weeks with steatosis resolution (<5% liver fat) in >85% of the two highest-dose groups; no hepatotoxicity signals were seen through 48 weeks and beta-hydroxybutyrate rose without associated ketoacidosis; the obesity program also showed improvements in blood pressure, lipids, and glycemia.
RCTTriple hormone receptor agonist retatrutide for MASLD: a randomized phase 2a trial.PMID 38858523 ↗
- 06
Retatrutide is an investigational triple agonist evaluated in Phase 2 with 24.2% weight loss at 48 weeks (12 mg) and is not yet an approved therapy; it belongs to the incretin/GLP-1-based drug class.
ReviewPMID 38356208 ↗
Last reviewed 2026-07-06 · Verified against PubMed · Educational, not medical advice