What Is Retatrutide? What the Research Actually Shows

Retatrutide is an experimental once-weekly injection that activates three receptors: GIP, GLP-1 and glucagon.^[1][2] It has produced very large weight loss in company-run trials. It is still investigational, though, and long-term safety is unknown.^[3][1][4]

What it is

• Retatrutide (also called LY3437943) is being tested by Eli Lilly for obesity, type 2 diabetes and other conditions.^[1][2]
• FDA says retatrutide is not a component of any FDA-approved drug and has not been found safe and effective for any condition (page updated 2026).^[4]

What the trials show

All of the human efficacy data below come from Lilly-funded trials.^[2]

• Phase 2 obesity trial (double-blind RCT, 338 adults, weekly injections): at 48 weeks body weight fell 8.7% on 1 mg, 17.1% on pooled 4 mg, 22.8% on pooled 8 mg and 24.2% on 12 mg, versus 2.1% on placebo.^[1]
• TRIUMPH-1 (phase 3, 2,339 adults with obesity and no diabetes, 80 weeks): weight fell 17.6% on 4 mg, 23.7% on 9 mg and 25.0% on 12 mg, versus 3.9% on placebo.^[3]
• In the TRIUMPH-1 sleep apnoea subgroup, events per hour fell by 22.9 to 34.3 on retatrutide versus 9.9 on placebo. Knee osteoarthritis pain scores also improved more than on placebo.^[3]
• TRIUMPH-2 (phase 3, 1,152 adults with obesity and type 2 diabetes, 80 weeks): weight fell 11.9% on 4 mg, 16.8% on 9 mg and 18.8% on 12 mg, versus 5.1% on placebo.^[2]
• Phase 2 trial in type 2 diabetes (RCT, 281 adults, 24 weeks): HbA1c fell by up to 2.02 points on retatrutide 12 mg versus 0.01 on placebo and 1.41 on dulaglutide 1.5 mg.^[5]
• In a liver sub-study of the phase 2 obesity trial (RCT, 98 adults with fatty liver), liver fat fell 81-82% at 24 weeks on 8 and 12 mg versus a 0.3% rise on placebo.^[6]

Doses as studied

These are the doses researchers tested. They are not recommendations.

• Phase 2 obesity trial (subcutaneous, once weekly for 48 weeks): 1 mg, 4 mg (start 2 mg or 4 mg), 8 mg (start 2 mg or 4 mg) or 12 mg (start 2 mg). A 2 mg start gave fewer stomach side effects than a 4 mg start.^[1]
• TRIUMPH-1 and TRIUMPH-2 (phase 3, subcutaneous, once weekly for 80 weeks): 4 mg, 9 mg or 12 mg, against placebo.^[3][2]
• These are trial doses, not approved doses. No label exists.^[4]

Side effects and known risks

• In the phase 2 obesity trial, the most common side effects were stomach-related and dose-related. Heart rate rose in a dose-dependent way, peaking at 24 weeks and then declining.^[1]
• In TRIUMPH-2, diarrhoea affected 27-34% on retatrutide versus 13% on placebo, and nausea 14-28% versus 8%.^[2]
• Dysaesthesia (odd skin sensations) affected 4-7% versus 1%, and low blood pressure 1-6% versus under 1%.^[2]
• In the same trial, 12% on 9 mg and 8% on 12 mg stopped because of side effects or death, versus 5% on placebo.^[2]
• Two case reports involve online-sourced products labelled retatrutide. A man with type 1 diabetes developed severe vomiting, high blood sugar, ketones and kidney injury soon after self-injecting one. A woman in her mid-20s had five days of refractory nausea after four weekly escalating injections from an unverified product; it settled within a week of stopping. In both, what the vials contained was not confirmed.^[7][8]
• In isolated mouse atria (a lab prep), retatrutide at 100 nM sped up the beating rate of right atria through the glucagon receptor. It did not increase contraction force in left atria. This is a lab finding, not a safety conclusion for people.^[9]

Legal and regulatory status

• As of the FDA page updated in 2026, retatrutide cannot be used in compounding under federal law. FDA says it has warned telehealth companies, API distributors and outsourcing facilities over retatrutide.^[4]
• FDA warning letters to sellers of 'research use only' or compounded retatrutide include Swisschems and Xcel Research (2024-12-10), Amazing Meds and GLP-1 Solution (2025-09-09), GenoGenix (2026-01-20), Gram Peptides and Prime Sciences (2026-03-31) and Peptide Partners (2026-08-24).^{[10][11][12][13][14][15][16][17]}
• DailyMed shows ten 'retatrutide' and 'Salkalli (retatrutide)' listings from Guangzhou Yixin Cross-Border E-Commerce Co., Ltd. Their marketing category is 'Export only'. These are self-submitted listings, not FDA approvals. They do not mean retatrutide is approved or that those products were reviewed.^[18][19][20]

What the evidence does not show

• Interactions have not been studied or labelled, because there is no approved product or prescribing information.^[4]
• Nothing in our sources shows what 'research' vials sold online actually contain or whether they behave like the trial drug.^[7][8]
• All human efficacy data come from Lilly-funded trials of 80 weeks or less. Long-term safety and effects on heart, kidney or death outcomes are unknown.^[2]
• TRIUMPH-Outcomes is registered but has no results in the records we hold.^[21]

The bottom line

Retatrutide has shown very large weight loss in company-run phase 2 and phase 3 trials.^[3][1] It is still investigational, and FDA says it has not been found safe and effective for any condition.^[4] In the phase 2 obesity trial, the most common side effects were stomach-related and dose-related.^[1] Long-term safety is unknown.^[2] Products sold online as retatrutide are unapproved.^[4] In the two case reports of online-sourced products, what the vials contained was not confirmed.^[7][8]