Tirzepatide (sold as Mounjaro and Zepbound) is the most effective weight-loss medicine you can get with a prescription today. Retatrutide is the experimental drug from the same company, Eli Lilly, that has posted even bigger numbers in phase 3. This guide puts the two side by side using published trial data, so you can see where retatrutide actually pulls ahead, where the picture is still incomplete, and why one is at your pharmacy while the other is not.
Key takeaways
- Receptors: tirzepatide activates two hormone receptors (GIP and GLP-1). Retatrutide activates those two plus a third, the glucagon receptor.
- Weight loss: in TRIUMPH-1, retatrutide 12 mg produced an average 28.3% loss at 80 weeks. In SURMOUNT-1, tirzepatide 15 mg produced about 22.5% at 72 weeks. Different trials, so the comparison is indirect.
- Side effects: both cause mostly stomach-related side effects. Retatrutide’s top dose showed higher rates of vomiting and a skin-sensation effect called dysesthesia, and more people stopped treatment.
- Status: tirzepatide is FDA-approved. Retatrutide is not approved; Lilly has said it plans to file with the FDA in the first quarter of 2027.
How the two drugs work
Both medicines are long-acting peptides injected once a week. They copy gut hormones that your body releases after a meal.
- GLP-1 slows how fast the stomach empties, increases fullness signals in the brain and helps the pancreas release insulin when blood sugar rises.
- GIP also supports insulin release and appears to make GLP-1 drugs more tolerable and more effective for weight loss, though scientists still debate exactly why.
- Glucagon is the piece only retatrutide adds. Glucagon tells the liver to burn stored fuel and may raise energy expenditure. In early studies this showed up as large drops in liver fat.
So the simplest way to frame it: tirzepatide mainly reduces how much you eat. Retatrutide aims to reduce intake and push up how much energy you burn. For the bigger family picture, see our guide to semaglutide, tirzepatide and retatrutide.
Weight-loss results side by side
The fairest comparison uses each drug’s main obesity trial in people without type 2 diabetes. The numbers below use the “efficacy estimand,” which reflects people who stayed on treatment. Results that count everyone, including those who stopped, are a few points lower for both drugs.
| Retatrutide (TRIUMPH-1) | Tirzepatide (SURMOUNT-1) | |
|---|---|---|
| Participants | 2,339 adults with obesity or overweight plus a related condition | 2,539 adults with obesity or overweight plus a related condition |
| Length | 80 weeks | 72 weeks |
| Weekly doses tested | 4, 9 and 12 mg | 5, 10 and 15 mg |
| Average loss, lowest dose | 19.0% | about 16% |
| Average loss, highest dose | 28.3% | about 22.5% |
| Placebo | 2.2% | about 2.4% |
| Lost 30% or more (top dose) | 45.3% | not a headline endpoint |
Retatrutide’s other phase 3 readouts point the same way. TRIUMPH-4 (people with obesity and knee osteoarthritis) reported 28.7% loss at 68 weeks with less knee pain. TRIUMPH-2 (people with type 2 diabetes) reported up to 20.8%, and TRIUMPH-3 (people with heart disease) up to 22.6%. People with diabetes tend to lose less weight on every drug in this class, so those lower numbers are expected.
The big caveat: there is no published head-to-head trial of retatrutide against tirzepatide. Cross-trial comparisons are useful for a rough sense of scale, but differences in trial length, starting weight and population can shift results by several points.
Side effects compared
For both drugs, the most common problems are nausea, diarrhea, constipation and vomiting. They are usually worst in the weeks after each dose increase and fade once the dose is stable.
| Side effect | Retatrutide 12 mg | Tirzepatide 15 mg |
|---|---|---|
| Nausea | 42.4% | about 31% |
| Diarrhea | 32.0% | about 23% |
| Vomiting | 25.3% | about 12% |
| Constipation | 26.1% | about 12% |
| Stopped due to side effects | 11.3% | about 6% |
Two retatrutide-specific signals are worth knowing about:
- Dysesthesia (unusual skin sensations such as tingling or sensitivity) was reported in about 12% of people on the higher doses versus about 1% on placebo. Most cases were mild to moderate and went away during treatment.
- Heart rate rose modestly in earlier retatrutide studies, which researchers link partly to the glucagon component. Tirzepatide also raises resting heart rate a little. Long-term cardiovascular outcome data will matter for both.
Shared warnings for this class include gallbladder problems, pancreatitis, dehydration from GI losses, and a boxed warning about thyroid C-cell tumors seen in rodents, which is why these drugs are not used by people with a personal or family history of medullary thyroid cancer or MEN 2.
Dosing: label versus trial
Pep Guide only lists doses that come from an FDA label or a published human trial.
- Tirzepatide (FDA label): starts at 2.5 mg once weekly for 4 weeks, then increases in 2.5 mg steps at least 4 weeks apart. Maintenance doses are 5, 10 or 15 mg weekly.
- Retatrutide (trial only): TRIUMPH-1 participants were escalated gradually to fixed weekly doses of 4, 9 or 12 mg. There is no approved dose, and trial schedules are not a substitute for a label.
Availability, cost and legal status
Tirzepatide is approved as Mounjaro for type 2 diabetes and as Zepbound for chronic weight management and obstructive sleep apnea in adults with obesity. It is available through pharmacies, and Lilly sells self-pay vials directly. Prices change often, so check the manufacturer and your insurer.
Retatrutide is investigational. As of October 2026, Lilly has said its data package is complete and plans to submit an application in Q1 2027, which would put a decision around late 2027 or 2028 under a standard review. Anything sold online as “retatrutide” today is not an approved medicine. Those products are typically labeled for research use only, and their identity, purity and sterility are not verified by any regulator. Read our explainer on what research use only means and how to spot sketchy vendors.
Who each drug may fit (as a conversation with your doctor)
- Tirzepatide has years of real-world use, a full safety label, insurance pathways and approved uses beyond weight loss.
- Retatrutide may eventually suit people who need more weight loss than current drugs deliver, or who have fatty liver disease, but that depends on what the FDA concludes from the full data. For now, the legitimate way to access it is a clinical trial.
Frequently asked questions
Is retatrutide stronger than tirzepatide?
In separate trials, retatrutide 12 mg produced more average weight loss (28.3% at 80 weeks) than tirzepatide 15 mg (about 22.5% at 72 weeks). There is no published head-to-head trial yet, so the exact difference is not known.
Is retatrutide FDA-approved?
No. As of October 2026 retatrutide is investigational. Eli Lilly has said it plans to file for approval in the first quarter of 2027.
Does retatrutide have more side effects than tirzepatide?
At the highest doses tested, retatrutide showed higher rates of nausea, vomiting and treatment discontinuation, plus a skin-sensation effect called dysesthesia. Lower doses were closer to tirzepatide.
Can you switch from tirzepatide to retatrutide?
Not legally outside a clinical trial, because retatrutide is not an approved drug. No official switching protocol exists.
Who makes retatrutide and tirzepatide?
Both are developed by Eli Lilly. Tirzepatide is sold as Mounjaro and Zepbound.
Sources
- Eli Lilly, TRIUMPH-1 topline results press release, May 21, 2026
- Jastreboff AM et al., Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1), N Engl J Med, 2022
- Jastreboff AM et al., Triple-hormone-receptor agonist retatrutide for obesity, N Engl J Med, 2023
- Zepbound and Mounjaro U.S. prescribing information
Medical disclaimer: Pep Guide is for educational purposes only. It is not medical advice, diagnosis or treatment. Research changes over time. Talk to a licensed healthcare provider before starting, stopping or changing any medication or compound.