Retatrutide vs. Tirzepatide: How Are They Different?

The core difference: two receptors versus three

Both drugs are made by Eli Lilly, and both are single peptides engineered to hit more than one hormone receptor.

The glucagon receptor is the addition. Glucagon raises blood sugar by telling the liver to release glucose, which sounds like the wrong direction for a diabetes or obesity drug. But glucagon also increases energy expenditure and fat burning. The idea behind retatrutide is that GLP-1 and GIP activity keep blood sugar in check while glucagon activity adds calorie burn on top of appetite reduction.

Weight loss in clinical trials

These numbers come from different trials with different participants and lengths, so they are not a direct comparison.

Lilly’s press release also reported 28.3% for TRIUMPH-1 and 20.8% for TRIUMPH-2 at 12 mg, estimates of the effect if everyone had stayed on the drug. Those estimates use a different analysis from the published treatment-regimen results above.

These are separate trials with different participants, lengths, and analysis methods, so their results cannot be compared directly. TRIUMPH-5 is the head-to-head trial that can test retatrutide against tirzepatide.

Blood sugar

Tirzepatide is approved for type 2 diabetes. In SURPASS-2, tirzepatide 15 mg lowered A1c by 2.3 percentage points at 40 weeks, versus 1.86 for semaglutide 1 mg.

Retatrutide lowered A1c by 1.4 to 1.6 percentage points at 80 weeks in TRIUMPH-2. Its phase 2 diabetes trial reported reductions of up to about 2 points at 36 weeks. Different baselines and time points make a direct comparison unreliable.

Side effects

Both drugs cause the gastrointestinal effects common to this class: nausea, diarrhea, vomiting, and constipation, most often during dose increases.

Retatrutide trials have reported two effects that are less typical of tirzepatide:

• Dysesthesia, an abnormal skin sensation, in up to 20.9% of participants on 12 mg in TRIUMPH-4 versus 0.7% on placebo.
• Heart rate increases in the phase 2 trial that rose with higher doses, peaked around 24 weeks, then declined.

Lilly’s press release reported that 11.3% of participants on retatrutide 12 mg in TRIUMPH-1 stopped because of side effects. In SURMOUNT-1 it was 6.2% on tirzepatide 15 mg. Again, different trials.

The prescribing information for semaglutide and tirzepatide carries a boxed warning about thyroid C-cell tumors seen in rodents. Not every GLP-1 medicine has one: short-acting exenatide (Byetta) does not. Retatrutide’s full safety profile will be known only after regulatory review.

Dosing and devices

Tirzepatide is approved at 2.5, 5, 7.5, 10, 12.5, and 15 mg once weekly. It starts at 2.5 mg for four weeks and increases in 2.5 mg steps at least four weeks apart. It comes in single-dose pens and vials, and in a multi-dose vial or KwikPen that holds four doses.

Retatrutide trials tested 4, 9, and 12 mg maintenance doses reached by stepwise increases; this is reported here as trial design, not a schedule for anyone to follow. There is no approved strength, device, or schedule.

Availability and legality

Tirzepatide is available by prescription from licensed pharmacies. Retatrutide is available only to trial participants. The FDA has stated that retatrutide cannot be used in compounding, and products sold online as “research” retatrutide are unapproved drugs of unknown quality.

Lilly has said it plans to file for FDA approval in the first quarter of 2027.

Which one is “better”?

That question cannot be answered yet. No trial has compared retatrutide with tirzepatide: each drug’s weight-loss figures come from its own trial, and TRIUMPH-5, the head-to-head trial, has not reported. Retatrutide’s side-effect profile is still being characterized, and it is not available outside research. Tirzepatide has years of real-world use, an approved label, and cardiovascular outcome data. A person choosing a treatment today is choosing among approved options, and that conversation belongs with a prescriber.