Does Retatrutide Burn More Fat Than Tirzepatide?

Retatrutide acts on three hormone receptors (GIP, GLP-1 and glucagon), and tirzepatide acts on two (GIP and GLP-1). The extra glucagon action is meant to raise how many calories you burn at rest, but no study in people has shown yet that retatrutide does this. In a human study, tirzepatide did not prevent the usual metabolic slowdown that comes with weight loss, but it shifted the body toward burning fat and cut appetite. Retatrutide has produced large average weight loss in its own trials (up to 24.2% in its phase 2 trial), but it has never been tested head to head against tirzepatide, and it is not FDA-approved. Tirzepatide is the one your clinical team can prescribe today.

What does retatrutide's third receptor do that tirzepatide doesn't?

Retatrutide adds the glucagon receptor to the two receptors tirzepatide already uses.

Tirzepatide's two targets are described in this tirzepatide metabolism study. Retatrutide does not hit its three receptors equally. In the retatrutide phase 2 trial, researchers compared it with the body's own hormones. Retatrutide was 8.9 times as potent at the GIP receptor and less potent at the GLP-1 receptor (0.4 times) and the glucagon receptor (0.3 times). Its half-life is about 6 days, which means the body takes about 6 days to clear half of each dose. That is why one shot a week is enough.

The glucagon part is the smallest piece. One review of glucagon research puts retatrutide's glucagon-to-GLP-1 action at roughly 1 to 7.5.

The developers added glucagon on the idea that it might cut eating further, raise the calories you burn, or both, and make the drug work better as a result. That was the starting idea behind retatrutide. Nobody has tested it in people yet.

Does retatrutide make you burn more calories at rest than tirzepatide?

Nobody knows yet. No published study has measured resting calorie burn in people taking retatrutide. A trial of how retatrutide affects eating and calorie burn (ClinicalTrials.gov NCT06313528) is still underway.

The research on glucagon itself is mixed. In most human studies, though not all, single doses of glucagon raised calorie burn. The increase ranged from nothing to 240 calories a day, and it showed up only when glucagon was given before a meal. A 72-hour, high-dose glucagon drip did not raise calorie burn in people with obesity. What glucagon does to calorie burn over months is largely unstudied in humans.

Resting burn matters because of metabolic adaptation. When you lose weight, your body often burns even fewer calories than your smaller size explains. By one estimate in the tirzepatide metabolism study, this slowdown accounts for up to 40% of the drop in calorie burn when you eat less. It is part of why weight comes back: about a third of lost weight typically returns within a year of finishing treatment.

That same study tested whether tirzepatide prevents the slowdown. In calorie-restricted mice with obesity, it did soften it. In people with obesity, it did not. Tirzepatide still changed how their bodies used fuel. They burned more fat, they felt less hungry, and they ate less at lunch and dinner.

So in people, tirzepatide's fat loss comes mostly from eating less while the body burns more fat for fuel. Retatrutide's promise of a higher resting burn is still being tested, and its human data is on the way.

Which one leads to more fat loss, retatrutide or tirzepatide?

Retatrutide's phase 2 trial posted the biggest average weight loss in the table below. These trials were run separately, though, so you can't line them up as a direct contest.

These trials enrolled different people, ran for different lengths of time and used different doses. No trial has put retatrutide and tirzepatide head to head, so neither one can be called better at fat loss yet.

On fat versus muscle, tirzepatide has the data. A review of tirzepatide and body composition found that over 72 weeks, fat mass fell by about a third (33.9%, versus 8.2% on placebo). Fat-free mass, which includes muscle, fell 10.9%, versus 2.6% on placebo. The reviewers say tirzepatide's effect on lean mass is still uncertain. Tirzepatide also cut total body fat, belly (visceral) fat and waist size more than semaglutide or dulaglutide over the same period.

The retatrutide results above report body weight, not how much of it was fat. Whichever drug you take, ask your clinical team how to hold on to muscle while you lose.

What do the side effects feel like on each one?

On retatrutide, stomach side effects were the most common. In TRIUMPH-2, 34% of people on 12 mg had diarrhea and 28% had nausea, compared with 13% and 8% on placebo. Some people had dysesthesia, meaning odd skin sensations such as tingling (7% versus 1%). Low blood pressure affected 6%, versus under 1% on placebo.

More people stopped retatrutide because of side effects at the higher doses: 12% on 9 mg and 8% on 12 mg, compared with 4% on 4 mg and 5% on placebo.

Tirzepatide (sold as Zepbound) carries a boxed warning. It caused thyroid C-cell tumors in rats, and nobody knows whether it does in humans. It is not for anyone with a personal or family history of medullary thyroid cancer or MEN 2 (multiple endocrine neoplasia type 2). Don't combine it with other tirzepatide products or any GLP-1 medicine. It may harm an unborn baby, so tell your clinical team if you are pregnant or planning to be. Pancreatitis and gallbladder problems are also listed warnings.

Not everyone tolerates these drugs. One person on Reddit stopped Zepbound after a little over two months on the lowest dose (2.5 mg). The side effects got worse and led to two ER trips in three weeks, though they lost 23 pounds. They wrote: "Listen to your body while it whispers, before it starts to yell." If side effects are building, tell your clinical team early.

Can you get retatrutide yet, or should you start tirzepatide?

You can't get retatrutide legally yet. Lilly says it is an investigational drug that cannot be legally sold or marketed for human use, so anything sold online as retatrutide is outside the law.

It is coming. Lilly plans to submit retatrutide to the FDA for approval in the first quarter of 2027, and it is the triple-receptor drug to watch.

Tirzepatide is available now. It is FDA-approved as Zepbound for adults with obesity, or adults with overweight plus at least one weight-related health problem. It is approved to help them lose weight and keep it off long term, alongside eating less and moving more. Our clinical team can prescribe tirzepatide if it fits your goal.

You want to lose fat, not muscle, and you want a medication you can start now. Take the quiz to see whether tirzepatide fits your goal and to start a plan with our clinical team.