Retatrutide week by week

How does retatrutide work?

Retatrutide is one peptide that works three hormone receptors⁴. Two of them, GLP-1 and GIP, are the incretin signals tirzepatide works through (semaglutide uses GLP-1 alone): they turn appetite down, and GLP-1 slows the stomach, so meals feel bigger and hunger returns later. The third is glucagon, the hormone the body leans on between meals.

Glucagon is what sets it apart. In obese mice, the glucagon arm raised energy expenditure, so the animals burned more on top of eating less, and they lost more weight than appetite alone explained⁴. That extra burn is a mouse result. In people, the trials measure the result where it counts, on the scale.

It also lasts. Its half-life in people is about six days, measured in a phase 1b trial⁵, which is why it is a once-weekly shot. A single dose in the first human study kept weight down for up to 43 days⁴. A drug with a six-day half-life takes roughly four to five half-lives, about a month of weekly doses, to settle at a steady level, so each new dose step keeps building for weeks after it starts. The chart shows that build-up.

What doses did the trials use, and how fast did they climb?

The obesity trials step the dose up every four weeks, because stomach side effects rise with the dose¹⁶. The phase 3 TRIUMPH trials start at 2 mg and climb through 4, 6 and 9 mg to 12 mg. People assigned to 4 mg stop after one step; people assigned to 9 mg stop after three³.

Phase 2 used bigger jumps, 2, 4, 8 and 12 mg, with every increase finished by week 12⁷. It also tested the start itself: beginning at 2 mg instead of 4 mg partly eased the stomach effects¹. The phase 3 trials all open at 2 mg³⁹.

These are the schedules the trials used for an investigational drug, set and monitored by trial clinicians. They describe what was studied, not a dose for any one person.

What happens week by week?

The table follows the phase 3 dose steps³ and lines them up with the weight results from phase 2¹¹⁰ and phase 3²³. Weight loss is the average change from starting weight.

Two numbers explain the shape. By week 48 of phase 2, 83% of people on 12 mg had lost at least 15% of their weight¹, and the trial's first author, Ania Jastreboff, said they 'had not yet reached a weight plateau' when it ended¹⁰. Phase 3 ran longer, to 80 weeks: on 12 mg, 45.3% of people lost at least 30% of their starting weight by week 80³.

Weight loss runs smaller in type 2 diabetes. In TRIUMPH-2, adults with obesity and type 2 diabetes lost 18.8% on 12 mg at 80 weeks, against 5.1% on placebo⁸.

When does appetite change?

Early. The incretin signals act on hunger from the first dose, well before the scale has much to show, and every step-up turns the dial further. The trials track weight rather than hunger, so the clearest picture of the first weeks comes from the people taking it.

Commonly reported: appetite dropping in the first week or two, often described as stronger than on tirzepatide; nausea or heavy fullness after each step-up that settles within one to two weeks; some tiredness; and a higher resting heart rate on a watch or ring.

What side effects come, and when?

The common ones are the stomach effects every drug in this class shares, and they follow the dose steps. In phase 2 they were mostly mild to moderate, passed with time and came mainly during the weeks of dose escalation¹⁶. Phase 3 put numbers on them for each dose, 4, 9 and 12 mg, against placebo³:

• Nausea: 28.6%, 38.4% and 42.4%, against 14.8% on placebo.
• Diarrhoea: 25.2%, 34.1% and 32.0%, against 13.5%.
• Constipation: 23.8%, 25.9% and 26.1%, against 10.9%.
• Vomiting: 10.6%, 22.8% and 25.3%, against 4.8%.

Most people stayed the course: 4.1% on 4 mg stopped because of side effects, against 4.9% on placebo, with 6.9% on 9 mg and 11.3% on 12 mg³. What helps in the step-up weeks is ordinary: smaller meals, eating slowly and stopping at the first sign of fullness, steady fluids through the day, and more fibre and water when constipation shows up.

Two more effects stand out in the retatrutide trials. Resting heart rate rose with the dose in phase 2, peaked around week 24 and eased after that¹. And some people in phase 3 noticed dysesthesia, a tingling or extra-sensitive feeling in the skin: 5.1% on 4 mg, 12.3% on 9 mg and 12.5% on 12 mg, against 0.9% on placebo, mostly mild to moderate and mostly passing while people stayed on treatment³.

What changes besides weight?

Liver fat drops fast. In a phase 2 substudy of people with fatty liver, 86% of those on 12 mg had normal liver fat by week 24, against none on placebo⁶. In TRIUMPH-1, people with knee osteoarthritis reported less knee pain, and people with sleep apnoea had fewer breathing pauses per hour of sleep².

Is retatrutide approved?

Not yet. Eli Lilly has published its first two phase 3 trials, TRIUMPH-1 in the New England Journal of Medicine² and TRIUMPH-2 in The Lancet⁸, and plans to file with the FDA in the first quarter of 2027⁹. Until a regulator approves it, Lilly states that it 'cannot be legally sold or marketed for human use'⁹.