Retatrutide studies moved from Phase 1 to Phase 3

Retatrutide research grew from small tests to large ones

Eli Lilly first tried retatrutide in small groups. The team watched for early harm.

In Phase 1, healthy adults received rising amounts under close care ^[7]. No problem stopped the team from doing the next study.

Phase 1b was another early test in adults with type 2 diabetes ^[4]. The 6-day half-life supported one study shot each week.

Those adults lost weight, and their blood sugar fell. The small study couldn't show what most people should expect.

Phase 2 then studied adults with obesity, fatty liver, or diabetes. Each Phase 2 study measured a different benefit and its side effects.

Phase 2 gives the best finished results so far. The main numbers appear lower on this page.

Phase 3 was the next step. Those studies are larger and run longer.

They ask whether benefits last and what harm appears. Phase 3 hasn't finished.

The stages show what your doctor knows now. They also show what your doctor still can't answer.

Retatrutide isn't approved while those answers are missing. Outside a study, your doctor has no legal prescription to offer.

Phase 1 found no harm that stopped further retatrutide tests

The first human Phase 1 study began in 2019. Healthy adults received retatrutide under close care ^[7]. Staff watched for harm while amounts rose.

The study didn't prove a benefit. It found no safety problem that stopped the next test.

Phase 1b, another early safety test, included 72 adults with type 2 diabetes ^[4]. Their three-month blood-sugar results were within the allowed 7.0–10.5% range.

That test shows blood sugar over several months. Lower is better, but your doctor sets your goal.

Groups received 0.5 mg, 1.5 mg, or 3 mg. Another plan was written 3/6/9/12 mg over 12 weeks.

The slashes mean staff increased the amount little by little. The drug had a 6-day half-life, so blood levels fell by half about every 6 days.

The paper also gives the time as 6 days. The highest plan beat the placebo group's weight loss by 8.96 kg.

The placebo shot held no retatrutide. Researchers were 90% sure retatrutide caused more weight loss than the placebo.

That finding still wasn't a promise for one person. Daily blood sugar fell in the 3 mg group.

That fall describes the group, not a target for you. Side effects appeared in 63% of the adults.

Most adults had nausea or other digestive trouble. Phase 2 then tested larger groups.

After one weekly shot, blood levels stayed up for 4–5 days. Retatrutide half life explains that timing.

These are group findings. They won't predict your blood sugar, weight, or side effects.

The small study explains why testing continued. It doesn't prove retatrutide would be safe or useful for you.

Phase 2 found the largest weight loss in the longest study

A 2023 study followed 338 adults with obesity in a 48-week test ^[1]. About 51.8% were men.

Body mass index compares weight with height. Each adult had a score of at least 30, which counted as obesity.

Some could enter at 27 with a related illness. Staff used 1 mg, 4 mg, 8 mg, 12 mg, or a placebo.

People were placed into groups by chance. Neither they nor the staff knew which shot they received.

Retatrutide results by amount:

At 12 mg, 26% of the adults lost at least 25%. Weight was still falling at week 48.

Nausea affected 45% of the 12 mg group. Diarrhea, vomiting, and constipation also occurred.

Study doctors called most cases mild or moderate. At 12 mg, 18% left, mainly because of those problems.

Pulse rose more as the amount rose. The largest rise came near week 24.

Retatrutide vs tirzepatide needs one direct test. Tirzepatide copies two hormones, including GLP-1, the gut hormone that eases hunger.

Retatrutide copies GLP-1 plus glucagon, a third hormone. Tirzepatide studies found about 20–22% weight loss over 72 weeks.

Retatrutide reached 24.2% at 12 mg over 48 weeks. Separate Phase 2 studies used different groups.

Those Phase 2 results can't name a winner. A Phase 3 study now compares the drugs directly ^[11].

That study can give a fairer answer. Even then, no group average can foretell your result.

Phase 2 lowered type 2 diabetes blood sugar and liver fat

The blood-sugar study came first. In 2023, it followed 281 adults with type 2 diabetes for 36 weeks ^[2]. Each stayed on usual care.

Groups received 0.5–12 mg once each week. Staff increased it little by little.

At 12 mg, the three-month blood-sugar test fell 2.02 points. Lower is better, but your doctor sets your goal.

The placebo group's test fell 0.01 point at week 24. Body weight fell 16.94%, against 3.00% with the placebo.

That weight result came at week 36. No one had very low blood sugar or died.

A 2026 review also found that the blood test improved ^[15]. It combined results differently, so it doesn't give your healthy result.

The fatty-liver study came next. In 2024, it followed 98 people with obesity or extra weight ^[5]. Each had at least 10% liver fat. None had diabetes.

Groups received 1 mg, 4 mg, 8 mg, or 12 mg for 48 weeks. Results also covered 1, 4, 8, and 12 mg.

The largest change came at 12 mg. A scan measured liver fat at week 24 without an X-ray.

The falls were 42.9%, 57.0%, 81.4%, and 82.4%. The placebo group rose 0.3%.

At 12 mg, 86% got below 5% liver fat. The study called that normal.

The drop was still 86.0% at week 48. These numbers describe the groups.

They don't tell your doctor which care fits you. They also can't predict your own tests.

Phase 3 must answer several safety questions

Retatrutide reached Phase 3, but the work wasn't done. As of mid-2026, the large studies hadn't reported results.

• One study follows obesity with type 2 diabetes ^[8].
• A second follows obesity with heart disease ^[9].
• A third looks for lasting heart and kidney harm ^[10].
• A fourth compares retatrutide with tirzepatide ^[11].

The type 2 diabetes study also tracks weight. Phase 2 couldn't show what years of use may do.

Larger studies ask whether lost weight stays off. The heart-and-kidney study checks harm over time.

The Phase 3 results weren't public when we checked. A 2025 review gathers earlier Phase 1/2 work ^[6].

No single new result will answer every question. Each Phase 3 study must answer the question named above.

A 2024 review compares drugs that copy several hormones ^[13]. Another 2024 review covers heart and blood-sugar illness ^[12].

If you're waiting for approval, the missing answers matter. Results that aren't public can't guide your care.

Reviews can sum up finished work. They can't replace the large studies still running.