Retatrutide Starting Dose: What Week 1 Looked Like in the Trials

The retatrutide starting dose in all six Phase 3 trials Lilly has reported is 2 mg, injected once a week for 4 weeks, before the first step up to 4 mg. Nobody has tested a lower start followed by a climb. The only doses under 2 mg in published weekly-dosing trials were fixed doses in early trials, where 0.5 mg did about as much for weight as placebo and 1 mg averaged 8.67% weight loss over 48 weeks.

Retatrutide is still in Phase 3 trials, and no approved version exists yet. Lilly's October 2026 FAQ says it "has not been reviewed or approved by the FDA or any regulatory agency in the world, and no one can legally sell it for human use." Lilly last said, on September 29, 2026, that it plans to submit retatrutide to the FDA in the first quarter of 2027 as a Biologics License Application (BLA). On October 2, 2026, the FDA decided that retatrutide is a drug, not a biological product, so an application would need to be filed under section 505 of the Federal Food, Drug, and Cosmetic Act. Lilly's appeal on that question is pending in the Seventh Circuit, and we found no ruling as of October 31, 2026. So there's no label and no official first dose, only what the trials did and what the tirzepatide and semaglutide labels say.

The retatrutide starting dose in Lilly's trials

Six Phase 3 results are public, and all six started people the same way. TRIUMPH-1 (obesity), TRIUMPH-2 (obesity with type 2 diabetes), TRIUMPH-3 (obesity with heart disease), TRIUMPH-4 (knee osteoarthritis), TRANSCEND-T2D-1 (type 2 diabetes) and TRANSCEND-T2D-2 (type 2 diabetes, against Ozempic 2 mg) gave 2 mg once a week, then raised the dose every 4 weeks until each person reached a target dose: 4, 9 or 12 mg in most, 9 or 12 mg in TRIUMPH-3, TRIUMPH-4 and TRANSCEND-T2D-2. Weeks 1 to 4 were 2 mg for everyone on retatrutide. The first 4 mg shot landed in week 5. The retatrutide dosage chart lays out the whole ladder.

Lilly states no combined total, but its per-trial randomized figures add up to 7,720 people, placebo and Ozempic groups included. No other starting dose comes close.

The Phase 2 obesity trial had tried two ways in, and the NEJM paper says the lower 2 mg start partly reduced stomach side effects. Among people aiming for 8 mg, 21 of 35 who began at 4 mg reported nausea, against 6 of 35 who began at 2 mg, according to the results posted on ClinicalTrials.gov. The retatrutide dosing schedule page walks through that comparison and every step after week 4.

Every weekly starting dose tested in people so far

Lower doses have been given to people, just never as the first rung of a climb. Here's every starting dose from the published weekly-dosing trials.

In Phase 3 the 2 mg start was only ever a 4-week stop, and no trial has kept anyone on 2 mg for months. The fixed 1 mg dose wasn't nothing, though: in the Phase 2 obesity trial, 1 mg took off 7.18% of body weight by week 24, and ClinicalTrials.gov's results show 59% of that group had lost at least 5% by then, against 26% on placebo (64% against 27% by week 48).

Is 0.5 mg or 1 mg a gentler way to start?

The trial answer on 0.5 mg is thin but clear. In the Phase 2 type 2 diabetes trial, 47 people were assigned a fixed 0.5 mg for 36 weeks. They lost 3.31 kg on average and the placebo group lost 3.28 kg, and 0.5 mg was the only retatrutide dose whose drop in long-term blood sugar (HbA1c) didn't beat placebo. Nausea hit 2 of 47 on 0.5 mg and 2 of 45 on placebo.

1 mg sits in between. In the Phase 2 obesity trial, 10 of 69 people on 1 mg reported nausea (14.5%), against 8 of 70 on placebo (11.4%). The group that began at 2 mg and moved up to 4 mg reported it in 6 of 33 (18.2%), counted over the whole study (up to 52 weeks with follow-up), not just the first month. That's about 1 in 7 people on 1 mg against about 1 in 5 on the 2 mg path. Among those who began straight at 4 mg, it was 12 of 33 (36.4%).

Our read: in these Phase 2 numbers, moving from a fixed 1 mg to the 2 mg start added a few points of nausea, while starting at 4 mg doubled it. A start under 2 mg is an untested plan, and these comparisons rest on groups of 33 to 70 people, so it's one for your doctor to weigh.

That matters when you see a forum ramp that opens at 0.5 mg: in the only trial that gave it, it did not beat placebo on weight. We found no weekly-dosing trial that started anyone at 0.25 mg or split 2 mg into smaller shots through the week. The microdosing retatrutide page covers what's known about splitting doses.

How 2 mg compares with Zepbound and Wegovy starting doses

Approved drugs in the same family start low too. The Zepbound (tirzepatide) label says its 2.5 mg starting dose "is for treatment initiation and is not approved as a maintenance dosage," and that the climb is there "to reduce the risk of gastrointestinal adverse reactions." The Wegovy (semaglutide) label starts injections at 0.25 mg for 4 weeks. Both are class evidence from other drugs' labels, not retatrutide data.

Milligrams don't carry over between these drugs, because each molecule acts on its receptors with a different strength. In cell tests reported in Nature Medicine in 2024, retatrutide was about 0.4 times as active as natural GLP-1 at the GLP-1 receptor and 8.9 times more potent than natural GIP at the GIP receptor. What carries over is the shape: 4 weeks on a starter dose, then steps.

What 2 mg looks like on a syringe

Outside a trial, the dose on paper has to become a line on an insulin syringe, and 10 units means very different things depending on how much water went into the vial.

That's our arithmetic: on a U-100 syringe, 10 units is 0.1 mL, so the same 10 units runs from an eighth of the trial starting dose to three quarters of it. Here's the working for 2 mg from a 10 mg vial mixed with 1 mL.

Try your own numbers below. The retatrutide units page has every trial dose, the 10 mg vial page has the vial math, and how to reconstitute retatrutide covers the water step.

All of this assumes the vial holds what its label says. In September 2026, Australia's TGA reported testing a vial sold as retatrutide after a serious reaction: it held no retatrutide, only undeclared semaglutide at about 8 times the level in approved products. If you're looking at listings of retatrutide for sale, that page covers what a batch-specific test report can and can't tell you.

When does the starting dose start working?

We couldn't find a published weight figure for the end of the 2 mg month, but there are three clues. In an early Phase 1 study of healthy volunteers, weight stayed down for up to 43 days after a single injection, according to the Cell Metabolism paper that first described retatrutide. Lilly Medical Information says blood levels peak within 12 to 72 hours of a dose. The Phase 1b trial measured a half-life of about 6 days (see retatrutide half-life), so each weekly shot lands on top of what's left of the last one.

Working from that half-life, and assuming the same share of the drug clears each day, about 45% of a dose is still in the body a week later. Here's how the level just after each 2 mg shot builds (ignoring the time it takes to absorb), as a share of where it would settle if you stayed on 2 mg.

That's our calculation, and real levels vary. Just after the fourth 2 mg shot there's about 1.7 times as much drug on board as after the first, and then the dose goes up. The Zepbound label reports tirzepatide levels settling after 4 weekly doses too (class evidence). The best time to take retatrutide page works through what Lilly's 12 to 72 hour peak means for the day and hour of the shot.

In TRANSCEND-T2D-1, Lilly said stomach side effects came mainly while the dose was going up, so the first month and the weeks after each step are the window. The retatrutide side effects page has the rates by dose.

One first-month point comes from the tirzepatide label, which tells people on birth control pills to switch to a non-oral method, or add a barrier method, for 4 weeks after starting and for 4 weeks after each dose increase. Retatrutide's own study with a combined birth control pill (46 postmenopausal women) finished in July 2024 but hasn't posted results, so ask your doctor whether the same precaution applies (our retatrutide and pregnancy page has the rest).

Who the trials didn't start on retatrutide

The trials screened people out before the first 2 mg shot, which makes a useful checklist for a doctor's visit. TRIUMPH-1's ClinicalTrials.gov record excluded anyone who:

• had a personal or family history of medullary thyroid cancer or MEN 2
• had ever had pancreatitis
• had diabetes (TRIUMPH-2 and TRANSCEND-T2D-1 studied it separately)
• had taken a weight-loss drug, over-the-counter ones included, in the 90 days before screening
• had gained or lost more than 5 kg (11 lb) in the previous 90 days
• had had, or planned, weight-loss surgery

So nobody in TRIUMPH-1 went straight from tirzepatide or semaglutide onto retatrutide, and TRIUMPH-9 uses the same 90-day rule. Whether your tolerance carries over, or whether 2 mg can be skipped, nobody has published. The switching from tirzepatide page covers what is and isn't known about the handover.

Insulin or a sulfonylurea changes the picture. In the Zepbound label's type 2 diabetes trial, 10.3% of people also on a sulfonylurea had glucose under 54 mg/dL, against 2.1% of those who weren't. That's tirzepatide data, and the label tells prescribers to consider lowering the insulin or sulfonylurea dose.

The next real evidence on how to start is TRIUMPH-9, which compares three dose-escalation schemes in about 600 people without diabetes. ClinicalTrials.gov doesn't describe the schemes and lists primary completion as October 2028 (estimated), well after Lilly's planned filing in early 2027.