Retatrutide 10mg Vial (Bac Water, Units and How Long It Lasts)

A retatrutide 10mg vial holds 10 mg of freeze-dried powder in total, not one dose. Mixed with 1 mL of bacteriostatic (bac) water it makes 10 mg per mL, so 20 units on a U-100 insulin syringe hold 2 mg, the starting dose in Lilly's Phase 3 trials. At 2 mg a week the vial holds 5 doses, and the fifth one lands exactly 28 days after the first, the limit FDA recommends for a multi-dose vial.

Retatrutide is still in Phase 3 trials, and no approved version exists yet. Lilly says it's legally available only through its clinical trials and that it "cannot be legally sold or marketed for human use." So any 10 mg vial you can buy isn't Lilly's, and Lilly has published no mixing instructions for one. The numbers below are arithmetic plus what the trials did. What you take, and when you change it, is a conversation for your doctor.

Is retatrutide 10mg a dose or a vial size?

In search results and forum threads, 10 mg nearly always means the vial, not a weekly dose.

In TRIUMPH-1, Lilly's 80-week Phase 3 trial that randomized 2,339 adults, everyone on retatrutide started at 2 mg once a week and stepped up every 4 weeks to a target of 4 mg, 9 mg or 12 mg, passing through 6 mg on the way to the two higher doses. The 2023 Phase 2 obesity trial in The New England Journal of Medicine used 1, 4, 8 and 12 mg. The Phase 1b trial in The Lancet tested fixed doses of 0.5, 1.5 and 3 mg, a step-up from 3 to 6 mg, and one group that climbed through 3, 6 and 9 mg to 12 mg. We found no 10 mg dose in any of them.

Part of the mix-up is that 10 mg is a real weekly dose of a different drug. The Zepbound (tirzepatide) label lists 10 mg as one of its three maintenance doses, alongside 5 mg and 15 mg, but tirzepatide milligrams and retatrutide milligrams aren't interchangeable, so "I was on 10 mg of tirzepatide" says nothing about a retatrutide dose. The trial ladder and what each step did are on our retatrutide dosage page.

How much bac water for 10 mg retatrutide?

There's no single right amount. The water sets the strength, and the strength decides how many units hold a dose; the 10 mg in the vial stays 10 mg whether you add 1 mL or 3 mL. Divide 10 by the milliliters you add to get mg per mL.

The dose columns are TRIUMPH-1 steps, shown for the math. A U-100 syringe holds 1 mL across 100 marks, so anything past 100 units needs two draws.

Lilly does make a 10 mg multi-dose vial, just not of retatrutide. The Zepbound 2.5 mg multi-dose vial holds 10 mg of tirzepatide in 2.4 mL (4.17 mg per mL), split into 4 weekly doses of 0.6 mL. That's a ready-made liquid with its own preservative, so it tells you nothing about retatrutide's chemistry, but it shows the scale Lilly chose for a month of a starting dose.

Our read: for a 10 mg vial, 1 mL is the mix that keeps the math simple, because every 10 units is 1 mg and both early trial doses sit mid-barrel at 20 and 40 units. 2 mL makes the 2 mg draw bigger and easier to see, at the cost of pushing 4 mg to 80 units and 6 mg past a full syringe.

Bac water is sterile water with benzyl alcohol added as a preservative (0.9% or 1.1% on Hospira's label), which is why people use it for a vial they'll puncture every week. For the hands-on steps, see how to reconstitute retatrutide.

Units for every trial dose from one vial

Here are the five Phase 3 dose steps at the two common mixes, with how much of the vial each dose uses up.

Worked through once: at 2 mL the strength is 10 ÷ 2 = 5 mg per mL. A 4 mg dose is 4 ÷ 5 = 0.8 mL, and 0.8 mL is 80 units. The same 80 units from a vial mixed with 1 mL would be 8 mg, twice the dose. A units number copied from someone else's vial only means the same dose if their water volume matches yours.

A 12 mg dose can't come from a 10 mg vial at all. It needs a second vial or a bigger one, and our retatrutide units guide covers syringe sizes for large draws. For any other vial and water combination, the retatrutide calculator does the division for you.

How long does a 10 mg vial of retatrutide last?

At 2 mg a week, 5 weeks. Once the dose climbs, the vial runs short fast.

These are best-case numbers. A little liquid always stays behind in the vial and the syringe hub, so the last draw can come up slightly short.

Follow the TRIUMPH-1 ladder and the fit gets awkward. Four weeks at 2 mg uses 8 mg, leaving 2 mg, which is half of the next step's 4 mg dose. On the path to 12 mg, the first 16 weeks add up to 84 mg: 8 mg at 2 mg, 16 mg at 4 mg, 24 mg at 6 mg and 36 mg at 9 mg. That's more than eight 10 mg vials before the top dose even starts. The week-by-week version lives on our retatrutide dosing schedule page.

Time is the other limit. Nobody has published how long retatrutide stays stable once it's mixed with bac water. FDA's advice for multi-dose vials of compounded GLP-1 drugs is to use them within 28 days of first use and throw them away after that, even if liquid is left and even if the instructions say it lasts longer. Lilly's Zepbound multi-dose vial carries its own limit: throw it away 30 days after first use or after 4 weekly doses, whichever comes first.

Our read: at 2 mg, a 10 mg vial and the 28-day rule line up almost exactly, which makes it the one dose where this vial size fits cleanly. At 4 mg and above, the vial empties long before day 28, and the real problem becomes the odd leftovers rather than the calendar.

What to do with the leftover milligrams

At 4 mg a week, the third dose from a 10 mg vial comes up 2 mg short, so people either throw the 2 mg away or pull the rest from a second vial.

Drawing one dose from two vials means two separate draws, each a chance to misread, and FDA's advice is a new sterile needle and syringe for every dose. It gets riskier if the second vial was mixed with a different amount of water, because the same number of units then holds a different number of milligrams. If you do it, work out the milligrams from each vial separately and write both numbers down before you draw.

Bigger vials cut the leftovers but bring the opposite problem: a 20 mg vial at 2 mg a week holds 10 doses, about twice what the 28-day window allows. Our retatrutide vial sizes page compares every size against each trial dose.

Is there a retatrutide 10 mg pen?

No. As of September 2026, Lilly sells no retatrutide product in any form, pen or vial, and says anything sold to consumers outside its trials comes "with no way to verify its safety, purity or dosing." Lilly plans to file its US application (a Biologics License Application) in the first quarter of 2027, so an approved pen, if there is one, would come after that.

What's actually sold online is freeze-dried powder in small vials from peptide sellers, sometimes marked with a code instead of the drug's name. The Victoria Department of Health's June 2026 alert described six cases of acute liver injury in Victoria since January 2026 in people who used unapproved products labeled Retatrutide, and the product names it listed include Reta, R-10 and R-20. Officials said the harm was possibly linked to a contaminant and that all such products are at risk.

Every number on this page assumes the vial holds 10 mg. If you're comparing retatrutide for sale listings, only a batch-specific third-party test report, with a batch number that matches the vial and a measured amount per vial, backs up the label. A vial that holds 8 mg or 12 mg makes every unit on your syringe wrong by the same share.

Mix-ups that change your dose

FDA's July 2024 alert on compounded semaglutide shows what goes wrong when people measure doses from a vial at home. That's semaglutide data, not retatrutide, but the syringe and the math are the same. Most reports described people drawing more than their prescribed dose, and they took 5 to 20 times the intended amount; in one example, people told to inject 5 units drew up 50. Prescribers slipped too, writing 20 units instead of 2.

The mix-ups that are specific to 10 mg vials:

• Confusing vial sizes. One Reddit thread asks how to tell a 10 mg vial from a 30 mg one after the two got mixed up. Mark each vial the day it arrives.
• Using a units chart built for a different water volume.
• Treating "10" as the dose, because the vial says 10.
• Keeping a mixed vial past 28 days because there's still liquid in it.

Retatrutide's half-life is about 6 days, according to the Phase 1b trial in The Lancet, so an accidental overdose won't wear off by the next morning.

If you also take insulin or a sulfonylurea, tell your doctor before starting any drug in this class. In the Zepbound label's type 2 diabetes trial, low blood sugar hit 10.3% of people also taking a sulfonylurea, against 2.1% of those who weren't. That's tirzepatide data, but nobody has shown retatrutide behaves differently. And if a mixed vial turns cloudy, changes color or shows particles, the bac water label's own advice applies: don't inject it.