Tirzepatide Reconstitution and Dosing: Syringe Units for Every Vial

Branded tirzepatide (Zepbound and Mounjaro) comes ready to inject. The pens and the single-dose vials hold one dose in 0.5 mL of solution. Zepbound also comes in a multi-dose vial and a KwikPen that each hold four weekly doses in 2.4 mL, so one dose is 0.6 mL. You add nothing to these, and the dose is set by which strength you were prescribed.

Vials of freeze-dried powder work differently. You reconstitute them with bacteriostatic water yourself, and the draw depends on how much you add. Compounded and research tirzepatide are often sold this way. The FDA declared the tirzepatide shortage resolved in December 2024, which narrowed the conditions under which pharmacies may compound it, so check where a vial came from before you rely on its label.

Some compounded vials come as a liquid with the strength printed on the label, for example 10 mg in 2 mL. You can use the same calculator for those: enter the total mg as the vial size and the total mL as the water volume.

The Zepbound prescribing information starts every patient at 2.5 mg once weekly for 4 weeks, then raises the dose to 5 mg. After that, the dose may go up by 2.5 mg at a time, with at least 4 weeks on each dose before the next increase. The maximum is 15 mg once weekly. The Mounjaro prescribing information uses the same starting dose, the same 2.5 mg steps, and the same 15 mg maximum.

The weeks shown are the fastest pace the label allows, and a prescriber may keep a dose longer before stepping up. For weight reduction, Zepbound lists 5, 10, and 15 mg as maintenance doses; for obstructive sleep apnea it lists 10 and 15 mg. Which step you are on is a decision for your prescriber.

Dividing the vial's milligrams by the millilitres of water gives the concentration. Divide your dose by that concentration to get millilitres, then multiply by 100 to get units on a U-100 insulin syringe. The three steps combine into one formula:

For a 10 mg vial mixed with 2 mL, the concentration is 5 mg/mL. A 2.5 mg dose is 0.5 mL, which is 50 units. The reconstitution math blog breaks each step down further, with practice problems.

Values are units on a U-100 syringe, rounded to one decimal place. Select one to open the calculator with that vial, water, and dose filled in.

More water means more units for the same dose: 2.5 mg is 25 units from a 10 mg vial with 1 mL and 75 units from the same vial with 3 mL. At the higher doses a weak mix outgrows the syringe. Anything over 100 units is more than 1 mL and needs a second draw, so 10 to 15 mg doses call for a stronger mix.

A 15 mg vial with 1, 2, or 3 mL gives fractions like 16.7 and 33.3 units, which no syringe marks exactly. Mixed with 1.5 mL instead, the same vial is 10 mg/mL and every dose lands on a whole unit.

The number of full doses in a vial depends only on the vial size and the dose. Water volume changes how many units each dose takes, but the total milligrams stay the same.

A 10 mg vial holds four 2.5 mg doses, which covers the four starting weeks. In practice a little liquid stays behind in the vial, so the last dose can come up slightly short.

Storage limits how long a large vial can last. The CDC says to discard an opened multi-dose vial within 28 days unless the manufacturer gives another date. Twelve weekly 2.5 mg doses from a 30 mg vial would take 12 weeks, far past that limit, so a smaller vial or a check with the supplier on stability may make more sense.

Every label dose is a multiple of 2.5 mg. If you mix the vial to 10 mg/mL, each 2.5 mg step is exactly 25 units: 25 for 2.5 mg, 50 for 5 mg, 75 for 7.5 mg, and so on. To get 10 mg/mL, add 1 mL of water for every 10 mg in the vial.

At this strength 10 mg is 100 units, a full 1 mL syringe, so 12.5 and 15 mg doses need two draws or a stronger mix. A 30 mg vial with 1.5 mL gives 20 mg/mL, where 15 mg is 75 units and every 2.5 mg step is 12.5 units, a half-unit mark that only some 30 unit syringes have.

The tradeoff runs the other way for small doses: the stronger the mix, the fewer units each dose takes, and a reading error of half a mark becomes a bigger share of the dose. The BAC water volume blog goes through that balance, and the syringe units blog explains which barrel reads each draw most clearly.

A unit count belongs to one vial mixed one way. People run into trouble when they carry a number they know by heart over to a new situation.

• Switching vial size changes the strength. 5 mg from a 10 mg vial with 2 mL is 100 units. From a 30 mg vial with 2 mL, 100 units holds 15 mg, three times as much.
• Mixing the next vial with 1 mL instead of 2 doubles its strength, so last month's unit count delivers twice the milligrams.
• Moving from 2.5 to 5 mg doubles the units from the same vial, and a step from 5 to 7.5 mg adds half again. The units rise in proportion to the dose.
• Prescriptions are written in mg and syringes are marked in units. Drawing to the 5 unit mark from a 10 mg vial with 2 mL gives 0.25 mg, a tenth of the starting dose.