Drug facts

Injection sites

Reviewed August 2026

Quick answer

The label names the abdomen and the thigh for a self-administered injection, and the back of the upper arm where another person is doing the injecting. It also says to rotate injection sites with each dose. In the approved product’s own pharmacokinetic data, the three sites gave similar exposure.

The sites the label names1

Among its administration instructions, section 2.4 states that the product is injected subcutaneously in the abdomen or thigh, or in the back of the upper arm if another person is administering it. The same section says the injection is given once weekly, at any time of day, with or without meals.

Section 2.4, as written

Self-administered
Abdomen, thigh
Given by another person
Back of the upper arm
Rotation
Rotate sites with each dose
Timing
Once weekly, any time of day, with or without meals

Whether the site changes the dose you get2

It does not, for the approved product. The absorption subsection reports a mean absolute bioavailability of 80% following subcutaneous administration, with peak plasma concentration at a median of 24 hours (range 8 to 72 hours), and states that similar exposure was achieved with subcutaneous administration in the abdomen, thigh or upper arm.

So the choice between the three named sites is not a dosing decision. The number of milligrams is settled elsewhere — and if the arithmetic between milligrams and the markings on a syringe is what is actually troubling you, that is unpacked in why a vial is dosed in units but prescribed in milligrams.

Injection site reactions, and how common they were3

Injection site reactions appear in the adverse reactions table for Studies 1 and 2 at 6% on 5 mg, 8% on 10 mg and 8% on 15 mg, against 2% on placebo. The footnote to that row explains that the entry pools several related terms — injection site bruising, erythema, pruritus, pain, rash and reaction.

A separate paragraph in the same section reports that these reactions were more frequent in patients who developed anti-tirzepatide antibodies (11.3%) than in those who did not (1%). The wider picture of what was reported, and at what rate against placebo, is in the side effects reported in 5% or more of trial participants.

What a compounded vial inherits from this1

On the label — Mounjaro / Zepbound

Dispensed as a defined presentation — vial, single-dose pen, autoinjector or KwikPen — with FDA-approved Instructions for Use for that presentation, and patients trained on it before initiation. Section 2.4 also instructs that the solution be inspected before use and not used if particulate matter or discoloration is seen.

In a compounded vial

Not established.

A compounded vial arrives with no FDA-approved Instructions for Use, because there are none to arrive with. The site guidance above was written about the approved product, and the practical instructions that govern your own vial come from your prescriber and the pharmacy that made it. Handling it — how it is kept, and for how long — is a separate question, covered in storage and the beyond-use date on a compounded vial. What the injected drug then does is covered in how tirzepatide acts on GIP and GLP-1 receptors.

Sources

  1. FDA prescribing information for ZEPBOUND (tirzepatide), section 2.4 Important Administration Instructions, revision 4/2026 DailyMed
  2. FDA prescribing information for ZEPBOUND (tirzepatide), section 12.3 Pharmacokinetics (Absorption), revision 4/2026 DailyMed
  3. FDA prescribing information for ZEPBOUND (tirzepatide), section 6.1 Clinical Trials Experience, revision 4/2026 DailyMed