Active ingredient
Tirzepatide
Last reviewed · Written by the PepCycle team
Tirzepatide is a dual agonist: it activates both the GLP-1 receptor and the GIP receptor, two different incretin hormone pathways. That makes it mechanistically distinct from semaglutide, which acts on GLP-1 alone. It is the active ingredient in Mounjaro, licensed for type 2 diabetes, and Zepbound, licensed for weight management.
What tirzepatide is
Two gut hormones are released after eating and act on insulin secretion: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). They are together called the incretins. Semaglutide targets one of them. Tirzepatide is a single engineered peptide that activates both receptors.
This is why calling tirzepatide "a GLP-1" is a useful shorthand but technically incomplete — and why comparisons between the two molecules are about genuinely different pharmacology, not brand preference. Head-to-head trial data comparing the two exists and is worth reading with a clinician rather than summarised second-hand.
- Incretin
- A gut hormone released in response to eating that increases insulin secretion. GLP-1 and GIP are the two principal human incretins; tirzepatide acts on both receptors, semaglutide on one.
Mounjaro and Zepbound
| Product | Form | Licensed for |
|---|---|---|
| Mounjaro | Weekly injection | Type 2 diabetes |
| Zepbound | Weekly injection | Weight management (and, in some markets, obstructive sleep apnoea in adults with obesity) |
As with semaglutide, record the product name from the packaging rather than the ingredient. Insurance records, pharmacy records and clinical notes are all keyed to the brand.
Pens and single-dose vials
Tirzepatide products have appeared in more than one presentation, including single-dose auto-injector pens and, for some strengths and markets, single-dose vials used with a separate syringe. What you record differs slightly:
- Single-dose pen: the amount is fixed by the device. Record the product, strength and lot; there is nothing to derive.
- Single-dose vial: the vial is a manufacturer-filled solution at a stated concentration, not something you reconstitute. Record the stated amount and the syringe used — again, no reconstitution arithmetic applies.
- Multi-dose dial pens where supplied: record the setting used, exactly as the instructions express it.
Keeping a useful record
- Product name, strength and lot exactly as printed.
- Date and time of each weekly injection, so the real interval is visible rather than assumed.
- Injection site, rotated and recorded.
- Every titration change as its own dated event.
- Side effects with timestamps — particularly gastrointestinal effects, whose relationship to injection day is often the clinical question.
- Weight or other measurements on a fixed schedule, so the trend is comparable rather than opportunistic.
PepCycle keeps injections, titration changes, side effects and weight in one timeline, which is the format a review appointment can actually use.
Frequently asked questions
Is tirzepatide a GLP-1 drug?
Partly. Tirzepatide activates both the GLP-1 receptor and the GIP receptor, so it is a dual incretin agonist rather than a pure GLP-1 receptor agonist like semaglutide. The shorthand is common but mechanistically incomplete.
What is the difference between Mounjaro and Zepbound?
Both contain tirzepatide. Mounjaro is licensed for type 2 diabetes and Zepbound for weight management, with sleep apnoea added in some markets. They are separate products with separate labelling, so record the brand printed on your box.
Do I use a reconstitution calculator for a tirzepatide vial?
No. A manufacturer single-dose vial contains a finished solution at a fixed concentration — you are not dissolving powder, so there is nothing to calculate. Follow the product instructions and record the stated amount.
How is tirzepatide different from semaglutide?
Mechanistically, tirzepatide activates two incretin receptors (GIP and GLP-1) while semaglutide activates one (GLP-1). They are separate molecules from separate manufacturers with separate trial programmes. Which is appropriate for any individual is a clinical decision.
Related pages
Keep a Tirzepatide record your prescriber can read
Log every injection with its real date and time, mark late and missed entries instead of hiding them, record side effects alongside dose changes, and export the whole timeline before an appointment.
Free to download · No account required · Price shown before you enter anything personal