Tirzepatide is a synthetic, chemically modified peptide made from an amino-acid chain combined with a fatty-acid component. The 2025 FDA label lists its molecular weight as approximately 4,813.53 daltons. Tirzepatide is the active ingredient in Mounjaro and Zepbound.
What Does the Tirzepatide Molecule Contain?
Tirzepatide is based on the sequence of glucose-dependent insulinotropic polypeptide (GIP). Its structure contains:
- A peptide chain made of amino acids
- Two modified amino acids called aminoisobutyric acid (Aib) at positions 2 and 13
- A C-terminal amide
- A lysine residue at position 20 connected through a linker to 1,20-eicosanedioic acid, a long-chain fatty diacid
- The molecular formula C₂₂₅H₃₄₈N₄₈O₆₈
- A molecular weight of approximately 4,813.53 daltons
The fatty-diacid component helps tirzepatide bind to albumin in the bloodstream. This contributes to its long duration of action. Tirzepatide activates both the GIP receptor and the GLP-1 receptor.
What Else Is in Mounjaro and Zepbound Injections?
The inactive ingredients depend on the injection presentation.
| Product presentation | Other ingredients |
|---|---|
| Single-dose pen or single-dose vial | Sodium chloride, sodium phosphate dibasic heptahydrate, and water for injection |
| Multi-dose vial or single-patient-use KwikPen | Sodium chloride, sodium phosphate dibasic heptahydrate, water for injection, benzyl alcohol, glycerin, and phenol |
| pH adjustment | Hydrochloric acid solution and/or sodium hydroxide solution may be used |
For the single-dose formulation, each 0.5 mL dose contains 4.1 mg of sodium chloride and 0.7 mg of sodium phosphate dibasic heptahydrate, along with the prescribed amount of tirzepatide.
Is Tirzepatide Insulin or a GLP-1 Drug?
Tirzepatide is a dual GIP and GLP-1 receptor agonist, not insulin. It acts on the GLP-1 pathway, like semaglutide, but its structure is based on GIP. The added fatty-acid side chain extends its activity in the body.
Does the Formulation Matter?
Yes. Mounjaro and Zepbound contain the same active drug, tirzepatide, but their inactive ingredients can vary by presentation. These include single-dose pens, single-dose vials, multi-dose vials, and KwikPens.
Anyone with an allergy or sensitivity should check the label for the specific product and ask a pharmacist to review its ingredients.