Mounjaro’s active ingredient is tirzepatide, a synthetic peptide made up of 39 amino acids. The rest of the injection is a short list of simple stabilizers: salt, a buffering agent, and water. Each prefilled pen delivers a small 0.5 mL dose of this clear, preservative-free solution under the skin.
The Active Ingredient: Tirzepatide
Tirzepatide is a lab-built protein chain with the chemical formula C₂₂₅H₃₄₇O₆₆N₄₉. It’s manufactured using a hybrid process that combines two forms of peptide synthesis, one on a solid support and one in liquid, to assemble the chain at commercial scale. The molecule is modeled after two natural gut hormones your body already produces: GIP and GLP-1. What makes tirzepatide unusual is that it activates the receptors for both hormones at the same time, which is why it’s called a dual agonist.
The 39-amino-acid backbone has a few intentional modifications. At one position, a standard amino acid is swapped for a non-natural one (aminoisobutyric acid), which helps the molecule resist being broken down too quickly. At another position, a fatty acid chain is attached to a lysine amino acid through a chemical linker. This fatty acid grabs onto albumin, a protein in your blood, which slows tirzepatide’s clearance from the body and is the reason you only need one injection per week.
How the Two Hormone Pathways Work
GLP-1, one of the two hormones tirzepatide mimics, triggers insulin release from pancreatic beta cells in a glucose-dependent way. That means it primarily works when blood sugar is actually elevated, which lowers the risk of hypoglycemia. GLP-1 also suppresses glucagon (a hormone that raises blood sugar) and slows the rate at which food leaves the stomach, which reduces appetite and contributes to weight loss.
GIP, the second hormone pathway, also stimulates insulin secretion after meals. Beyond that, lab studies suggest GIP acts as a growth factor for beta cells and may help protect them from dying off. In animal research, combining GLP-1 and GIP signaling produced stronger effects on energy intake and body weight than either hormone alone. This synergy between the two pathways is the core idea behind tirzepatide’s design.
Inactive Ingredients in the Single-Dose Pen
The single-dose pen, which is the most commonly prescribed format, contains a minimal list of excipients alongside tirzepatide:
- Sodium chloride (4.1 mg): ordinary salt, used to match the solution’s tonicity to your body so the injection doesn’t sting or damage tissue.
- Sodium phosphate dibasic heptahydrate (0.7 mg): a buffering agent that keeps the solution’s pH between 6.5 and 7.5, close to the body’s natural range.
- Water for injection: highly purified water that makes up the bulk of the 0.5 mL solution.
Small amounts of hydrochloric acid or sodium hydroxide may also be added during manufacturing to fine-tune the pH. The single-dose pen is preservative-free.
Multi-Dose Vials Have Extra Preservatives
Mounjaro also comes in multi-dose vials and multi-dose KwikPens, each containing 2.4 mL of solution (enough for four separate doses of 0.6 mL each). Because the vial or pen is opened and used more than once, preservatives are needed to prevent bacterial contamination between uses. These formulations add three ingredients the single-dose pen does not have:
- Benzyl alcohol (5.4 mg per dose): an antimicrobial preservative.
- Phenol (1.08 mg per dose): another antimicrobial preservative commonly used in injectable medications.
- Glycerin (4.8 mg per dose): a stabilizer that also helps maintain the solution’s consistency.
The multi-dose version still contains sodium chloride, sodium phosphate, and water, though in slightly different amounts per dose. If you have a known sensitivity to benzyl alcohol, the single-dose pen avoids that ingredient entirely.
Available Dose Strengths
Both the single-dose pens and multi-dose formats come in six strengths: 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg of tirzepatide. The inactive ingredients stay the same across all strengths; only the concentration of tirzepatide in the solution changes. Treatment typically starts at 2.5 mg and increases in steps over several months.
Storage and Stability
Unused pens should be refrigerated between 2°C and 8°C (about 36°F to 46°F). If refrigeration isn’t available, an unused pen can be kept at room temperature, up to 30°C (86°F), for a maximum of 30 days. Once you’ve started using a multi-dose pen, the same 30-day, 30°C rule applies. Mounjaro should never be frozen. If a pen has been frozen, it needs to be discarded, because ice crystals can damage the peptide’s structure and make the dose unreliable.