Zepbound’s active ingredient is tirzepatide, a synthetic peptide made up of 39 amino acids with a molecular weight of about 4,813 daltons. Beyond that active compound, each injection contains a short list of inactive ingredients that keep the solution stable and safe to inject. Here’s a full breakdown of everything in the pen.
The Active Ingredient: Tirzepatide
Tirzepatide is a lab-made peptide, not extracted from human or animal tissue. It’s manufactured using a hybrid process that combines two types of peptide synthesis (solid-phase and liquid-phase) to build the 39-amino-acid chain at industrial scale. The result is a fully synthetic molecule designed to mimic two natural gut hormones your body already produces.
Specifically, tirzepatide activates receptors for both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). These are hormones released after you eat that help regulate blood sugar, appetite, and how quickly your stomach empties. What makes tirzepatide unusual is that it doesn’t activate both receptors equally. Research published in JCI Insight found that tirzepatide engages the GIP receptor with roughly the same strength as your body’s natural GIP hormone, but it’s about 5 times weaker at binding the GLP-1 receptor and 20 times less potent at triggering signaling through it. It’s also only about 51% as effective at the GLP-1 receptor compared to natural GLP-1.
This imbalance is intentional. Tirzepatide behaves as a full agonist at the GIP receptor (meaning it turns it on completely) but only a partial agonist at the GLP-1 receptor. It also triggers far less receptor internalization at the GLP-1 receptor, a process where the receptor gets pulled inside the cell and temporarily stops responding. The practical result is a dual-action drug that leans more heavily on the GIP pathway than most people realize.
Inactive Ingredients in Single-Dose Pens
If you’re using a standard single-dose pen or single-dose vial, the formulation is simple. Each 0.5 mL injection contains tirzepatide plus just three other ingredients:
- Sodium chloride (4.1 mg), ordinary salt, which makes the solution match the salt concentration of 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, the sterile water that makes up the bulk of the liquid.
Small amounts of hydrochloric acid or sodium hydroxide may also be added during manufacturing to fine-tune the pH. The final solution is clear and colorless to slightly yellow.
Extra Ingredients in Multi-Dose Vials
Multi-dose vials and KwikPens contain 2.4 mL of solution (enough for four doses of 0.6 mL each) and include three additional ingredients not found in the single-dose pens:
- Benzyl alcohol (5.4 mg per dose), a preservative that prevents bacterial growth once the vial has been opened and used more than once.
- Phenol (1.08 mg per dose), another antimicrobial preservative serving the same purpose.
- Glycerin (4.8 mg per dose), which stabilizes the protein and helps maintain the solution’s consistency.
These preservatives are necessary because multi-dose containers get punctured repeatedly, creating opportunities for contamination. Single-dose pens don’t need them because they’re used once and discarded.
Available Dose Strengths
Zepbound comes in six strengths, all delivered in the same 0.5 mL volume for single-dose pens: 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg. The difference between a 2.5 mg pen and a 15 mg pen isn’t the volume you inject. It’s the concentration of tirzepatide dissolved in the same half-milliliter of liquid. Treatment typically starts at 2.5 mg once weekly and increases over time.
How the Formulation Affects Storage
Because tirzepatide is a peptide (essentially a small protein), it’s sensitive to heat. Unused pens should stay refrigerated between 2°C and 8°C (about 36°F to 46°F). If you need to travel or can’t refrigerate it, an unused pen can be stored at room temperature up to 30°C (86°F) for up to 30 days. The same 30-day, 30°C rule applies to a multi-dose KwikPen that’s already in use. After 30 days at room temperature, it should be discarded regardless of how much solution remains.
How Long It Stays Active in Your Body
Tirzepatide’s structure gives it a long half-life of roughly 5 days, which is why Zepbound is a once-weekly injection rather than a daily one. After injection, it reaches peak levels in your bloodstream somewhere between 8 and 72 hours. That wide window varies from person to person depending on factors like injection site and individual metabolism.