Zepbound contains tirzepatide, a synthetic peptide that mimics two gut hormones your body naturally produces after eating. It comes as a clear injectable solution, and beyond tirzepatide itself, the formulation includes a short list of inactive ingredients that keep the medication stable and at the right pH. Here’s what’s in each pen and how these components work together.
The Active Ingredient: Tirzepatide
Tirzepatide is a lab-made protein chain built from 39 amino acids. It works by activating two hormone receptors at once: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). Both of these hormones are normally released by your gut when you eat, signaling your brain that you’re full and prompting your pancreas to release insulin. Tirzepatide hits both targets simultaneously, which is what sets it apart from older weight loss injections like semaglutide (Wegovy, Ozempic) that only activate GLP-1.
This dual action does several things. It slows stomach emptying so food stays in your digestive tract longer, reduces appetite at the brain level, improves how your body handles blood sugar, and increases levels of adiponectin, a hormone involved in fat metabolism. The combined effect on both receptors appears to produce greater appetite suppression and weight loss than targeting GLP-1 alone.
Inactive Ingredients in Each Pen
The inactive ingredients differ slightly depending on whether you have a single-dose or multi-dose pen. In the single-dose pens and vials (which hold 0.5 mL of solution), each injection contains:
- Sodium chloride (4.1 mg) to match the salt concentration of your body and make the injection comfortable
- Sodium phosphate dibasic heptahydrate (0.7 mg) as a buffer to keep the solution at the right pH
- Water for injection as the base liquid
Hydrochloric acid or sodium hydroxide may also be added during manufacturing to fine-tune the pH. That’s the full list for single-dose pens. No preservatives are needed because each pen is used once and discarded.
Multi-dose pens and vials hold 2.4 mL of solution (enough for four doses) and include two additional preservatives to keep the medication safe between uses: benzyl alcohol (5.4 mg per dose) and phenol (1.08 mg per dose). They also contain glycerin (4.8 mg per dose), which helps stabilize the solution. If you have a known sensitivity to benzyl alcohol, the single-dose format avoids that ingredient entirely.
Zepbound vs. Mounjaro: Same Formula
Zepbound and Mounjaro are the same drug, tirzepatide, made by the same manufacturer (Eli Lilly) in the same concentrations. The difference is purely regulatory. Zepbound is approved for chronic weight management, while Mounjaro is approved for type 2 diabetes. The liquid inside the pens is identical, and both come in the same dose strengths.
Available Doses
Zepbound comes in six strengths: 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg, each delivered in a 0.5 mL injection. Everyone starts at 2.5 mg once weekly for the first four weeks. That starting dose isn’t meant as a long-term treatment; it’s designed to let your body adjust and minimize side effects. After four weeks, the dose increases to 5 mg, and from there it can go up in 2.5 mg steps every four weeks or longer until you reach a maintenance dose.
For weight loss, the approved maintenance doses are 5 mg, 10 mg, or 15 mg weekly, with 15 mg as the maximum. In clinical trials, higher doses produced more weight loss: participants lost an average of 15% of their body weight on 5 mg, 19.5% on 10 mg, and 20.9% on 15 mg over 72 weeks. Your prescriber will help find the dose that balances effectiveness with tolerability for you.
Common Side Effects From the Formula
The most frequent side effects are digestive: nausea, diarrhea, vomiting, and reduced appetite. These tend to be worst during dose increases and often improve as your body adjusts. The gradual titration schedule exists specifically to ease you through this. Injection site reactions like redness or itching occur in roughly 2% to 4.5% of users and are typically mild. Allergic reactions are uncommon, reported in under 3% of trial participants.
How to Store It
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, an unused pen can be kept at room temperature (up to 30°C or 86°F) for a maximum of 30 days. Once you’ve started using a multi-dose pen, you can also store it at room temperature for up to 30 days. Don’t freeze Zepbound, and don’t use it if it’s been frozen or if the solution looks cloudy or contains particles.