Tirzepatide is delivered through pre-filled, single-use pens that make weekly injections straightforward for most people. The pens are designed so patients never need to draw up a dose or calculate units themselves—the dial or click mechanism automatically sets the correct amount of medication. This built-in safety feature eliminates guesswork and reduces the risk of dosing errors that can occur with vial-and-syringe insulin regimens.
Even so, many new users see “units” printed on the pen dial or read about units in forums and wonder how the milligram strength they were prescribed translates to the number they dial up. The confusion is understandable: tirzepatide is measured in milligrams (mg) on the label and prescription, but the pen display shows units. Knowing the exact relationship helps patients double-check that they are receiving the intended dose and builds confidence during the early weeks of treatment.
This article explains precisely how many units correspond to the 2.5 mg starting dose, why the pen is calibrated this way, how the units-to-milligrams conversion works across all available strengths, and what to watch for to ensure accurate dosing every week. With clear information, you can focus on the benefits of treatment rather than worrying about the mechanics of the pen.
Why Tirzepatide Pens Use “Units” Instead of Milligrams
The tirzepatide pen (Mounjaro or Zepbound) is engineered by Eli Lilly to deliver a fixed volume of solution for each dose strength. The concentration of tirzepatide in the solution is intentionally set so that the dial reads in “units” rather than milligrams. This design choice mirrors insulin pens, where units reflect a standardized volume of liquid rather than the actual weight of the active drug.
Each pen contains 0.5 mL of solution per dose click, but the amount of active tirzepatide dissolved in that 0.5 mL varies by strength. The pen’s internal gearing translates the dialed units into the correct volume of solution, ensuring the patient receives exactly 2.5 mg (or any other prescribed strength) when the dose is delivered. This system prevents users from having to perform manual conversions at home.
Because the concentration changes between strengths, the number of units needed to deliver 2.5 mg is not the same as the number needed for 5 mg, 7.5 mg, and so on. The pen automatically accounts for this difference through its mechanical design.
How Many Units Is 2.5 mg of Tirzepatide
The 2.5 mg starting dose of tirzepatide is delivered by dialing to 25 units on the pen. Every Mounjaro and Zepbound pen that contains the 2.5 mg strength is calibrated so that setting the dose selector to 25 units expels exactly 0.5 mL of solution containing 2.5 mg of active drug. The pen does not allow dialing beyond 25 units on the 2.5 mg pen.
This 25-unit setting is fixed and cannot be changed by the user—the pen is mechanically locked to deliver only the labeled strength when dialed to the maximum position. After injecting, the dose window returns to zero, and the pen locks until the next weekly use. Once the pen has been used for the first injection, it remains stable for 21 days (refrigerated or at room temperature up to 30°C).
Patients should never attempt to dial less than 25 units on a 2.5 mg pen in an effort to take a “smaller starting dose.” Doing so would deliver less than the intended 2.5 mg and is not how the pen is designed or tested.
Units-to-Milligrams Conversion Across All Tirzepatide Strengths
Each strength of the tirzepatide pen has its own fixed units setting that corresponds to the labeled milligram amount. The pen is engineered so that the maximum dial setting always delivers the correct dose for that specific pen.
- 2.5 mg pen: Dial to 25 units = 2.5 mg
- 5 mg pen: Dial to 50 units = 5 mg
- 7.5 mg pen: Dial to 75 units = 7.5 mg
- 10 mg pen: Dial to 100 units = 10 mg
- 12.5 mg pen: Dial to 125 units = 12.5 mg
- 15 mg pen: Dial to 150 units = 15 mg
The pattern is simple: the number of units equals the milligram strength multiplied by 10. This consistent 10:1 ratio (units to mg) applies across the entire product line and makes it easy to verify that the correct pen is being used for the prescribed dose.
Why the Units Are Fixed and Cannot Be Adjusted
The pens are intentionally designed as fixed-dose devices rather than variable-dose pens like some insulin products. This eliminates the risk of dialing an incorrect number of units and receiving the wrong milligram amount. Each pen is color-coded and labeled clearly with both the milligram strength and the corresponding unit number, reducing confusion at the time of injection.
Fixed dosing also simplifies manufacturing, storage, and pharmacy dispensing. Patients never need to perform math or remember conversion factors—the pen itself ensures that dialing to the maximum setting delivers the exact prescribed dose. This safety feature is especially valuable for patients who may have visual or cognitive challenges.
If a dose other than the labeled strength is needed (for example, during very slow titration in highly nausea-sensitive patients), the prescriber must write for a different pen strength rather than instructing the patient to dial a partial dose on a higher-strength pen.
Comparison of Tirzepatide Strengths and Their Corresponding Units
Here is a clear reference table showing every approved tirzepatide pen strength, its corresponding dial setting in units, and typical clinical use:
| Pen Strength (mg per dose) | Units Dialed on the Pen | Volume Delivered per Dose | Typical Use in Treatment Schedule | Average HbA1c Reduction (add-on therapy) | Average Weight Loss at 72 Weeks (obesity trials) | Common Time to Reach This Strength |
|---|---|---|---|---|---|---|
| 2.5 mg | 25 units | 0.5 mL | Starting dose (4 weeks) | 1.6–1.9 % | 5–8 % | Weeks 1–4 |
| 5 mg | 50 units | 0.5 mL | First maintenance dose | 1.8–2.1 % | 10–13 % | Weeks 5–8 |
| 7.5 mg | 75 units | 0.5 mL | Second escalation level | 2.0–2.3 % | 13–17 % | Weeks 9–12 |
| 10 mg | 100 units | 0.5 mL | Common maintenance dose | 2.1–2.4 % | 15–20 % | Weeks 13–16 |
| 12.5 mg | 125 units | 0.5 mL | Higher maintenance for max effect | 2.2–2.5 % | 18–21 % | Weeks 17–20 |
| 15 mg | 150 units | 0.5 mL | Maximum approved dose | 2.3–2.6 % | 19–22 % | Weeks 21+ |
Note: All pens deliver 0.5 mL of solution per dose. The concentration of tirzepatide increases with each higher strength, so the units-to-milligram ratio remains consistent (10 units = 1 mg).
Common Dosing Mistakes to Avoid
Never dial less than the maximum units on a pen in an attempt to take a “partial dose.” For example, dialing 12 units on a 2.5 mg (25-unit) pen would deliver only half the intended 2.5 mg. The pen is not designed or tested for partial dosing—doing so is off-label and may lead to inconsistent blood sugar or weight outcomes.
Do not use a higher-strength pen to take a lower dose by dialing fewer units. A 5 mg pen dialed to 25 units would deliver only half the intended 5 mg (2.5 mg), but the concentration is different from the 2.5 mg pen, so the delivered volume and active drug amount would not match exactly. Always use the pen that matches your prescribed milligram strength.
If you accidentally dial past the intended units or inject an incorrect dose, contact your doctor or pharmacist immediately. Do not attempt to compensate for an overdose by skipping the next dose—follow professional guidance to avoid hypoglycemia or other complications.
Practical Tips for Accurate Dosing Every Week
Store unopened pens in the refrigerator (2–8 °C / 36–46 °F) in their original carton to protect from light. After first use, you may keep the pen at controlled room temperature (up to 30 °C / 86 °F) for up to 21 days or continue refrigerating it—choose whichever is more convenient.
Always check the pen label and carton before injecting to confirm the strength matches your prescription. Look through the pen window to ensure the solution is clear, colorless to slightly yellow, and free of particles or discoloration.
Inject into the abdomen, thigh, or upper arm, rotating sites each week to minimize irritation. Pinch the skin lightly, insert the needle at a 90-degree angle, press the button fully, and hold for 5–10 seconds to ensure the full dose is delivered.
If you miss a dose, inject it as soon as possible within 4 days. If more than 4 days have passed, skip the missed dose and resume your regular weekly schedule on the next planned day. Do not take two doses within 3 days of each other.
Summary
The 2.5 mg starting dose of tirzepatide is delivered by dialing to exactly 25 units on the dedicated 2.5 mg pen. Every higher strength follows the same 10:1 ratio (10 units = 1 mg), so 5 mg = 50 units, 7.5 mg = 75 units, 10 mg = 100 units, 12.5 mg = 125 units, and 15 mg = 150 units. The pens are mechanically fixed-dose devices designed to eliminate user calculation and reduce dosing errors.
All strengths are priced the same per carton, so escalating the dose does not increase monthly costs. Always verify the milligram strength on the pen label matches your prescription, store correctly, and use the full dialed dose each week. With proper technique and storage, every injection delivers the exact intended amount of medication for consistent blood sugar control and weight-management support.
FAQ
How many units do I dial for the 2.5 mg dose of tirzepatide?
You dial to exactly 25 units on the 2.5 mg pen. The pen is mechanically locked so it cannot deliver more or less than 2.5 mg when dialed to the maximum setting.
Do higher doses require more units on the pen?
Yes—the units increase proportionally with the milligram strength. 5 mg = 50 units, 7.5 mg = 75 units, 10 mg = 100 units, 12.5 mg = 125 units, and 15 mg = 150 units. The pen for each strength is pre-calibrated to deliver the correct amount at its maximum dial setting.
Can I dial fewer units on a higher-strength pen to take a lower dose?
No—the pens are fixed-dose devices. Dialing fewer units on a higher-strength pen would deliver less than the intended milligram amount for that pen’s concentration. Always use the pen that matches your prescribed milligram strength.
Why does the pen show units instead of milligrams?
The pen is designed like insulin pens, where units reflect a standardized volume of liquid rather than the weight of the active drug. The internal gearing ensures that dialing the maximum units delivers the exact labeled milligram dose for that pen.
What should I do if I accidentally dial the wrong number of units?
If you dial past the maximum or inject an incorrect dose, do not attempt to compensate by skipping or doubling the next dose. Contact your doctor or pharmacist immediately for guidance. Do not reuse or adjust a pen that has already been injected.
Is the 2.5 mg pen different from the 5 mg pen besides the dose?
Yes—each strength has a different concentration of tirzepatide in the same 0.5 mL volume. The pen mechanics are calibrated specifically for that concentration so that the maximum dial setting (25 units for 2.5 mg, 50 units for 5 mg, etc.) always delivers the correct milligram amount.
How do I know I’m using the correct pen for my prescribed dose?
Check the label on the pen and carton—it clearly states the milligram strength and the corresponding units (e.g., “2.5 mg / 25 units per dose”). The pen color and labeling also differ by strength. Your pharmacy label should match the pen you receive. If anything looks incorrect, do not use the pen and contact your pharmacy immediately.
Dr. Hamza is a medical content reviewer with over 12+ years of experience in healthcare research and patient education. He specializes in evidence-based health information, medications, and chronic conditions. His reviews are grounded in trusted medical sources and current clinical guidelines to ensure accuracy, transparency, and reliability. Content reviewed by Dr. Hamza is intended for educational purposes and is not a substitute for professional medical advice.