Compounds / Tirzepatide
Tirzepatide
Mounjaro, Zepbound
Mounjaro and Zepbound are the FDA-approved US brands of tirzepatide1–2. It's a once-weekly injection that acts on two gut hormone receptors, GIP and GLP-11. Compounded tirzepatide vials are not FDA-approved, so FDA doesn't review them for safety, effectiveness or quality3.
- Taken
- Once a week, injection
- Half-life
- About 5 days
- US status
- FDA-approved (Mounjaro, Zepbound)
- Type
- GIP and GLP-1 agonist
What is tirzepatide and how does it work?
Tirzepatide is a weekly injection that switches on the receptors for two gut hormones, GIP and GLP-1. GLP-1 helps control appetite and how much you eat. Lab studies suggest GIP may add to that, and both receptors are found in the parts of the brain that regulate appetite2.
A fatty acid chain on the molecule makes it stick to albumin, a protein in your blood, which slows its removal. That gives a half-life of about 5 days, long enough for one dose a week1. Tirzepatide also slows how fast your stomach empties, which can change how some tablets are absorbed1.
What is tirzepatide approved for?
In the US, tirzepatide is approved as Mounjaro for type 2 diabetes and as Zepbound for weight reduction and obstructive sleep apnea1–2. In the UK, one brand, Mounjaro, is licensed for both type 2 diabetes and weight management5.
| Use | US | UK |
|---|---|---|
| Type 2 diabetes | Mounjaro: adults and children 10 and older | Mounjaro: adults and children 10 and older |
| Heart attack, stroke and heart death risk | Mounjaro: adults with type 2 diabetes at high risk | Not a listed use |
| Weight management | Zepbound: adults with obesity, or overweight with at least one weight-related condition | Mounjaro: adults with a BMI of 30 or more, or 27 to under 30 with a weight-related condition |
| Obstructive sleep apnea | Zepbound: moderate to severe, in adults with obesity | Not a listed use |
Sources: Mounjaro and Zepbound US labels, section 1; UK Mounjaro SmPC, section 4.11–2,5. Compounded tirzepatide isn't FDA-approved for any use3.
The UK SmPC adds a review point for weight: if someone hasn't lost at least 5% of their starting weight 6 months after reaching their highest tolerated dose, the prescriber decides whether to carry on5.
What are the tirzepatide doses?
Tirzepatide starts at 2.5 mg once a week for 4 weeks, then goes up in 2.5 mg steps, with at least 4 weeks on each, to a maximum of 15 mg1–2. The 2.5 mg dose is only for starting. It isn't a maintenance dose2.
| Weeks (earliest) | Weekly dose | What the label says |
|---|---|---|
| 1 to 4 | 2.5 mg | Starting dose |
| 5 to 8 | 5 mg | Zepbound maintenance option |
| 9 to 12 | 7.5 mg | Step up |
| 13 to 16 | 10 mg | Zepbound maintenance option, and the lowest sleep apnea dose |
| 17 to 20 | 12.5 mg | Step up |
| 21 onward | 15 mg | Maximum for adults |
Each step can last longer than 4 weeks. Mounjaro goes up only if more blood sugar control is needed, and children stop at 10 mg1.
A pen delivers the dose it's set to. If you draw from a vial with a syringe, the tirzepatide dosage chart turns each dose into mL and units on a U-100 syringe.
What are the side effects of tirzepatide?
The most common side effects of tirzepatide are nausea, diarrhea, vomiting and constipation. In the Zepbound weight trials, 56% of people at each dose had a stomach or gut side effect, compared with 30% on placebo, and most nausea, vomiting and diarrhea happened while the dose was going up2.
| Side effect | Placebo | 5 mg | 10 mg | 15 mg |
|---|---|---|---|---|
| Nausea | 8% | 25% | 29% | 28% |
| Diarrhea | 8% | 19% | 21% | 23% |
| Vomiting | 2% | 8% | 11% | 13% |
| Constipation | 5% | 17% | 14% | 11% |
| Stomach (abdominal) pain | 5% | 9% | 9% | 10% |
| Indigestion (dyspepsia) | 4% | 9% | 9% | 10% |
| Injection site reactions | 2% | 6% | 8% | 8% |
| Tiredness (fatigue) | 3% | 5% | 6% | 7% |
| Burping (eructation) | 1% | 4% | 5% | 5% |
| Hair loss | 1% | 5% | 4% | 5% |
Between 4.8% and 6.7% stopped Zepbound because of side effects, vs 3.4% on placebo2. In the Mounjaro diabetes trials, nausea was 12% to 18% vs 4% on placebo1.
Both labels carry a boxed warning: tirzepatide caused thyroid C-cell tumors in rats, and it isn't known whether it does in people. It isn't used by anyone with a personal or family history of medullary thyroid cancer or with MEN 2. The other serious warnings are pancreatitis, gallbladder problems, kidney injury from dehydration, serious allergic reactions, low blood sugar when taken with insulin or a sulfonylurea, eye problems in people with type 2 diabetes, and food entering the lungs under anesthesia2. The GLP-1 side effects guide covers how long each one tends to last.
How do you take tirzepatide, and what if you miss a dose?
Tirzepatide is injected under the skin once a week, at any time of day, with or without food. The labels list the belly and thigh, or the back of the upper arm if someone else gives it, and say to rotate sites each week1–2.
- Missed dose: take it within 4 days (96 hours). After that, skip it and take the next one on your usual day1–2.
- Changing your day: fine, as long as there are at least 3 days (72 hours) between two doses1–2.
- Birth control pills: the Zepbound label advises switching to a non-oral method, or adding a barrier method, for 4 weeks after starting and 4 weeks after each dose increase2.
Store it in the fridge at 2°C to 8°C (36°F to 46°F) and never freeze it. Single-dose pens and vials can stay out at up to 30°C (86°F) for 21 days in total. Multi-dose vials and KwikPens are thrown away 30 days after first use or after 4 doses, whichever comes first1. The missed dose guide compares this rule with other GLP-1 medicines.
How much weight do people lose on tirzepatide?
In the main 72-week weight trial, people without diabetes lost an average of 15.0% of their body weight on 5 mg, 19.5% on 10 mg and 20.9% on 15 mg, compared with 3.1% on placebo2. Everyone in that trial also had diet and activity counseling.
| Trial | Who took part | On tirzepatide | Comparison |
|---|---|---|---|
| Study 1 (SURMOUNT-1), 72 weeks2 | Adults with obesity or overweight, no diabetes | −15.0% (5 mg), −19.5% (10 mg), −20.9% (15 mg) | −3.1% on placebo |
| Study 2, 72 weeks2 | Adults with overweight or obesity and type 2 diabetes | −12.8% (10 mg), −14.7% (15 mg) | −3.2% on placebo |
| Study 3, 72 weeks after a 12-week diet lead-in2 | Adults who had already lost at least 5% with diet | −18.4% more (10 or 15 mg) | +2.5% on placebo |
| Study 4, weeks 36 to 882 | Adults who had taken 10 or 15 mg for 36 weeks | −5.5% more if they kept going | +14.0% regained on placebo |
| SURPASS-1, 40 weeks1 | Adults with type 2 diabetes on diet and exercise only | HbA1c down 1.7 to 1.8 points (5 to 15 mg) | HbA1c down 0.1 on placebo |
Results vary a lot from person to person. On 15 mg, 56.7% lost at least a fifth of their weight, while 9.1% lost less than 5%2. In the sleep apnea trials, breathing pauses per hour (the apnea-hypopnea index) fell by 50.7% and 58.7% on tirzepatide, vs 3% and 2.5% on placebo2. Week-by-week averages are in GLP-1 weight loss by week.
For heart risk, a trial of 13,294 adults with type 2 diabetes and heart disease compared Mounjaro with dulaglutide (Trulicity), another GLP-1 medicine. Mounjaro was no worse at preventing heart attacks, strokes and heart deaths, but it wasn't shown to be better1.
How does tirzepatide compare with semaglutide?
In SURMOUNT-5, the one head-to-head trial, adults with obesity lost an average of 20.2% of their weight on tirzepatide and 13.7% on semaglutide after 72 weeks. The trial was open-label and funded by Eli Lilly, which makes tirzepatide7. Wegovy vs Zepbound sets out the doses and side effects side by side.
Mounjaro and Zepbound are the same medicine in the same strengths, licensed for different uses in the US1–2. The Mounjaro vs Zepbound guide explains where they differ.
Tirzepatide fact sheet
- Also called
- Mounjaro (type 2 diabetes)1 and Zepbound (weight and sleep apnea)2 in the US. Mounjaro KwikPen in the UK5.
- Type
- GIP and GLP-1 receptor agonist, given as an injection under the skin1
- Status (US)
- Mounjaro: type 2 diabetes in adults and children 10 and older, and lowering heart and stroke risk in high-risk adults with type 2 diabetes1. Zepbound: weight reduction and moderate to severe obstructive sleep apnea in adults with obesity2. Compounded versions are not FDA-approved3.
- Status (UK)
- Mounjaro KwikPen is licensed for type 2 diabetes (age 10 and up) and for weight management in adults with a BMI of 30 or more, or 27 or more with a weight-related condition5
- How it's taken
- Once a week, under the skin of the belly, thigh or back of the upper arm, at any time of day, with or without food1
- Doses on the label
- Start at 2.5 mg once weekly. Go up in 2.5 mg steps, at least 4 weeks apart. Maximum 15 mg once weekly for adults.1
- Storage
- Fridge, 2°C to 8°C (36°F to 46°F). Single-dose pens and vials can be kept up to 30°C (86°F) for a total of 21 days. Multi-dose vials and KwikPens: throw away 30 days after first use or after 4 doses.1
Mixing math for Tirzepatide
For a vial you mix yourself. Example numbers only: change them to match your own vial and dose. This does the math; it doesn't recommend a dose.
The mg printed on the vial label.
Bacteriostatic (bac) water, in mL.
Check whether it says mcg or mg. They differ by 1,000 times.
It's printed on the packet: 0.3, 0.5 or 1 mL.
10 mg vial + 2 mL water → 2.5 mg dose
Fill your syringe to
50units
The 50 line on a 1 mL syringe, which is 0.5 mL.
- Strength after mixing
- 5 mg/mL
- Each unit holds
- 50 mcg
- Doses in this vial
- 4
Show the math
- 10 mg × 1,000 = 10,000 mcg in the vial
- 10,000 mcg ÷ 2 mL = 5,000 mcg/mL
- 2,500 mcg ÷ 5,000 mcg/mL = 0.5 mL
- 0.5 mL × 100 units/mL = 50 units
Not what you expected? Check the numbers above, and confirm with your prescriber or pharmacist before you inject.
Tirzepatide guides
- Tirzepatide dosage chart: mg, mL and unitsPrescriptions say mg and syringes say units. This chart lines up each Mounjaro and Zepbound label dose with its mL and U-100 units.
- Mounjaro pen doses in the UKEvery UK Mounjaro pen gives 4 fixed doses, and the liquid left at the end isn't a 5th.
- The Mounjaro "golden dose": what the label, MHRA and Lilly saySome medicine is meant to be left in a Mounjaro KwikPen after the 4th dose. Every official source we checked says it isn't a 5th dose.
- Zepbound vials vs pens: single-dose, multi-dose and KwikPen comparedZepbound now comes four ways in the US. The medicine and doses are the same in each. What changes is the volume, what else you need, and how long it keeps.
Units for each dose: 2.5 mg · 5 mg · 7.5 mg · 10 mg · 12.5 mg · 15 mg
Also useful: Dose schedule planner · Missed dose checker · Units to dose
Questions
Are Mounjaro and Zepbound the same medicine?
Yes. Both contain tirzepatide and come in the same strengths, from 2.5 mg to 15 mg. Mounjaro is approved for type 2 diabetes and Zepbound for weight reduction and obstructive sleep apnea with obesity. The Zepbound label says using it with other tirzepatide products or any GLP-1 medicine isn't recommended.
Is compounded tirzepatide FDA-approved?
No. FDA says compounded drugs are not FDA-approved and are not reviewed for safety, effectiveness or quality before they're sold. FDA declared the tirzepatide shortage resolved on December 19, 2024, and the grace periods for compounders ended in 2025. In April 2026 FDA also proposed leaving tirzepatide off the list of ingredients outsourcing facilities may compound from.
How many units is 2.5 mg of tirzepatide?
At 10 mg/mL, 2.5 mg is 25 units on a U-100 insulin syringe. At 5 mg/mL it's 0.5 mL, or 50 units (100 units = 1 mL), so the number depends on your vial's strength. Our tirzepatide dosage chart lists every label dose at common strengths.
What if I miss a tirzepatide dose?
The Mounjaro and Zepbound labels say to take the missed dose as soon as possible within 4 days (96 hours). If more than 4 days have passed, skip it and take the next dose on your usual day. Keep at least 3 days (72 hours) between two doses.
How long does tirzepatide stay in your body?
The labels give a half-life of about 5 days (5 to 6 days in people with overweight or obesity). So the amount in your body drops by about half every 5 days. Our how-long-in-system tool shows the curve for weekly doses.
My compounded vial is already liquid. Do I add water?
If the label shows a strength in mg/mL, the vial is already mixed. In the reconstitution calculator, choose "Already liquid" and enter that strength. Only add water if your pharmacy's written instructions say to, and ask the pharmacy if the label is unclear.
Sources
- 1Mounjaro (tirzepatide) injection: US prescribing information, Eli Lilly (DailyMed, revised 08/2026). sections 1, 2.1, 2.2, 3, 6.1, 12.3, 14 and 16.2
- 2Zepbound (tirzepatide) injection: US prescribing information, Eli Lilly (DailyMed, revised 08/2026). sections 1, 2.1 to 2.4, 3, 5, 6.1, 7.2, 12.1, 12.3 and 14
- 3FDA: FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. content current as of October 1, 2026
- 4FDA: FDA clarifies policies for compounders as national GLP-1 supply begins to stabilize. updates of December 19, 2024, March 10, 2025 and April 1, 2026
- 5Mounjaro KwikPen solution for injection in pre-filled pen: Summary of Product Characteristics (emc, revised August 29, 2026). sections 4.1 and 4.2
- 6FDA news release: FDA Proposes to Exclude Semaglutide, Tirzepatide, and Liraglutide on 503B Bulks List (April 30, 2026)
- 7Aronne LJ, et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5). N Engl J Med 2025;393:26-36. doi:10.1056/NEJMoa2416394; abstract
This page explains what labels and published studies say. It isn't medical advice and doesn't replace your prescriber or pharmacist.
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