# Tirzepatide Explained: Mounjaro vs Zepbound, Label Doses and Results

> Tirzepatide is a once-weekly injected peptide from Eli Lilly that activates both GIP and GLP-1 receptors. It is sold as Mounjaro for type 2 diabetes and Zepbound for weight management and obstructive sleep apnea. Both start at 2.5 mg and can rise in 2.5 mg steps, at least 4 weeks apart, to a maximum of 15 mg, as your prescriber decides.

- URL: https://glpcalendar.app/blog/tirzepatide
- Published: 2026-09-29
- Updated: 2026-09-29
- Publisher: GLP Calendar (https://glpcalendar.app)

Tirzepatide is the active ingredient in Mounjaro and Zepbound, two once-weekly injections made by Eli Lilly. It activates two hormone receptors, GIP and GLP-1, rather than GLP-1 alone. This guide covers how it works, how the two brands differ, the dose steps and devices on the US labels, the main trial results and the side effects, as of September 2026.

## How does tirzepatide work?

Tirzepatide is a single peptide that activates both the GIP and GLP-1 receptors, the targets of two natural gut hormones. Both receptors are found in brain areas that regulate appetite, and the drug decreases food intake. It also raises insulin and lowers glucagon when blood sugar is high, and it slows stomach emptying.

The molecule is built on the GIP hormone sequence and carries a fatty-acid chain that binds to albumin in the blood, which stretches its half-life to about 5 days. The label calls GLP-1 a physiological regulator of appetite and calorie intake and says nonclinical studies suggest GIP may add to that effect. The delay in stomach emptying is largest after the first dose and diminishes over time, which matters for birth control pills (see below).

## Mounjaro vs Zepbound: what is the difference?

Mounjaro and Zepbound contain the same drug, in the same strengths and devices, from the same company. The difference is the approved use: Mounjaro is for type 2 diabetes and, since August 2026, heart-risk reduction in diabetes, while Zepbound is for weight management and, since December 2024, obstructive sleep apnea.

| | Mounjaro | Zepbound |
|---|---|---|
| Approved uses | Type 2 diabetes (ages 10+); heart-risk reduction in type 2 diabetes | Weight reduction and long-term maintenance; sleep apnea with obesity |
| First US approval | May 13, 2022 | Nov 8, 2023 |
| Later additions | Children 10+ (Dec 2025); heart risk (Aug 2026) | Obstructive sleep apnea (Dec 20, 2024) |
| Maximum dose | 15 mg weekly (children: 10 mg) | 15 mg weekly |

- **Zepbound's weight indication** covers adults with obesity, or with overweight plus at least one weight-related condition. Its sleep apnea indication covers moderate to severe obstructive sleep apnea in adults with obesity; the FDA called it the first medicine approved for that condition.
- **Mounjaro's heart indication** rests on SURPASS-CVOT, which compared it with dulaglutide (Trulicity) in 13,299 adults with type 2 diabetes over a median of 4.1 years. Mounjaro had an 8% lower rate of heart attack, stroke or cardiovascular death (hazard ratio 0.92). That showed it was not worse than dulaglutide, but it did not prove it better.
- The Zepbound label says not to use it with other tirzepatide products or with any GLP-1 receptor agonist.

## What are the tirzepatide doses on the US label?

Both labels start tirzepatide at 2.5 mg once a week. The dose can then rise in 2.5 mg steps, each after at least 4 weeks on the current dose, to a maximum of 15 mg. For Zepbound, 2.5 mg is a starter dose only; maintenance doses are 5, 10 or 15 mg for weight and 10 or 15 mg for sleep apnea.

The Zepbound label's schedule:

1. Weeks 1–4: 2.5 mg once weekly, for starting treatment only.
2. From week 5: 5 mg once weekly.
3. If your prescriber decides to go higher: 7.5, 10, 12.5 and then 15 mg, with at least 4 weeks at each dose.

At that pace, the earliest week at 15 mg is week 21. Other label rules:

- The maintenance dose is chosen by response and tolerability. If a maintenance dose is not tolerated, the label says to consider a lower one.
- Mounjaro is increased only if more blood-sugar control is needed, up to 15 mg in adults and 10 mg in children aged 10 and older.
- Injections can be taken at any time of day, with or without meals. A missed dose can be given within 4 days (96 hours); after that, skip it. The weekly day can change if doses stay at least 3 days apart. Our [missed-dose guide](https://glpcalendar.app/blog/missed-glp-1-dose) has the details.

These are label facts, not instructions. Your prescriber decides whether and when to move up.

## Which tirzepatide devices are sold in the US?

The US labels list four presentations: a single-dose pen, a single-dose vial, a four-dose KwikPen and a four-dose multi-dose vial. For Zepbound, Lilly sells the single-dose pen, single-dose vials and, since February 23, 2026, the KwikPen. We could not confirm that the multi-dose vial is stocked by US pharmacies as of September 2026.

| Device | Doses per device | Dose volume | Label limit out of the fridge |
|---|---|---|---|
| Single-dose pen | 1 | 0.5 mL | Up to 21 days total at up to 86°F |
| Single-dose vial | 1, drawn with a syringe | 0.5 mL | Up to 21 days total at up to 86°F |
| KwikPen | 4 weekly doses | 0.6 mL | 30 days after first use, or 4 doses |
| Multi-dose vial | 4 weekly doses | 0.6 mL | 30 days after first use, or 4 doses |

The label's full rule for the KwikPen and multi-dose vial: throw it away after a total of 30 days at room temperature, 30 days after first use, or after 4 weekly doses, even if medicine is left. An unopened one kept at room temperature is discarded after 30 days.

Each KwikPen holds one strength, from 2.5 mg to 15 mg per dose. The instructions say to prime it before each weekly injection and not to count clicks when setting the dose. After four doses the pen will not dial a full dose, and the leftover medicine must not be injected or moved into a syringe. There is no clinical evidence for "microdosing" with partial doses. The label also says the KwikPen is not recommended for self-injection by people who are visually impaired, and a pen must never be shared, even with a new needle.

Lilly launched the Zepbound KwikPen through its LillyDirect self-pay pharmacy, alongside single-dose vials, and Kroger announced retail availability in March 2026. For sites, rotation and storage, see our [injection sites and storage guide](https://glpcalendar.app/blog/glp-1-injection-sites-and-storage).

## How much weight do people lose on tirzepatide?

In SURMOUNT-1, adults without diabetes lost an average of 15.0% of body weight on 5 mg, 19.5% on 10 mg and 20.9% on 15 mg over 72 weeks, versus 3.1% on placebo. People with type 2 diabetes lost less. These are averages, and individual results vary widely.

| Trial | Who and how long | Tirzepatide | Comparator |
|---|---|---|---|
| SURMOUNT-1 | No diabetes, 72 weeks | 5 mg −15.0%; 10 mg −19.5%; 15 mg −20.9% | Placebo −3.1% |
| SURMOUNT-2 | Type 2 diabetes, 72 weeks | 10 mg −12.8%; 15 mg −14.7% | Placebo −3.2% |
| SURMOUNT-5 | No diabetes, 72 weeks | 10 or 15 mg: −20.2% | Semaglutide 1.7 or 2.4 mg: −13.7% |
| SURMOUNT-4 | Weeks 36–88, after 36 weeks on tirzepatide | Continued: −5.5% | Switched to placebo: +14.0% |

- In SURMOUNT-1, 57% of people on 15 mg lost at least 20% of their weight, versus 3% on placebo.
- SURMOUNT-5 was an open-label, Lilly-funded, head-to-head trial in 751 adults, each group on its maximum tolerated dose. Waist size fell 18.4 cm (about 7.2 inches) with tirzepatide versus 13.0 cm (about 5.1 inches) with semaglutide. Our [three-way comparison](https://glpcalendar.app/blog/semaglutide-vs-tirzepatide-vs-retatrutide) covers approval status as well.
- In SURMOUNT-4, everyone first took tirzepatide for 36 weeks and lost 20.9% on average. By week 88, 89.5% of those who continued had kept at least 80% of that loss, versus 16.6% of those switched to placebo.

### What did the 2026 maintenance trial find?

SURMOUNT-MAINTAIN, reported by Lilly in May 2026, tested whether a lower dose can hold weight loss. After 60 weeks at 10 or 15 mg, adults who stayed on that dose for 52 more weeks kept all of their prior loss on average, while those moved to 5 mg regained an average of 5.6 kg (about 12 lb). Both groups did better than placebo. These are company-reported results, and any dose reduction is your prescriber's decision.

## What are the side effects of tirzepatide?

Stomach and bowel symptoms are the most common. In the Zepbound weight trials, 56% of people on each dose had a gastrointestinal side effect versus 30% on placebo. The label says most nausea, vomiting and diarrhea happened during dose escalation and decreased over time.

| 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% |
| Injection-site reactions | 2% | 6% | 8% | 8% |
| Hair loss | 1% | 5% | 4% | 5% |

- Hair loss was reported in 7.1% of women versus 0.5% of men on Zepbound (1.3% versus 0% on placebo), and the label links it to weight reduction.
- Stomach side effects led 1.9%, 3.3% and 4.3% of people on 5, 10 and 15 mg to stop treatment, versus 0.5% on placebo.
- With Mounjaro in type 2 diabetes, nausea affected 12%, 15% and 18% on 5, 10 and 15 mg, versus 4% on placebo.

Our [side-effects timeline](https://glpcalendar.app/blog/glp-1-side-effects-timeline) explains how these usually change week by week.

### Boxed warning and red flags

Both labels carry a boxed warning: tirzepatide caused thyroid C-cell tumors in rats, and it is unknown whether it does in humans. The labels say not to use it if you or a family member has had medullary thyroid carcinoma, or if you have multiple endocrine neoplasia syndrome type 2.

The Zepbound Medication Guide says to contact your healthcare provider right away for:

- Severe stomach pain that will not go away, with or without vomiting (possible pancreatitis).
- Upper stomach pain, fever, yellow skin or eyes, or clay-colored stools (possible gallbladder problems).
- Nausea, vomiting or diarrhea that does not go away, which can cause dehydration and kidney problems.
- Swelling of the face, lips, tongue or throat, trouble breathing or swallowing, fainting or a very rapid heartbeat. Get medical help right away for a possible serious allergic reaction.
- Signs of low blood sugar, such as shakiness, sweating or confusion, if you also use insulin or a sulfonylurea.

Tell every care team that you take tirzepatide before any surgery or sedation.

### Birth control pills

The US labels say tirzepatide may reduce the effectiveness of birth control pills because of delayed stomach emptying, which is largest after the first dose. They advise switching to a non-oral method, or adding a barrier method, for 4 weeks after starting and for 4 weeks after each dose increase. The Zepbound label also says to discontinue it when pregnancy is recognized.

## How long does tirzepatide stay in your body?

Tirzepatide's half-life is about 5 days; the Zepbound label gives 5 to 6 days in people with overweight or obesity. That is why it is dosed once a week. Blood levels reach a steady state after 4 weeks on a dose and peak a median of 24 hours after each injection.

Semaglutide, by comparison, has a half-life of about one week. Our [semaglutide guide](https://glpcalendar.app/blog/semaglutide) covers Ozempic, Wegovy and the pills.

## What to do next

A few practical steps help you use tirzepatide as labeled and get more out of prescriber visits.

- **Know your product and device.** Mounjaro and Zepbound share one drug but have different approved uses, and KwikPen doses (0.6 mL) differ in volume from single-dose pens and vials (0.5 mL).
- **Log every dose and step-up date.** A tracker such as GLP Calendar can log weekly shots, suggest the next injection site from your history and build a PDF for appointments.
- **Mark contraception windows.** If you take birth control pills, note the 4-week backup periods after starting and after each increase.
- **Watch discard dates.** A KwikPen or multi-dose vial goes after 30 days or 4 doses, whichever comes first.
- **Act on red flags and ask before changes.** Call your prescriber about warning signs, and talk to them before changing doses, devices or brands.

## Frequently asked questions

### Are Mounjaro and Zepbound the same drug?

Yes. Both contain tirzepatide made by Eli Lilly, in the same strengths. The difference is the approved use: Mounjaro for type 2 diabetes and heart-risk reduction in diabetes, Zepbound for weight management and obstructive sleep apnea.

### Why is the 2.5 mg dose only used for the first month?

The Zepbound label says 2.5 mg is for starting treatment and is not approved as a maintenance dose. Its step-up schedule is meant to reduce the risk of stomach side effects, and it places the first increase, to 5 mg, after 4 weeks.

### Do I need backup birth control on tirzepatide?

If you take birth control pills, the US labels advise switching to a non-oral method or adding a barrier method for 4 weeks after starting and for 4 weeks after each dose increase. The label says hormonal contraceptives that are not taken by mouth should not be affected.

### Can I count clicks on the Zepbound KwikPen to take a smaller dose?

No. The KwikPen instructions say not to count clicks when setting a dose, and the pen is designed to deliver four full, fixed doses. There is no clinical evidence supporting 'microdosing' with partial doses.

### Does tirzepatide cause hair loss?

In the Zepbound weight-loss trials, hair loss was reported in 4% to 5% of people on tirzepatide versus 1% on placebo, and far more often in women (7.1%) than men (0.5%). The label says it was associated with weight reduction.

### What happens to weight after stopping tirzepatide?

In SURMOUNT-4, people switched to placebo after 36 weeks on tirzepatide saw their weight rise by an average of 14% over the next year, while those who continued lost another 5.5%. Stopping or changing treatment is a decision to make with your prescriber.

## Sources

- [Zepbound (tirzepatide) US prescribing information and Medication Guide (rev. 08/2026), DailyMed](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=487cd7e7-434c-4925-99fa-aa80b1cc776b)
- [Mounjaro (tirzepatide) US prescribing information (rev. 08/2026), DailyMed](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=d2d7da5d-ad07-4228-955f-cf7e355c8cc0)
- [Drugs@FDA application and approval records for Mounjaro and Zepbound, FDA openFDA (accessed September 2026)](https://api.fda.gov/drug/drugsfda.json?search=products.active_ingredients.name:TIRZEPATIDE&limit=50)
- [FDA: FDA approves first medication for obstructive sleep apnea (December 20, 2024)](https://www.fda.gov/news-events/press-announcements/fda-approves-first-medication-obstructive-sleep-apnea)
- [Eli Lilly: FDA approves Mounjaro to reduce cardiovascular risk in adults with type 2 diabetes (press release, August 28, 2026)](https://www.biospace.com/press-releases/fda-approves-lillys-mounjaro-tirzepatide-to-reduce-cardiovascular-risk-in-adults-with-type-2-diabetes)
- [Eli Lilly: Zepbound now available in multi-dose KwikPen (press release, February 23, 2026)](https://www.prnewswire.com/news-releases/zepbound-tirzepatide-the-most-prescribed-weight-management-medication-in-2025-now-available-in-multi-dose-kwikpen-302693937.html)
- [Kroger: Zepbound KwikPen at retail pharmacy for self-pay patients (press release, March 13, 2026)](https://www.prnewswire.com/news-releases/kroger-brings-zepbound-kwikpen-to-retail-pharmacy-expanding-access-and-savings-for-self-pay-patients-302713400.html)
- [Jastreboff et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). NEJM, 2022](https://pubmed.ncbi.nlm.nih.gov/35658024/)
- [Garvey et al. Tirzepatide once weekly for obesity in people with type 2 diabetes (SURMOUNT-2). Lancet, 2023](https://doi.org/10.1016/S0140-6736(23)01200-X)
- [Aronne et al. Continued tirzepatide for maintenance of weight reduction (SURMOUNT-4). JAMA, 2024](https://jamanetwork.com/journals/jama/fullarticle/2812936)
- [Aronne et al. Tirzepatide as compared with semaglutide for the treatment of obesity (SURMOUNT-5). NEJM, 2025](https://www.nejm.org/doi/abs/10.1056/NEJMoa2416394)
- [Eli Lilly: SURMOUNT-MAINTAIN and ATTAIN-MAINTAIN results (press release, May 12, 2026)](https://www.prnewswire.com/news-releases/lillys-foundayo-and-lower-dose-zepbound-helped-people-maintain-weight-loss-after-switching-from-higher-doses-of-injectable-incretin-therapy-in-two-late-phase-trials-302769938.html)

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Educational content, not medical advice. GLP Calendar is a tracking app; it does not recommend or calculate doses.