Tirzepatide: Zepbound and Mounjaro Compared
One molecule, two brand names, two separate approvals. It produces the largest weight reduction of any approved drug in this class — and the trial nobody quotes shows what happens when people stop.
- Two receptorsGIP as well as GLP-1 — the only dual agonist approved for weight management
- −20.9%Average body weight change at 72 weeks on 15 mg, from the FDA label
- +14%Weight regained over a year by people who stopped, in the label’s own withdrawal trial
- Sleep apneaThe only one of these drugs also approved to treat moderate to severe OSA
Regulatory status
Tirzepatide is FDA-approved and prescription-only, sold as Zepbound for weight management and obstructive sleep apnea, and as Mounjaro for type 2 diabetes. Initial United States approval 2022. It carries a boxed warning about thyroid C-cell tumours. Compounded tirzepatide is a separate matter and is not the approved product.
The short version
Semaglutide acts on one receptor. Tirzepatide acts on two — GLP-1, and a second gut hormone receptor called GIP. That is the entire structural difference between them, and it is the most plausible explanation for why the weight reductions in its trials are larger.
Like semaglutide, it is sold under two names for two different jobs. Zepbound is the weight-management product. Mounjaro is the diabetes product. Same molecule, separate applications, and insurance treats them as unrelated — which is where most of the practical difficulty lives.
It is also the most expensive way to buy a GLP-1 direct from its manufacturer, and the only one carrying a refill deadline that changes the price if you miss it. Both of those are covered further down.
Every approved tirzepatide product
Read from the FDA approvals record and the approved prescribing information, not from marketing material. Each application is a separate approval with its own indication.
- Zepbound — injection. NDA 217806. Eli Lilly and Company. The weight-management product, and since December 2024 also approved to treat moderate to severe obstructive sleep apnea in adults with obesity. The same application covers the KwikPen. Supplied as single-dose vials, single-dose pens and the multi-dose KwikPen.
- Mounjaro — injection. NDA 215866. The type 2 diabetes product. Same molecule as Zepbound, different approval, and not approved for weight management — so prescribing it for weight loss is off-label, exactly as with Ozempic and semaglutide.
- Strengths. Six, from 2.5 mg to a maximum of 15 mg injected once weekly. Which strength applies to a given person, and how anyone moves between them, is set by the prescribing information and the prescriber, not by this page.
- A limitation of use. The label states that taking it alongside another tirzepatide product, or any GLP-1 receptor agonist, is not recommended. Relevant to anyone considering moving between drugs in this class.
On the boxed warning
The label states plainly that in rats, tirzepatide causes thyroid C-cell tumours, and that whether it does so in humans is unknown. That is a stronger statement than the equivalent warning on orforglipron, whose label records that it produced no tumours in rodents and is not even pharmacologically active in them. The warnings look identical at a glance; the evidence behind them is not.
What the weight trials showed
Study 1 (NCT04184622) ran 72 weeks and enrolled 2,539 adults with obesity, or overweight plus a weight-related condition, excluding people with type 2 diabetes. Study 2 covered people who did have type 2 diabetes. Every participant in both also received diet and activity counselling, so the placebo column is not “nothing”.
| Study 1 — no diabetes | Placebo | 5 mg | 10 mg | 15 mg |
|---|---|---|---|---|
| Average body weight change | −3.1% | −15.0% | −19.5% | −20.9% |
| Lost 10% or more | 18.8% | 68.5% | 78.1% | 83.5% |
| Lost 15% or more | 8.8% | 48.0% | 66.6% | 70.6% |
| Lost 20% or more | 3.1% | 30.0% | 50.1% | 56.7% |
The difference from placebo at 15 mg was 17.8 percentage points, with a 95% confidence interval of 16.3 to 19.3. More than half of that group lost at least a fifth of their body weight, which no other approved drug in this class has matched in its own trials.
| Study 2 — with type 2 diabetes | Placebo | 10 mg | 15 mg |
|---|---|---|---|
| Average body weight change | −3.2% | −12.8% | −14.7% |
| Lost 10% or more | 9.5% | 60.5% | 64.8% |
| Lost 20% or more | 1.0% | 21.5% | 30.8% |
Weight reduction is consistently smaller in people with type 2 diabetes — roughly six percentage points here. The same gap appears with orforglipron and across the class, so the headline figure is not the one to expect if you have diabetes.
How solid are these numbers
At week 72 in Study 1, body weight was missing for 21.6% of the placebo group and about 10% of each treatment group. Those values were filled statistically rather than measured, which the label states openly. A dropout gap that lopsided is worth knowing about, because the people who leave a weight-loss trial are unlikely to be a random sample of it.
What happens when people stop
This is the trial that rarely makes the headlines, and it is arguably the most useful one on the label. Study 4 (NCT04660643) gave everyone tirzepatide openly for 36 weeks, then split them: half carried on, half switched to placebo without knowing. It then followed both for another 52 weeks.
Mean weight at the start was 107.3 kg. By week 36, on the drug, it was around 85 kg. Over the following year the group that stopped regained 14.0% of their body weight. The group that continued lost a further 5.5%. The gap between them was 19.4 percentage points.
What this means in practice
This is not a course of treatment that finishes. On the label’s own evidence, stopping reverses a substantial part of the result within a year. Anyone weighing the monthly cost should price it as an ongoing commitment rather than a temporary one — which is exactly why the annual arithmetic further down matters more than the monthly figure.
One further detail from the same trial: of the 783 people who started, 14.4% stopped before the randomisation point at week 36, and adverse events were the most common reason, at 6.8%. Tolerability is not a footnote in this class — on orforglipron, the oral option, one in ten stopped at the highest dose for the same reason.
The sleep apnea approval, which is genuinely unusual
In December 2024 Zepbound gained a second indication: treating moderate to severe obstructive sleep apnea in adults with obesity. No other drug in this class has it, and it is the clearest case of a weight-management drug being approved to treat something other than weight.
Two 52-week placebo-controlled trials support it (NCT05412004), enrolling 469 adults between them. One studied people unable or unwilling to use PAP therapy; the other studied people already on PAP, who suspended it for seven days before the measurement. The measure was the apnea-hypopnea index — breathing interruptions per hour of sleep.
| Apnea-hypopnea index at 52 weeks | Not using PAP | Using PAP |
|---|---|---|
| Baseline events per hour, on the drug | 52.9 | 46.1 |
| Change on tirzepatide | −25.3 | −29.3 |
| Change on placebo | −5.3 | −5.5 |
| Reduced their AHI by half or more | 61.2% | 72.4% |
| Reached remission or mild, non-symptomatic OSA | 42.2% | 50.2% |
Roughly half the events per hour, and between four and five in ten participants reaching remission or mild non-symptomatic disease. For anyone who cannot tolerate PAP, that is a materially different proposition from a weight-loss drug that happens to help.
What it costs without insurance
Lilly publishes self-pay prices for every strength, and they now apply equally to the vial, the single-dose pen and the KwikPen. The higher strengths carry a condition attached to nothing but the calendar.
| Strength | Refilled within 45 days | Refilled later |
|---|---|---|
| 2.5 mg | $299 | — |
| 5 mg | $399 | — |
| 7.5 mg | $449 | $499 |
| 10 mg | $449 | $699 |
| 12.5 mg | $449 | $699 |
| 15 mg | $449 | $699 |
A year costs more than twelve months
Lilly’s own terms define a month here as 28 days — one KwikPen, or four weekly vials. Refilling every 28 days means about 13 fills in a year, not 12. At $449 that is roughly $5,850 a year rather than the $5,388 a twelve-month assumption produces. The oral drugs in this class are billed in 30-day months, so they genuinely are about 12.
To hold the $449 rate you must complete each refill within 45 days of the previous one. Against a 28-day supply that leaves a little over two weeks of slack — workable, but a delayed delivery, a lapsed prescription or a pause for side effects can cost the difference. Our GLP-1 cost comparison puts every product’s self-pay pricing side by side.
Where it sits in the class
On the evidence available, tirzepatide produces the largest weight reduction of the approved options, and costs the most. Semaglutide sits below it on both. Orforglipron sits below both on effect and well below on price, and is a tablet rather than an injection.
Tirzepatide against semaglutide is the one comparison that does not rely on reading across separate trials, because the two have been tested head to head. We cover that directly in Wegovy vs Zepbound. Everything else in this class is inference across trials with different populations, and should be read that way.
What we have looked at in detail
- Tirzepatide side effects. Every reaction the label reports in 2% or more of patients, dose by dose against placebo, how many people stopped and when, and the warning that does not appear on every label in this class.
- Compounded tirzepatide. The shortage that made it lawful ended in December 2024, the compounders lost in court, and the FDA has logged more than 730 adverse event reports it says are underreported.
- Cost without insurance. Zepbound’s self-pay ladder, the 45-day refill condition, and why a year is about 13 fills rather than 12.
- Wegovy vs Zepbound. The one comparison in this class that rests on a head-to-head trial rather than on reading across separate ones.
What this site will not tell you
- What amount to inject, or how long to spend at each strength. The label sets that out for prescribers, and reproducing it here would function as instruction however carefully it were worded.
- Whether to ask for Zepbound or Mounjaro. They are approved for different conditions, and which applies to you is a clinical question rather than a pricing one.
- Anything about compounded tirzepatide dosing. Concentrations vary by pharmacy, so a figure taken from an approved pen can be wrong and dangerous applied to a compounded vial.
- Where to buy it without a prescription. Every approved tirzepatide product is prescription-only, and anything sold otherwise is not one of them.
Sources and method
- Indications, strengths, boxed warning and every trial figure. The approved prescribing information for Zepbound (tirzepatide), NDA 217806, read in full from the FDA’s own document archive. Weight, withdrawal and sleep apnea figures are taken from the clinical studies section of that label.
- Trial registrations. NCT04184622 (weight, Study 1), NCT04660643 (randomised withdrawal, Study 4) and NCT05412004 (the sleep apnea master protocol).
- Application numbers. Drugs@FDA, queried by brand name, for Zepbound and Mounjaro.
- Self-pay pricing and the refill condition. LillyDirect — Zepbound, read from the manufacturer’s own pharmacy pages rather than a price-comparison site, including the terms defining a month as 28 days.
- What we could not verify. Whether additional dispensing or shipping fees apply at checkout, current availability of individual strengths, and eligibility conditions for the insured pricing routes. We have not reproduced the label’s adverse-reaction tables; anyone weighing this drug up should read them with a prescriber.
- Method. Figures come from the approved label rather than from manufacturer announcements or press coverage, and the label’s intent-to-treat numbers are used throughout, including where a more flattering on-treatment analysis exists. Missing-data rates are reported alongside the results rather than omitted. Annual costs multiply the monthly price by the number of fills the manufacturer’s own definition of a month actually produces in a year.
GLP-1 medications → Wegovy vs Zepbound → Cost without insurance →
FDA label and LillyDirect pricing read 5 September 2026. We recheck this page when the label is revised, when self-pay pricing changes, or when a new indication is approved.