Side by side
| Mounjaro (tirzepatide) | Ozempic (semaglutide) | |
|---|---|---|
| Receptors | GIP and GLP-1 | GLP-1 |
| Manufacturer | Eli Lilly | Novo Nordisk |
| Cadence | Weekly injection | Weekly injection |
| Elimination half-life | About 5 days | About 7 days |
| Dose strengths | 2.5, 5, 7.5, 10, 12.5, 15 mg | 0.25 to 2 mg, across four pens |
| How the dose is set | A separate product per strength | Dialled on the pen |
| Pen designs | KwikPen (4 doses) or single-use | Dial pen, 4 doses |
| Days usable once started | 30 (KwikPen) | 56 |
| Weight-management sibling | Zepbound | Wegovy |
| Oral version | None | Rybelsus and other tablets |
Availability, indication and licensing vary by country. This table is about the devices and the pharmacokinetics, not about which is prescribed for what where you live.
One receptor or two
Semaglutide is a GLP-1 receptor agonist. Tirzepatide acts at both the GLP-1 and the GIP receptor. That is the headline pharmacological difference and it is genuinely a difference in kind rather than in degree, but it is also the part where the honest answer is: ask your clinician what it means for you. What it does not do is change how you track the medication.
The difference you can actually feel: the shape of the week
This is where the two drugs diverge in daily life, and it comes entirely from the half-life.
Semaglutide has a half-life of about seven days against a seven-day dosing interval. Half of each dose is still there when the next lands, the level rises to roughly twice a single dose at steady state, and the week is comparatively flat.
Tirzepatide has a half-life of about five days against the same seven-day interval. More is lost between doses, the level settles at around 1.6 times a single dose, and the peak-to-trough swing across the week is deeper.
In practice that means the tirzepatide week tends to have a shape: strongest in the first two or three days after the shot, lighter towards the end. Semaglutide is steadier. People who switch notice this before they notice anything else, and it is the most common thing they are told they are imagining. The full arithmetic is in tirzepatide half-life and semaglutide half-life.
How the dose is set, and why that matters more than it sounds
Ozempic is a dial pen. One device covers several doses and you turn the dial. Tirzepatide is dispensed as six separate products, one per strength, and a KwikPen delivers a single fixed volume that means whatever the strength in the box means.
- Changing dose on Ozempic can mean staying on the same pen, or moving to the next pen up when you outgrow its maximum.
- Changing dose on Mounjaro always means a different product, a new prescription line and a new box.
- Splitting or reducing a dose is mechanically simple on a dial pen and awkward on a fixed-dose one.
- A click is worth a different amount of medicine on nearly every pen in both families, because a detent moves a fixed volume rather than a fixed number of milligrams. The pen click chart sets out every one.
The pen clock: 30 days against 56
A started Ozempic pen may be used for 56 days. A started Mounjaro KwikPen, 30. Both hold four weekly doses, so the semaglutide pen has four weeks of slack and the tirzepatide one has two days.
That difference is invisible until the week you shift a shot, hold a dose or run a pen at a lower strength – and then it decides whether your pen expires with liquid still in it. It is one of the more common ways people lose medicine. How long a GLP-1 pen lasts covers every product, and Penwise counts both clocks so the collision is visible before it happens.
What a missed dose costs on each
Because semaglutide loses about nine per cent a day and tirzepatide about thirteen, lateness is not equally expensive.
| How late | Semaglutide | Tirzepatide |
|---|---|---|
| 1 day | Barely detectable | Small but real dip |
| 3 days | Still inside the normal weekly range | Approaching the bottom of a normal trough |
| A full week missed | About a quarter of a dose left, recovers over a fortnight | About an eighth left, the following week often feels like restarting |
Label guidance on what to do about a late dose differs by product and is on your leaflet. See missed doses.
If you switch
Switching between them is common – supply, tolerance, cost, response. Two things are worth knowing.
First, the milligram numbers have nothing to do with each other. 1 mg of semaglutide and 10 mg of tirzepatide are not two points on the same scale, and no arithmetic converts one into the other. Your prescriber sets the starting step on the new drug, usually near the bottom of its ladder regardless of where you were.
Second, the old drug is still there for weeks. Stopping semaglutide leaves half of the last dose after seven days and a quarter after fourteen. If you start the new one immediately, the first fortnight is both drugs overlapping, which is worth knowing before you interpret how you feel. A tracker that keeps both medications in one dated timeline makes that period readable instead of confusing.
Tracking either one
Penwise models both properly rather than treating them as one generic “GLP-1”: the right half-life, the right ladders, the right pens, the right in-use windows and the right supply arithmetic. It also holds a history that spans a switch, so the before and after can be compared honestly.
The medication-specific detail is in tracking Mounjaro and tracking Ozempic. Everything logged stays on your phone: no account, no server, nothing to leak.
One tracker that knows the difference
Both drugs, both pen families, the right curve for each. Free core tracker, no account.
Common questions
Is Mounjaro stronger than Ozempic?
Can I switch from Ozempic to Mounjaro directly?
Which has fewer side effects?
Why do Mounjaro pens expire faster once opened?
Do Zepbound and Wegovy work the same way as Mounjaro and Ozempic?
Does Penwise handle both in the same history?
Sources
Every factual claim on this page traces to the product information a regulator publishes, or to a published trial. Links open on the site of whoever issued the document.
- Mounjaro: EPAR product information European Medicines Agency
- Ozempic: EPAR product information European Medicines Agency
- Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes (SURPASS-2) New England Journal of Medicine, 2021
- Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5) New England Journal of Medicine, 2025
Penwise is a tracking and education tool, not a medical device. Nothing on this page is medical advice, a diagnosis or a recommendation to start, stop or change a dose. Dosing decisions belong to you and your clinician. Ozempic, Wegovy, Rybelsus, Saxenda and Victoza are trademarks of Novo Nordisk; Mounjaro, Zepbound and Trulicity are trademarks of Eli Lilly. Penwise is not affiliated with, endorsed by or connected to either company.