Semaglutide vs Tirzepatide — How They Differ and What Trials Show

Your prescriber mentions both names in the same breath — "we could try semaglutide, or tirzepatide" — and you leave the appointment unsure what actually separates them. Brand names make it more confusing still: four of them, two molecules. This page sets the brands aside and compares the medicines themselves.

What is the real difference between the two molecules?

Both are weekly injections that copy gut hormones involved in appetite and blood sugar. The difference is how many hormone receptors they act on.

Semaglutide is a GLP-1 receptor agonist. Tirzepatide is a GIP receptor and GLP-1 receptor agonist — it activates both. The Zepbound label notes that GLP-1 is a known regulator of appetite, and that animal studies suggest GIP may add to that effect.

Each molecule is sold under two brands with different approvals. Semaglutide is Ozempic for type 2 diabetes and Wegovy for weight, heart risk reduction and a form of fatty liver disease called MASH. Tirzepatide is Mounjaro for type 2 diabetes and Zepbound for weight and obstructive sleep apnea.

What did the head-to-head trials find?

Two large trials have compared them directly. Both were funded by Lilly, which makes tirzepatide, and both were open-label, meaning participants knew which medicine they were taking.

SURPASS-2 (type 2 diabetes, 40 weeks). 1,879 adults on metformin were randomized to tirzepatide 5, 10 or 15 mg or semaglutide 1 mg. Average HbA1c fell by 2.01, 2.24 and 2.30 percentage points on the three tirzepatide doses, and by 1.86 on semaglutide. Weight reductions were also larger with tirzepatide, by an estimated 1.9, 3.6 and 5.5 kg more than semaglutide.

SURMOUNT-5 (obesity without diabetes, 72 weeks). 751 adults were randomized to the highest dose they could tolerate of tirzepatide (10 or 15 mg) or semaglutide (1.7 or 2.4 mg). Average weight change was −20.2% with tirzepatide and −13.7% with semaglutide. Waist circumference fell by an average of 18.4 cm and 13.0 cm.

These results describe averages across hundreds of people. Within each group, some people lost much more and some much less, and a trial cannot tell you which group you would fall into.

Neither trial was designed to study switching from one medicine to the other, and SURMOUNT-5 did not include semaglutide's newer 7.2 mg Wegovy dose. If you are weighing a change, those gaps are worth raising with your prescriber.

How do side effects compare?

For both medicines, the most common side effects are stomach-related, and most happen while the dose is being increased.

In SURPASS-2, nausea was reported by 17% to 22% on tirzepatide and 18% on semaglutide; diarrhea by 13% to 16% and 12%; vomiting by 6% to 10% and 8%. In SURMOUNT-5, the authors reported that gastrointestinal events were the most common in both groups and were mostly mild to moderate.

The warnings on all four labels look very similar, including a boxed warning about thyroid C-cell tumors seen in rodents.

Are there practical differences day to day?

A few, and they are worth knowing before you set up reminders.

• Half-life. The Wegovy label puts semaglutide's half-life at about a week; the Zepbound label puts tirzepatide's at about 5 days.
• Missed doses. The rules differ by brand. Mounjaro and Zepbound allow a missed dose within 4 days. Ozempic allows 5 days. Wegovy injection uses a different rule: take it if the next dose is more than 2 days away.
• Forms. Semaglutide is also available as a daily tablet under the Wegovy brand. Tirzepatide comes in pens and vials, but no tablet.

What we'd track

Whichever molecule you end up on, these are the notes we would keep.

1. Brand, dose and shot day. The missed-dose rule depends on the brand, so the log should say which one.
2. Side effects against dose steps. It shows whether symptoms settle the way the labels describe.
3. Weight trend, weekly. A gentle line is more honest than daily numbers.
4. Meals and protein. Appetite often drops on both.
5. The date of any change. New medicine, new dose, new form — so you and your prescriber can compare fairly.

Holt handles shots or pills, reminders, side effects, meals, water and weight on your iPhone, with Apple Health sync and a report for appointments. It is free to download, with an optional subscription.

Track either medicine in Holt