Semaglutide vs Tirzepatide: Weight Loss, Dosing and Side Effects Compared

Semaglutide and tirzepatide are the two most discussed peptides in the GLP-1 family. They are studied for similar reasons but work through different receptor targets, follow different titration schedules and show different average weight loss figures in published trials. This is a neutral, third-person comparison of how the two compare on paper.

Both are research compounds, not approved for self-directed human use. The numbers below come from clinical studies and are reference information only. Nothing here is dosing guidance. Any decision about a protocol belongs with a licensed clinician. For the math side, the tirzepatide calculator and the semaglutide calculator handle the unit conversions covered later.

Mechanism: one receptor vs two

This is the core difference between the two molecules.

• Semaglutide is a single GLP-1 receptor agonist. It mimics one gut hormone, GLP-1.
• Tirzepatide is a dual agonist. It activates both the GLP-1 receptor and the GIP receptor, a second incretin hormone.

The added GIP activity is the main reason researchers study tirzepatide as a separate compound rather than a stronger version of the same thing. Both still share the same general half-life profile that supports once-weekly dosing in study settings.

Average weight loss in trials

Head-to-head and standalone trials report different averages. These are study population means, not promises, and individual results in any dataset vary widely around the average.

• Tirzepatide: the SURMOUNT-1 trial reported roughly 20% average body weight reduction at the highest dose over 72 weeks.
• Semaglutide: the STEP trials reported roughly 14 to 15% average body weight reduction at 68 weeks.
• Direct comparison: the SURMOUNT-5 head-to-head reported a larger average reduction for tirzepatide than semaglutide.

Dosing schedules side by side

Both follow a slow step-up structure in studies, starting low and increasing every four weeks to let the body adjust. The reference ranges differ.

• Semaglutide study range: typically starts around 0.25 mg weekly and steps up toward a 2.4 mg weekly maintenance figure.
• Tirzepatide study range: typically starts around 2.5 mg weekly and steps up toward 5, 10 or 15 mg weekly figures.
• Shared pattern: four weeks per step is the common interval in both protocols.

The milligram numbers are not comparable one to one. A 2.4 mg semaglutide figure and a 15 mg tirzepatide figure are different molecules at different potencies. See semaglutide vs tirzepatide framing and the tirzepatide to semaglutide conversion note for why direct mg swaps do not hold.

Worked example: drawing a dose

Once a vial is reconstituted, the syringe units depend on concentration, not on which peptide it is. The arithmetic is identical for both.

1. Take a 10 mg vial and add 2 mL of bacteriostatic water. Concentration is 10 mg / 2 mL = 5 mg/mL.
2. For a 2.5 mg target: 2.5 mg / 5 mg/mL = 0.5 mL = 50 units on a U-100 insulin syringe.
3. For a 5 mg target from the same vial: 5 / 5 = 1.0 mL = 100 units.

The reconstitution calculator does this both directions, and the mg to units calculator converts a milligram figure into syringe units once your concentration is set. For the water step itself, see how much bacteriostatic water to add.

Reported side effects

Both compounds share a similar reported side effect profile in study literature, driven mostly by the gut. Trials describe these as most common during the titration period.

• Nausea, the most frequently reported effect for both.
• Diarrhea, constipation and reduced appetite.
• Reactions that studies note often ease as the body adjusts to each step.

Across datasets the gastrointestinal effects look broadly comparable between the two, which is why the slow four-week titration exists in both protocols. The GLP-1 side effects overview covers what the literature describes in more detail.

Quick reference summary

• Targets: semaglutide hits GLP-1 only; tirzepatide hits GLP-1 and GIP.
• Trial average loss: about 14 to 15% vs about 20%.
• Maintenance figures studied: 2.4 mg weekly vs 5 to 15 mg weekly.
• Side effects: broadly similar, mostly gastrointestinal, mostly during titration.
• Math: identical once reconstituted, only concentration changes the units.

For tracking schedules, doses and vial concentrations in one place, the Stackr app keeps the running log. See the full disclaimer before using any figure here.