Switching From Semaglutide to Tirzepatide: The Maths

If you are switching from semaglutide to tirzepatide, the single most important thing to understand is that you cannot map your old dose onto the new drug milligram-for-milligram — they run on completely different number lines, and a switch almost always means restarting titration from a low tirzepatide dose. This guide is the conversion maths and the titration logic behind that decision: what the two scales are, why your prescriber resets the starting dose, and how to turn whichever dose you land on into syringe units.

TL;DR — switching in four points

• No mg-for-mg map. 1 mg of semaglutide is not 1 mg of tirzepatide. Semaglutide tops out at 2.4 mg weekly; tirzepatide goes to 15 mg. The numbers are not comparable across the two molecules.
• You usually restart low. The Zepbound label has no "switch-over" dose — tirzepatide begins at 2.5 mg weekly for 4 weeks regardless of what you took before, because that 2.5 mg dose is a non-therapeutic lead-in for tolerability.
• Your prescriber sets the new start. Whether you begin at 2.5 mg or your clinician individualises it is a clinical decision — this page is the maths, not the prescription.
• Units still follow concentration. Once you have the new mg dose, the units you draw depend only on your vial's mg/mL, not on the drug name.

Why you can't convert the dose mg-for-mg

Semaglutide is a single glucagon-like peptide-1 (GLP-1) receptor agonist. Tirzepatide is a dual agonist: it activates both the GLP-1 receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor. Because the two molecules bind different receptor sets with different potencies and have different molecular weights, there is no fixed ratio that turns a semaglutide milligram into a tirzepatide milligram. Anyone who tells you “X mg of sema equals Y mg of tirz” is inventing a number the labels do not support.

That is also why the approved ranges look so unalike. The Wegovy label climbs 0.25 → 0.5 → 1 → 1.7 → 2.4 mg weekly; the Zepbound label climbs 2.5 → 5 → 7.5 → 10 → 12.5 → 15 mg weekly. A person stable on 2.4 mg semaglutide is at their maximum; 2.5 mg tirzepatide is the lowest, non-maintenance rung. The raw numbers cross over near 2.5 mg purely by coincidence of scale — it does not mean the doses are equivalent in effect.

What a switch actually changes

Both drugs share a once-weekly schedule and a multi-week half-life, so the rhythm of dosing feels familiar. What changes is the dose ladder you are standing on. When you stop semaglutide and start tirzepatide, you step off one staircase and onto a different one — and the convention on the Zepbound label is to begin at the bottom (2.5 mg) for at least 4 weeks, then step up no faster than every 4 weeks. Your prescriber may keep you there longer or move differently based on tolerance; the label sets the floor on speed, not the ceiling on caution.

Side-by-side: the two dose scales

Figures reflect the FDA-approved Wegovy and Zepbound labels for chronic weight management. There is no row labelled “equivalent dose” on purpose — the labels do not define one.

The switch, visualised: two different staircases

You are not continuing up one ladder — you step off the semaglutide ladder (wherever you were) and start near the bottom of the taller tirzepatide ladder.

How this is calculated: dose to units

Whatever tirzepatide dose your prescriber sets, the syringe maths is the same two-step conversion used for any vialled drug. A U-100 insulin syringe holds 100 units per 1 mL. So: divide your dose (mg) by the vial concentration (mg/mL) to get volume (mL), then multiply by 100 to get units. There is no drug-specific constant — the arithmetic that worked for your semaglutide vial works identically for tirzepatide. Only the dose and the concentration change.

Tirzepatide dose-to-units reference

Tirzepatide ladder doses as units on a U-100 syringe at two common compounded concentrations. Always confirm against your own vial label and your prescriber's plan.

Notice the higher tirzepatide doses overflow a 1 mL syringe at 10 mg/mL — which is why stronger vials are typical higher up the ladder. If your draw exceeds 1 mL, ask your pharmacy about a higher-concentration vial.

Timing the changeover

Because both drugs have multi-day half-lives, semaglutide does not vanish the moment you stop — it washes out over several weeks. Prescribers commonly begin tirzepatide at the next weekly slot rather than waiting for full clearance, precisely because the new drug also starts at a low, non-therapeutic dose. The overlap is part of why tirzepatide restarts low rather than picking up where semaglutide left off. The exact timing is a clinical judgement your prescriber makes — this guide does not set it.

Frequently asked questions