Tirzepatide: Mounjaro, Zepbound & the math

What tirzepatide is

Tirzepatide (development code LY3298176) is a synthetic peptide from Eli Lilly that activates two incretin receptors at once: the receptors for glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). That makes it a dual agonist — one step beyond a single GLP-1 agonist like semaglutide, and one step short of the triple agonist retatrutide. The combined incretin action enhances glucose-dependent insulin secretion, reduces appetite and food intake, and delays gastric emptying.

Like other long-acting incretins, it carries a fatty-diacid chain that binds albumin reversibly, slowing clearance — which is why it is dosed once weekly.

The brands

• Mounjaro — subcutaneous, for type 2 diabetes (US, 2022). In the EU and Germany, Mounjaro is the single brand for BOTH diabetes and weight management (the obesity indication was added under the same authorization in 2024).
• Zepbound — the US brand for chronic weight management (2023); also approved for obstructive sleep apnea with obesity (2024). Zepbound is not marketed in the EU.

Both are the same active ingredient. In the US, tirzepatide comes as single-dose pens and single-dose vials; in the EU it is also available as a multi-dose KwikPen.

What the trials showed

In SURMOUNT-1 (obesity without diabetes, NEJM 2022; ~2,539 participants, 72 weeks), mean body-weight reduction was 16.0% at 5 mg, 21.4% at 10 mg and 22.5% at 15 mg, versus 2.4% on placebo. In the head-to-head SURMOUNT-5 trial (vs semaglutide 2.4 mg, NEJM 2025, 72 weeks), tirzepatide produced about 20.2% mean weight loss versus 13.7% for semaglutide. A three-year SURMOUNT-1 extension reported a 94% reduction in progression to type 2 diabetes versus placebo in people with prediabetes and obesity.

In the diabetes SURPASS program, tirzepatide produced large A1c reductions (up to roughly −2.1 to −2.5%), outperforming semaglutide 1 mg in the SURPASS-2 head-to-head. The SURPASS-CVOT cardiovascular outcomes trial reported non-inferiority to dulaglutide for major adverse cardiovascular events. These are published trial outcomes for specific protocol doses under medical supervision — cited as facts, not targets.

Tolerability is dominated by gastrointestinal effects (nausea, diarrhoea, vomiting, constipation, decreased appetite), generally mild-to-moderate and concentrated during dose escalation. Tirzepatide carries a boxed warning for thyroid C-cell tumours (rodent data) and is contraindicated with a personal/family history of medullary thyroid carcinoma or MEN 2. Read the primary sources at the foot of this page.

The titration ladder

This is the approved label schedule, listed as fact — increases come in 2.5 mg steps no sooner than every 4 weeks, because side effects concentrate during escalation and a ~5-day half-life means each step takes about 4 weeks to fully express:

Maintenance doses are 5, 10 or 15 mg weekly. Not everyone titrates to the top — the lowest dose that holds the result is the usual aim.

The math

Approved pens are pre-dosed, so the arithmetic only matters if you are dealing with a compounded or lyophilized vial that must be reconstituted — the same arithmetic the peptide reconstitution calculator does for any vial. Worked example, presented purely as arithmetic:

Concentration is just milligrams divided by millilitres, and units are just hundredths of a millilitre on a U-100 insulin syringe. Change the water volume and every number moves: the same 10 mg vial with 2 mL gives 5 mg/mL (so 1 mg = 20 units), with finer markings and larger draws. The reconstitution calculator converts vial size + water + target into exact syringe units (and works in reverse). None of this implies a dose you should take; it is the conversion math for whatever a clinician specifies.

Half-life & what a weekly schedule means

Tirzepatide's elimination half-life is about 5 days (≈117–120 hours), because the peptide binds albumin via a fatty-diacid chain and clears slowly. A ~5-day half-life supports once-weekly injection: by the time the next dose is due, a meaningful fraction of the prior dose remains, so levels stack toward a plateau. With weekly dosing the concentration climbs for about 4 weeks before it stabilizes at steady state — which is why titration steps are spaced ~4 weeks apart and why a new dose isn't fully expressed in week one. You can see this accumulation curve for any half-life and interval in the half-life visualizer.

Storage & handling

• Approved pens / vials (Mounjaro / Zepbound): stored refrigerated at 2–8°C (36–46°F); follow the product's own room-temperature window and instructions.
• Lyophilized (powder, unreconstituted): for compounded vials, freeze-dried peptide is the most stable form and is typically stored cold and away from light.
• Reconstituted (in bacteriostatic water): refrigerated at 2–8°C, not frozen — freeze/thaw cycles can damage peptide bonds. Published stability practice assigns GLP-class peptides in bacteriostatic water a beyond-use window of roughly four weeks.
• BUD honesty: a "beyond-use date" is a stability convention, not a sterility guarantee. With unregulated compounded material, neither label accuracy nor sterility can be assumed.

Tracking tirzepatide in OptiPin

Whether you're on a prescribed Mounjaro/Zepbound pen or tracking a weekly GLP-class compound, OptiPin treats it like any other entry:

• Dose log + weekly reminders — record each injection on a once-weekly cadence and get reminded on the due day, with site rotation.
• Titration schedule — set the 2.5 → 5 → 7.5 → 10 → 12.5 → 15 mg ramp and OptiPin tracks which step you're on and when the next is due.
• Weight & body metrics — log weight and measurements and see them plotted directly over your dose timeline.
• OptiInsight analysis — OptiPin's AI reads the full record (doses, weight, symptoms, labs) and surfaces what moved with what.
• Pen & vial tracking — log the pen or reconstituted vial; OptiPin estimates remaining doses and an empty date, and warns before you run out.