The escalation schedule
The calculator implements the FDA-label escalation for tirzepatide as marketed under Mounjaro (type 2 diabetes, 2022) and Zepbound (obesity, 2023; obstructive sleep apnea, 2024). The schedule is six steps of four weeks each:
weeks 1–4 2.5 mg initiation (not therapeutic, minimizes GI)
weeks 5–8 5.0 mg first therapeutic dose
weeks 9–12 7.5 mg continued escalation
weeks 13–16 10.0 mg higher therapeutic dose
weeks 17–20 12.5 mg near-maximum
weeks 21+ 15.0 mg maximum maintenance
All injections are once-weekly subcutaneous, into the abdomen, thigh, or upper arm, on the same day each week. Each escalation step holds for at least four weeks before moving up. The labels permit slower titration (every six or eight weeks) if gastrointestinal symptoms are intense, but never faster.
Calculator inputs: a week number (1+) and the brand context (Mounjaro vs Zepbound; same molecule, same pen, different label and indication). Calculator outputs: the dose in milligrams that corresponds to that week, the matching pen concentration from the label table, the equivalent injection volume in millilitres, and the purpose statement for that step. Pen concentrations are taken directly from the FDA labels and live in src/data/compounds/semaglutide.json under the tirzepatide entry.
Why those specific doses
Tirzepatide is a 39-amino-acid peptide engineered as a dual agonist of the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor. GLP-1 alone slows gastric emptying, suppresses glucagon, and enhances insulin secretion. The added GIP arm modulates adipose insulin sensitivity and amplifies satiety signalling, and it appears to blunt some GLP-1-driven nausea. The net effect at comparable GI burden is larger weight loss and larger HbA1c drops than any GLP-1 monoagonist.
The 2.5 mg starting dose is sub-therapeutic by design. SURPASS-2 (Frías et al., 2021, NEJM, PMID 34170647) and SURMOUNT-1 (Jastreboff et al., 2022, NEJM, PMID 35658024) both used the same 4-week-per-step ramp. SURMOUNT-1 reported −20.9% mean weight loss at 15 mg / 72 weeks vs −3.1% on placebo; SURPASS-2 head-to-head against semaglutide 1 mg in T2D showed HbA1c −2.30% at 15 mg vs −1.86% (5.5 kg additional weight loss). SURMOUNT-5 MAINTAIN (Aronne et al., 2025, NEJM, PMID 40353578) confirmed superiority over semaglutide 2.4 mg in obesity without diabetes (−20.2% vs −13.7% at 72 weeks). SURMOUNT-OSA (Malhotra et al., 2024, NEJM, PMID 38912654) drove the obstructive-sleep-apnea label expansion. Slow escalation is what makes the drug tolerable. The GI rate at 15 mg without the ramp would push trial dropout well above the 4–7% the registration trials reported.
Assumptions and limits
The calculator assumes weekly subcutaneous injection on the FDA-label schedule for an adult who is eligible under that label: adults with type 2 diabetes (Mounjaro), adults with obesity or overweight plus a weight-related comorbidity (Zepbound), or adults with moderate-to-severe obstructive sleep apnea plus obesity (Zepbound, OSA indication).
It assumes none of the listed contraindications apply: personal or family history of medullary thyroid carcinoma, Multiple Endocrine Neoplasia syndrome type 2, prior pancreatitis (relative), severe baseline gastroparesis, pregnancy or breastfeeding, or known hypersensitivity to tirzepatide or excipients.
It assumes the pen used matches the dose step. The label ships six pen strengths (2.5, 5, 7.5, 10, 12.5, 15 mg, all 0.5 mL), and the calculator surfaces the matching pen for each week. Mixing strengths to land on a custom dose is not on-label and is not modelled.
It does not cover compounded tirzepatide; 503A/B compounded vials have variable concentration, no pen, and require manual reconstitution math. None of the safety, efficacy, or pharmacokinetic data underwriting the chart applies to compounded material.
It is not a replacement for prescriber judgement. The chart assumes someone with a prescription is using it to plan and check. It does not titrate the dose for a given patient, recommend slowing the ramp for GI symptoms, or adjust around concurrent insulin or sulfonylurea therapy. Those decisions sit with the prescriber.
Unit conventions
Tirzepatide doses are in milligrams (mg), not micrograms or international units. The pen delivers a fixed volume of 0.5 mL per dose; the strength of the pen determines the mg delivered. So a "5 mg pen" is a pen whose 0.5 mL injection delivers 5 mg of tirzepatide; the pen does not have a dial, and the user does not measure the dose.
The cadence is once weekly. "Once weekly" in the FDA sense means a 7-day interval ±1 day, on the same calendar day each week. A missed dose can be taken within 4 days of the scheduled day; past 4 days, the dose is skipped and the next regular injection is taken on the original day. The chart does not adjust for missed doses; that is a label rule, not a calculation.
Vial-and-syringe formats (compounded material) flip the unit convention: the user reads concentration in mg/mL, draws a measured volume, and calculates the mg delivered themselves. Those conversions live in the peptide reconstitution calculator, not here. The chart on this page assumes the on-label pen.
Why this is documentary, not prescriptive
This page describes a published escalation schedule. It does not prescribe one. The FDA labels for Mounjaro and Zepbound, plus the SURPASS / SURMOUNT trial program, define what dose corresponds to what week and what outcome distribution to expect; the calculator presents the same mapping in one table.
What the calculator cannot do: decide whether tirzepatide is an appropriate drug for a given person, predict an individual's weight or HbA1c response, weigh the GI burden against the gallbladder and pancreatitis class signals, or substitute for the prescriber's judgement on slowing the ramp. The 2025 Cochrane review (Franco et al., 2025, Cochrane Database Syst Rev, PMID 41161687) summarized the durability data, moderate-certainty 16% body-weight reduction sustained out to 3.5 years, and also noted that all nine included trials reported "a major role of the drug manufacturer in their design, conduct, analysis, or writing." Independent replication outside the Eli Lilly program is limited. That caveat sits on top of the schedule, not under it.