The first four weeks at 2.5 mg: what the label says that dose is for

The first month on tirzepatide is the month most likely to be misread, in both directions: as proof it does not work, or as the time to push harder. The label is unusually direct about what 2.5 mg is for.

What the label says 2.5 mg is

Section 2.1: "The recommended starting dosage of ZEPBOUND for all indications is 2.5 mg injected subcutaneously once weekly for 4 weeks. The 2.5 mg dosage is for treatment initiation and is not approved as a maintenance dosage. Follow the dosage escalation below for all indications to reduce the risk of gastrointestinal adverse reactions."

So the label gives one reason for the low start, tolerability, and one job for the first four weeks, initiation. It does not describe 2.5 mg as a dose expected to produce the trial results, and none of the SURMOUNT trials had a 2.5 mg arm. The published weight numbers, from a 15.0% mean reduction at 5 mg to 20.9% at 15 mg over 72 weeks (PubMed 35658024), were measured after at least 52 weeks at the assigned maintenance dose.

What is happening in the blood

Section 12.3 says steady state is reached after 4 weeks of once-weekly dosing, and the half-life is about 5 days. The first injection produces a peak around 24 hours later and then decays; the second injection lands on top of what is left of the first; and so on. By the fourth injection the level has settled into the range it will hold at that dose. The prescriber's week-5 decision to move to 5 mg is therefore made with a full picture of how you handle 2.5 mg, which is the point of the four-week minimum at every step, not just the first.

What to expect and what not to

The label reports adverse reactions over the whole trial, not by week, so there is no honest "week 1 versus week 3" table. What it does say is that most nausea, vomiting and diarrhea occurred during dose escalation and decreased over time, and that most people who stopped for adverse reactions did so in the first few months. The first month is inside that window. The side-effects page has the full table by dose arm.

On weight, the honest statement is that the label and the trials do not report a four-week figure for 2.5 mg and this site will not invent one. Weighing yourself weekly in the first month is fine as a record; using it to judge the drug is not.

The one thing not to do

Do not accelerate. The Mounjaro and Zepbound labels both require at least four weeks at each dose before the next 2.5 mg step. FDA's page on unapproved GLP-1 drugs (current as of September 1, 2026) describes adverse event reports involving compounded semaglutide and tirzepatide in which dosing exceeded the approved label, including escalation faster than the label schedule, with nausea and vomiting as the result. Compounded tirzepatide is not FDA approved and is not interchangeable with Zepbound or Mounjaro; if that is what you are prescribed, the schedule your prescriber wrote is the one to follow, and the brand label's minimums are a reasonable floor to ask about.

Practical items for month one

• Put the four 2.5 mg dates and the first 5 mg date in your calendar with the titration planner. The planner marks the first injection of each step so you can confirm the increase with your prescriber before it happens rather than after.
• Learn the missed-dose rule now, while it is not urgent.
• If you are using a compounded vial, work out the units for 2.5 mg at your vial's concentration before the first draw with the units converter, and check it against the pharmacy's instructions.
• Read the storage limits for your presentation; a month is long enough for a pen to go on a trip.

For the broader week-zero orientation, including how to talk to a prescriber before starting, the FormBlends tirzepatide introduction covers the ground this page assumes.