First Dose of Semaglutide or Tirzepatide: What to Expect
In this guide
This article is for information only and is not medical advice. Talk to your prescriber or a qualified professional.
Your first dose of semaglutide (Wegovy or Ozempic) or tirzepatide (Zepbound or Mounjaro) is the smallest dose on the label. It is a starting step, chosen to reduce the risk of stomach side effects, and it is not meant to be your long-term dose. The labels list nausea, diarrhea, vomiting, constipation and stomach pain among the most common side effects. This guide restates what the current FDA labels say about the first shot and the days after it, so you know what to read, what to prepare and when to call someone.
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Key points
- Every label starts with a low dose for 4 weeks. Your prescriber decides when, and whether, it goes up.
- The labels say your healthcare provider should train you on your exact pen before you start.
- Stomach side effects are the most common ones listed. Severe stomach pain, signs of dehydration and allergic reactions are reasons to call.
- Pick a weekly day you can keep. The labels say once weekly, on the same day each week for semaglutide.
The starting dose on each label
The first dose of semaglutide and of tirzepatide is the same for every adult on a given product. According to each prescribing information:
| Product | Starting dose | How long, per the label |
|---|---|---|
| Wegovy (semaglutide) injection | 0.25 mg once weekly | Weeks 1 through 4 |
| Ozempic (semaglutide) | 0.25 mg once weekly | 4 weeks |
| Zepbound (tirzepatide) | 2.5 mg once weekly | 4 weeks |
| Mounjaro (tirzepatide) | 2.5 mg once weekly | At least 4 weeks before any increase |
The labels say to follow the step-up schedule to reduce the risk of gastrointestinal side effects. The Zepbound label adds that 2.5 mg is for starting treatment and is not approved as a maintenance dose. So if the first weeks feel like “nothing is happening,” that is how the schedule is written. The full step-up tables are in GLP-1 dosage: what the labels say. Your prescriber and pharmacist set your plan.
Before the first shot
Get trained on your exact pen
The Wegovy, Zepbound and Mounjaro labels tell healthcare providers to explain which presentation you will receive and to train you on it before you start. The pens are not interchangeable. A single-dose pen and a multi-dose pen have different steps. Our guide to GLP-1 pens explains the types, and how to inject a GLP-1 in the stomach walks through the steps the Instructions for Use describe.
Tell your prescriber about your health history
The Medication Guides ask you to tell your healthcare provider about all your medical conditions before you start. The Zepbound and Mounjaro guides list, among others, past pancreas problems, severe stomach problems such as slowed stomach emptying, a history of diabetic retinopathy, planned surgery or sedation, and pregnancy or plans to become pregnant.
The Zepbound and Mounjaro guides also say birth control pills taken by mouth may not work as well. If you use them, your healthcare provider may recommend another method for 4 weeks after you start and for 4 weeks after each dose increase.
Plan where and when
Choose a site from the ones your label allows, and decide how you will rotate it. Our injection site rotation guide covers that. Then pick a weekly day. For what the labels say about time of day, food and moving your day, see the best time to take a GLP-1 injection.
What the first days can bring
The labels do not promise a specific first-week experience, and people differ. What they do list is the most common side effects across their trials:
- Wegovy: nausea, diarrhea, vomiting, constipation, stomach pain, headache, tiredness, upset stomach, dizziness, bloating, belching and others.
- Ozempic: nausea, vomiting, diarrhea, stomach pain and constipation.
- Zepbound: nausea, diarrhea, vomiting, constipation, stomach pain, indigestion, injection site reactions, tiredness, allergic reactions, belching, hair loss and heartburn.
- Mounjaro: nausea, diarrhea, decreased appetite, vomiting, constipation, indigestion and stomach pain.
Two label details are worth knowing at the start. First, the labels tell prescribers to watch kidney function in people with side effects that could lead to fluid loss, especially when starting and when stepping up the dose. Second, the Zepbound label reports that most people who stopped the medicine because of side effects in its weight trials did so in the first few months, mainly because of stomach side effects. That is why the early weeks are a good time to keep notes.
The Medication Guides say diarrhea, nausea and vomiting can cause a loss of fluids (dehydration), which may cause kidney problems, and that drinking fluids helps reduce the chance of dehydration.
At the injection site
Some people notice the site itself. The Zepbound label lists injection site reactions in 6% to 8% of people on the medicine in its weight trials, compared with 2% on placebo. Our guide to GLP-1 injection site reactions has the numbers for each label and what they include.
When to call a clinician
When to call: The Medication Guides for Wegovy, Ozempic, Zepbound and Mounjaro say to stop using the medicine and get medical help right away for signs of a serious allergic reaction: swelling of the face, lips, tongue or throat, problems breathing or swallowing, severe rash or itching, fainting or feeling dizzy, or a very rapid heartbeat.
All four guides also say to stop using the medicine and call your healthcare provider right away for severe stomach pain that will not go away, with or without vomiting, which may be felt from the abdomen to the back. The Wegovy and Ozempic guides say to tell your healthcare provider about stomach problems that are severe or will not go away, and all four ask you to report side effects that bother you or do not go away.
Call your pharmacist if the pen does not behave the way its Instructions for Use describe, or if you are not sure you received the full dose. If a week goes by and you missed the shot, read missed a GLP-1 dose? what to read first. Your prescriber and pharmacist set your plan.
Frequently asked questions
What is the first dose of semaglutide?
According to the Wegovy and Ozempic labels, the first dose of semaglutide injection is 0.25 mg once weekly, for the first 4 weeks. The labels describe it as a starting dose, not a maintenance dose.
Will I feel side effects after my first GLP-1 shot?
Some people do and some do not. The labels list stomach side effects such as nausea, diarrhea and constipation as the most common ones. Keep a note of what you feel and when, and share it with your prescriber.
Why is my first dose so low?
The labels say the dose is increased in steps to reduce the risk of gastrointestinal side effects. The Zepbound label says its 2.5 mg starting dose is not approved as a maintenance dose. Your prescriber decides when to move to the next step.
Do I need training before my first injection?
The Wegovy, Zepbound and Mounjaro labels tell healthcare providers to train you on your specific pen before you start. Ask your pharmacist to show you the pen if no one has.
Keep a record from week one
The first weeks are when notes help most. In Dosinha you can log your first shot with its site on the body map, set a weekly reminder, and use the side-effect check-in to rate how you feel each day. When you see your prescriber, you have a clear record to share instead of trying to remember. The shot log, reminders and check-ins are free. Start your free log.
Sources
- DailyMed. Wegovy prescribing information, Medication Guide and Instructions for Use, sections 2.1, 2.2, 5.5 and 6. Novo Nordisk, label revised June 2026.
- DailyMed. Ozempic prescribing information and Medication Guide, sections 2.1, 2.2 and 6. Novo Nordisk, label revised June 2026.
- DailyMed. Zepbound prescribing information and Medication Guide, sections 2.2, 2.4 and 6.1. Eli Lilly, label revised August 2026.
- DailyMed. Mounjaro prescribing information and Medication Guide, sections 2.1, 2.2 and 6.1. Eli Lilly, label revised August 2026.
This article is for information only and is not medical advice. Talk to your prescriber or a qualified professional.