Your First Week on Mounjaro®: Side Effects and What People Notice

The prescription is in and the first shot is close. One person with type 2 diabetes in a tirzepatide subreddit wrote while waiting to pick theirs up: "I have spent a lot of time reading as many post in here as possible so I can see what to expect."

The short answer: Mounjaro® is tirzepatide, a once-weekly shot that its FDA label on DailyMed indicates for type 2 diabetes. The label lists nausea, diarrhea, decreased appetite, vomiting, constipation, indigestion and stomach pain as the most common side effects. Because it is a diabetes medicine, it also warns that low blood sugar is more likely if you take insulin or a sulfonylurea with it. In week one, notice what you feel, write it down next to your blood sugar notes, and bring both to your prescriber.

For a medication-neutral overview, see week one on any GLP-1; this page sticks to Mounjaro®.

What are Mounjaro® side effects in the first week?

Mostly stomach ones. The Mounjaro® label names nausea, diarrhea, decreased appetite, vomiting, constipation, indigestion and stomach pain as the most common. In its trials, most reports of nausea, vomiting and diarrhea came during dose escalation and decreased over time. It gives no day-by-day timeline for week one.

So your own dated notes are what show your pattern. In a tirzepatide subreddit, one person described the day after their first dose: "I had an incident, where I was … away from home in the park, and didn't make it until home." Not every first-week story is about the stomach. Another person wrote that the day after their first shot, "I got so knocked out I had to take a nap in my car at lunch for an hour." These are single stories, not a forecast for you.

MedlinePlus says to tell your doctor if nausea, vomiting, diarrhea, decreased appetite, constipation or upset stomach are severe or do not go away. The Medication Guide says to call your healthcare provider right away for severe stomach pain that will not go away, sometimes felt through to the back, and for nausea, vomiting or diarrhea that does not go away, because losing fluids can lead to kidney problems. Your log can wait; that call cannot.

What is Mounjaro® prescribed for?

The Mounjaro® label covers two uses: improving blood sugar, along with diet and exercise, in adults and children 10 and older with type 2 diabetes, and lowering the risk of major heart events in adults with type 2 diabetes who are at high risk for them. So on Mounjaro®, the log gets two extra lines: blood sugar readings and your other diabetes medicines, next to your shots and side effects. Which brand you were prescribed, and why, is a conversation for your prescriber.

How fast did your blood sugar drop?

That question is the title of a thread in a tirzepatide subreddit, and the Mounjaro® label does not answer it for week one. It describes tirzepatide helping the body release insulin when blood sugar is high and slowing how fast sugar from a meal is absorbed. It sets no first-week number to aim for.

People arrive with this question before the first shot. One person asked: "Are there diabetics in here that can give me any information on how your blood sugar is (before and now) and your A1c?" At the end of a hard first week, someone else wrote: "On top of all that, my blood sugar has remained high all week so far." (both in a tirzepatide subreddit)

Neither story tells you what your numbers will do. If your prescriber has asked you to check your blood sugar, write each reading with the time, whether you had eaten, and how many days it has been since your shot. A week of readings next to shot dates is something your prescriber can read at a glance.

Can Mounjaro® cause low blood sugar?

It can, and the Medication Guide says the risk may be higher if you use Mounjaro® with another medicine that can cause low blood sugar, such as a sulfonylurea or insulin. It also says to talk to your healthcare provider about low blood sugar and how to manage it before you start.

The guide lists possible signs, among them dizziness or light-headedness, sweating, shakiness, a fast heartbeat, confusion, headache, blurred vision, hunger, weakness and irritability. The label tells prescribers to consider those other diabetes medicines when Mounjaro® is started. So if you take insulin or a sulfonylurea, ask your prescriber before your first shot what the plan is for them, and what to do if a low happens.

Feeling off is not proof of a low. One person in a tirzepatide subreddit, whose blood sugar stayed high through their first week, wrote: "I'm frequently quite lightheaded, too," and described an eye spasm "complete with blurred vision when it happens." What was behind it is a question only a prescriber can answer. The Medication Guide asks you to tell your provider about any changes in vision during treatment. Log each episode with the time, and a reading if your prescriber has asked you to check.

What should you write down in your first week on Mounjaro®?

A few lines a day, with blood sugar next to everything else. The Medication Guide asks you to keep a list of the medicines you take to show your healthcare provider and pharmacist. The table below is a note template, not a forecast of what will happen.

Line in your log	What to note
Shot	Day, time and spot, so every other line can be counted in days since the shot
Blood sugar	Each reading with the time and whether you had eaten, if your prescriber asked you to check
Low blood sugar signs	Shakiness, sweating, light-headedness (Medication Guide): when, how long, any reading taken
Diabetes medicines	Each one you take, and any change your prescriber makes, with the date
Stomach and appetite	When each change started and stopped, strength from 0 to 10, including hunger
Vision	Any change, with the date

If nausea is your main story, how long GLP-1 nausea lasts goes deeper on logging it.

FAQ: first week on Mounjaro®

Why am I hungrier, not less hungry, in my first week on Mounjaro®?

Only your prescriber can say. The Mounjaro® label lists decreased appetite among common side effects, yet one person wrote that by day two they had "INTENSE hunger." Hunger is also on the Medication Guide list of low blood sugar signs. Log when it hits, with a reading if you were asked to check, and show your prescriber.

Should I keep taking metformin when I start Mounjaro®?

Someone asked exactly that in a tirzepatide subreddit thread, and it only has a personal answer. The Medication Guide says to tell your healthcare provider about all the medicines you take, including other diabetes medicines. Whether to keep, change or stop any of them, ask your prescriber, and log any change with its date.

Does Mounjaro® make birth control pills less effective?

It may. The Mounjaro® label says it may reduce how well oral hormonal contraceptives work, because it slows stomach emptying, an effect it calls largest after the first dose. It has specific advice for the weeks after starting and after each dose increase. Ask your prescriber what that means for you, ideally before your first shot.

Tinsy is a wellness diary, not a medical device. It does not diagnose, treat or give medical advice. Always talk to your prescriber about doses, side effects and anything that worries you.

Tinsy keeps your Mounjaro® shots, dose reminders, a body map that remembers your last injection spot, and your calendar and history free. Tinsy Plus adds side effects with severity levels next to each dose, charts including your estimated medication level over time, Apple Health sync and a doctor report (PDF). Keep blood sugar readings wherever your prescriber asked you to, and bring both. More on our GLP-1 tracker by medication page.