What to expect during your first month on Mounjaro

Before I get into what to expect in your first month taking Mounjaro, it will be helpful to first explain how the weight loss treatment works.

Mounjaro works by mimicking two hormones found in the body, GLP-1 and GIP. This helps you lose weight in three ways:

Now that you understand the science behind Mounjaro and weight loss, we can go over what to expect during the first month of your weight loss journey.

For a more detailed explanation, you can also check out my dedicated article on how Mounjaro works for weight loss.

Weight loss while using Mounjaro is unlikely to be very noticeable within the first month, but some initial change in body fat is possible.

One clinical trial on Mounjaro’s effectiveness found that most people lost 1-2% of their overall body weight within 4 weeks. That’s 0.85-1.7 kg if you weigh 85 kg.

While that may not sound very impressive, it’s important to keep in mind that the best results from Mounjaro are typically seen after taking it for at least 6 months.

Your first month using Mounjaro isn’t about instant results, it’s about getting used to the treatment and developing new habits. The following is a timeline of what to expect during this important first step in your weight loss journey.

Week 1

Everyone using Mounjaro for the first time will start on the lowest dose: 2.5 mg. This helps you get used to the medication and reduces the risk of side effects.

I've also written a dedicated guide on how to inject Mounjaro for the first time if you'd like a full walkthrough.

Effects of treatment

Mounjaro starts working within days to suppress your hunger, with research showing the medicine reaches peak effectiveness in 8-72 hours after you take it. However, the results are often modest to begin with.

During the first week, you may notice:

Remember, weight loss with Mounjaro happens gradually when used alongside a calorie-reduced diet. The first week is more about getting used to the treatment, not instant weight loss.

Side effects

Mild gastrointestinal side effects are common during the first week or two of treatment, but try not to worry. This just means your body is getting used to the medicine.

Note down any side effects you experience at this stage. That way, you can see if they improve or worsen later on.

Week 2

Week 2 marks your second injection. The dose will still be 2.5 mg, but with more time on Mounjaro, you might notice slightly more pronounced effects.

Weight change

You may notice a modest change in weight during your second week. However, this is largely water weight, not body fat.

This happens because of the way your body stores and burns energy. Glycogen is stored alongside water, so when those stores decrease, water is released and lost. Some fat loss can happen alongside this, but this is typically more noticeable further down the line.

Side effects

Side effects may persist into the second week, but they can also go away by this point. If side effects get worse or feel unmanageable, speak with a doctor as soon as possible.

Week 3

The third week is when most of my patients tell me they really notice a change in their appetite. While weight loss may not yet be noticeable, the effects certainly can be.

Effects of treatment

By this point you will have injected 3 doses of Mounjaro, and the active ingredient, tirzepatide, should now be built up in your system. For many people, this is the point when they get used to the medicine, and can look forward to enjoying the benefits of treatment.

This can make cravings and food noise far less noticeable than during the first 2 weeks, and you should be more comfortable developing your new eating habits going forward.

Week 4 (and beyond)

First off, if you’ve made it to the fourth week of treatment with Mounjaro, congratulate yourself on reaching a major milestone in your journey. If you haven’t yet, here’s what you can expect.

By this point, initial side effects should have subsided as your body has gotten used to the medicine. If they haven’t gone away yet, it’s important to speak with a doctor to get their advice. For some, all it takes to manage side effects is some lifestyle changes. For others, they may need to try an alternative weight loss treatment.

If you aren’t experiencing side effects, it’s almost time to increase your dosage to 5 mg. While this may be intimidating for some, keep in mind that the jump up to the next dose should be easier to manage than when you first started treatment.

What to focus on going forward

While the first month of treatment is about introducing Mounjaro to your system, the following months are about long-term habits, and real weight loss results.

With the first month done, you can start shifting your focus from getting used to treatment to building sustainable, long-term habits.

While Mounjaro is designed with ease in mind, there are some things you can do to make starting treatment go more smoothly.

Mild side effects in the first month are completely normal, and a sign that Mounjaro is working as intended.

According to a clinical study, the most common side effects in the first month of treatment are as follows:

You can find a full list of Mounjaro’s side effects in the patient information leaflet.

While side effects should go away after a few weeks, there are some things you can do to manage them in the meantime:

If these tips aren’t enough to manage your side effects, or if they persist past the first month, talk with a doctor for personalised advice. If you’re on a higher dose, they may lower it. If you are on the lowest dose already, they may suggest an alternative treatment.

If you experience severe or persistent stomach pain, signs of an allergic reaction, or gallbladder problems, speak to a doctor immediately.