Mounjaro® Side Effects (and How to Manage Them)

If you’re considering whether Mounjaro® might be the right prescription weight loss treatment for you, it’s important to be aware of the common side effects that many people experience while taking this medication.

In this guide, our UK pharmacy team explores what the Mounjaro® side effects are, when they happen, how long they last and how you can best manage them safely.

What is Mounjaro® and why do side effects occur?

Mounjaro® (tirzepatide) is a once-weekly injection that works by mimicking two naturally occurring gut hormones involved in appetite regulation, blood sugar control and digestion.

Because Mounjaro® changes how quickly your stomach empties and how strongly hunger signals are sent to your brain, your body needs time to adjust to the medication. Most side effects occur during this adjustment period, particularly when starting treatment or increasing your dose.

This is also why Mounjaro® is introduced using a gradual titration schedule, rather than starting on a higher dose. 

Find out more about Mounjaro® dosage.

What are the side effects of Mounjaro®? 

The most common side effects sometimes experienced when taking Mounjaro® are related to the digestive system. These usually occur early in treatment or after a dose increase during titration.

Common Mounjaro® side effects include:

• Nausea

• Reduced appetite

• Feeling full quickly

• Bloating

• Constipation

• Diarrhoea

• Mild stomach discomfort

• Heartburn or indigestion

• Fatigue

Less commonly, some people may experience:

• Vomiting

• Dizziness

• Headache

• Injection site irritation

• A fast pulse

• A change in the sense of taste

• A change in the sensitivity of the skin

Severe side effects are rare when Mounjaro® is prescribed appropriately and monitored, but medical attention is required if you experience:

• A severe allergic reaction – swelling of the face, lips, tongue or throat, having difficulty breathing 

• Pancreatitis – a severe and persistent pain in the upper belly that might radiate to the back, accompanied by vomiting

How long do Mounjaro® side effects last?

For most people, side effects when taking Mounjaro® are short-lived and soon start to fade away. Typically, these can be:

• Mild nausea or bloating that often settles within a few days

• Appetite suppression that tends to stabilise within 1–2 weeks

• Side effects after a dose increase, which usually improve within 7–14 days

If side effects persist beyond a few weeks or interfere with daily life, your prescriber may recommend staying on your current dose longer, adjusting your titration schedule, or providing additional support.

When do Mounjaro® side effects start?

Side effects most commonly start:

• Within the first few days of your first injection

• Within the first week after increasing your dose

Some people feel very little at first, while others notice appetite changes almost immediately. Both responses are normal because this medication can affect people in a variety of different ways.

Importantly, if you don’t feel any side effects, it DOES NOT mean the medication is not working.

Do Mounjaro® side effects get worse when the dosage changes?

If you are going through titration and gradually increasing your dose of Mounjaro®, your body needs to re-adjust each time the dosage changes, so you may experience some temporary side effects with every increase. 

Your prescriber minimises this risk by: 

• Increasing doses slowly

• Ensuring at least four weeks between dose changes

• Giving advice to eat smaller meals during transitions

If side effects are significant or causing you a lot of concern, dose escalation can be paused or reversed.

When should I seek help for Mounjaro® side effects?

You should contact your prescriber if:

• Side effects are severe or worsening

• You are unable to keep fluids down

• Vomiting is persistent

• Abdominal pain is severe or ongoing

• You feel faint or unwell

You should seek urgent medical advice if you experience symptoms such as severe abdominal pain with vomiting, signs of dehydration, or symptoms that feel sudden or alarming.

Tips for managing Mounjaro® side effects

Most Mounjaro® side effects are mild and temporary, but there are practical steps you can take to reduce their impact and help your body adjust more comfortably. Some of the tips that our pharmacy team find most effective include:

Eat smaller meals and slow down

Mounjaro® slows stomach emptying, which means large or fast meals can sometimes trigger nausea, bloating or discomfort. Eating smaller portions and taking your time allows your digestive system to keep up.

Stopping when you feel comfortably full, rather than finishing what’s on your plate, can significantly reduce symptoms.

Avoid very fatty or rich foods, especially early on

High-fat, greasy or heavily processed foods are more likely to worsen nausea and indigestion, particularly during the first few weeks of taking the medication or after a dose increase.

Opt for lighter meals such as lean protein, vegetables, soups and simple carbohydrates until your appetite stabilises.

Find out more about meal planning when taking weight loss injections.

Stay well hydrated

Dehydration can worsen nausea, constipation and fatigue. Aim to sip water regularly throughout the day rather than drinking large amounts at once.

If plain water feels unappealing, herbal teas, diluted squash or electrolyte drinks can help.

Choose your injection day carefully

Some people feel slightly tired or nauseous the day after their injection. Choosing a day when you can take things a bit easier, such as the evening before a quieter day, can make side effects feel more manageable.

Be extra cautious around dose increases

Side effects are most common when moving up a dose. During these weeks:

• Eat smaller meals

• Avoid alcohol

• Prioritise hydration

• Keep food choices simple

If side effects feel strong, speak to your prescriber. It may be appropriate to stay on your current dose longer instead.

Gentle movement can help digestion

Light activity such as walking can help to reduce bloating and constipation. 

Manage constipation early

If constipation occurs, increasing fibre gradually, drinking more fluids and staying active can help. Avoid suddenly increasing fibre intake too quickly, as this may worsen bloating.

Avoid lying down immediately after eating

Remaining upright for at least 30 minutes after meals can help reduce reflux and nausea.

Keep injections consistent

Taking Mounjaro® on the same day each week helps keep medication levels steady, which can reduce side effects over time.

Find out more about how to inject Mounjaro®.

Know when to ask for help

If side effects are persistent, worsening or affecting your quality of life, contact your pharmacist or clinician. Support, reassurance or dose adjustments are often all that is needed.

If you experience mild side effects on Mounjaro®, it usually means that the medication is doing what it is designed to do and the body will soon adjust. With correct titration, professional oversight from your prescriber and realistic expectations, most people find side effects settle quickly and become far less noticeable over time.

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FAQs about Mounjaro® side effects

Are there side effects if I stop taking Mounjaro®?

Mounjaro® does not cause withdrawal symptoms.

However, after stopping treatment:

• Appetite may gradually return

• Hunger signals may increase

• Weight regain can occur if lifestyle changes are not maintained

This is why many people work with their prescriber to establish a long-term maintenance dose and lifestyle strategy rather than stopping taking the medication suddenly.

Does drinking alcohol make Mounjaro® side effects worse?

Drinking alcohol while you are taking Mounjaro® can sometimes worsen certain side effects, particularly:

• Nausea

• Acid reflux

• Dizziness

• Fatigue

Alcohol also adds calories without promoting fullness, which may reduce weight-loss progress. 

Many people find they naturally drink less alcohol when on Mounjaro® because their tolerance decreases. If you do drink, pharmacists recommend moderation and avoiding alcohol during the first few weeks or around dose changes.

Does taking Mounjaro® affect fertility?

There is no evidence that Mounjaro® negatively affects fertility.

In fact, weight loss and improved metabolic health may improve fertility in some individuals. However, Mounjaro® should not be taken if you are trying to conceive and should not be taken during pregnancy. Effective contraception should be used while taking Mounjaro®.

If you are planning to conceive, speak to your prescriber about stopping treatment safely in advance of trying for a baby.

Are Mounjaro® side effects worse than Wegovy® side effects?

Mounjaro® and Wegovy® are both weight loss injections that work in similar but not identical ways, so side effects can differ between individuals.

Some people report:

• Less nausea on Mounjaro®

• Better appetite control

• Slower, steadier weight loss

Others will tolerate Wegovy® better and experience fewer side effects on it.

The right weight loss medication for you will depend on your individual response to the injections. 

Find out more if you’re considering switching from Mounjaro® to Wegovy®.

Does everyone get side effects when taking Mounjaro®?

Not everyone will have side effects when taking Mounjaro®. Many people experience very mild or no side effects at all.

Side effects are more likely if:

• Doses are increased too quickly

• Meals are large portions or high in fat

• Alcohol intake is high

• Hydration is poor

Following the guidance provided by your prescriber closely dramatically reduces the risk and severity of side effects.

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