If you've recently started taking Mounjaro, or you're thinking about beginning treatment, you may be wondering what side effects to expect.
Like all medicines, Mounjaro can cause side effects. Thankfully, most are mild and temporary, and many people find they improve as their body gets used to treatment.
The most common side effects affect the digestive system and can include nausea, diarrhoea, constipation, and stomach discomfort. And while serious side effects are possible, they're rare.
This guide explains the most common Mounjaro side effects, how long they last, and what you can do to manage them.
Three key takeaways
Most Mounjaro side effects are mild, temporary, and usually affect the digestive system.
You can manage most side effects by avoiding trigger foods, eating smaller portions, and staying hydrated.Mounjaro side effects treatments are also available if needed.
Side effects are most common when you first start treatment and after you increase your dose.
What are the 5 most common Mounjaro side effects?
The five most reported Mounjaro side effects (affecting more than one in 10 people) are:
Mounjaro side effects can start soon after your first dose. Most begin within a few days but go away just as quickly. They might also come back when you increase your dose, as your body needs to get used to more medication.
How long do Mounjaro side effects last?
Most Mounjaro side effects tend to go away on their own within a couple of days or weeks. If they don’t get better, get worse, or start preventing you from going about your day-to-day life, you should speak to your prescriber.
How to avoid Mounjaro side effects
“To avoid the most common Mounjaro side effects, we always recommend:
Staying hydrated. Getting enough water each day is key to avoiding headaches and constipation on Mounjaro. It’s also important to stay hydrated if you’re getting diarrhoea.
Eating a balanced diet containing plenty of fibre, whole foods, and protein. These foods keep you full and prevent some of the side effects.
Avoid spicy foods, fatty foods, alcohol, and caffeine. These foods can be hard on your stomach, leading to side effects. Alcohol and caffeine can make side effects worse and cause dehydration, too.
Move more! Regular exercise reduces some side effects and increases weight loss - it’s win-win. Small changes, like taking the stairs or a daily walk, can make a world of difference.”
Most Mounjaro side effects are linked to how the medicine affects your digestive system. This is because Mounjaro slows down how quickly food leaves your stomach, which can cause changes to digestion.
*Between 2022-2024, four studies with 5,808 Mounjaro patients revealed the 10 most common side effects they experienced.
Uncommon side effects of Mounjaro
Most Mounjaro side effects are mild, but there are some rarer side effects that can be more serious.
While these side effects only happen to less than 1% of people, it’s still important to know and understand them so you know what to do if they happen.
The uncommon side effects of Mounjaro may include:
inflamed pancreas (pancreatitis)
rapid pulse
increased levels of amylase (pancreatic enzyme)
low blood sugar
injection site pain
gallstones
inflammation of the gall bladder, affecting bile flow (cholecystitis)
Serious Mounjaro side effects
"It’s important to remember that serious side effects with Mounjaro are very rare, affecting fewer than 1% of people. While they sound worrying, they’re unlikely to happen. Most people only experience mild side effects as their body adjusts to treatment.
"All medicines can cause serious side effects, even everyday ones like paracetamol. But the health benefits of weight loss usually far outweigh the risks.
"Before prescribing, our clinicians always make sure treatment is safe and suitable for you. And if you’re ever worried about side effects, we’re here to help."
Ian Budd, Prescribing Pharmacist at Chemist4U
Gall bladder issues
While rare, Mounjaro can cause gallbladder problems, like gallstones or inflammation of the gallbladder. This could happen because Mounjaro reduces levels of a hormone called cholecystokinin, which helps your gallbladder work properly.
Rapid weight loss can also increase the risk of gallstones. When you lose weight quickly, your liver may release more cholesterol into bile, which can form stones in your gallbladder.
Gallbladder problems affect fewer than one in 100 people taking Mounjaro, but it’s important to know the symptoms. These can include:
pain in the upper right side of your abdomen or between your shoulder blades, especially after eating fatty foods
nausea or vomiting
fever
yellowing of your skin or eyes (jaundice)
If you experience anything like this, stop taking Mounjaro and speak to a doctor as soon as possible.
Low blood sugar (hypoglycaemia)
If you have type 2 diabetes and take medicines to lower your blood sugar alongside Mounjaro, you may be more likely to experience low blood sugar.
If you experience these symptoms, eat or drink something sugary, like:
fruit juice
a non-diet fizzy drink
sweets
glucose or dextrose tablets
If your symptoms improve, eat a longer-lasting carbohydrate, like biscuits, bread, or something similar. Speak to your doctor if this happens regularly.
Important: Very low blood sugar can cause loss of consciousness. If this happens to you or someone with you, seek emergency medical help.
Allergic reactions
Like with all medications, there's a possibility of experiencing an allergic reaction to Mounjaro. This is considered very rare, affecting fewer than 1% of people who take it.
If you notice symptoms of a severe allergic reaction, like:
difficulty breathing
swelling of the lips or tongue
feeling like you’re going to collapse
You should get emergency medical attention.
Inflammation of the pancreas (pancreatitis)
While uncommon, pancreatitis is a serious side effect of Mounjaro. Symptoms typically include:
severe stomach pain that may spread to your back
pain that doesn’t improve over time or when changing position
high temperature
feeling or being sick
If you experience these symptoms, stop using Mounjaro and seek immediate medical attention.
“In my experience, patients sometimes worry unnecessarily about every stomach twinge, but true pancreatitis pain is unmistakable.
"If you experience severe, persistent abdominal pain that doesn't improve with rest or position changes, especially if accompanied by vomiting or fever, that's when you need immediate medical attention.”
Are there long-term side effects from taking Mounjaro?
The longest Mounjaro clinical trial lasted for 72 weeks and didn't identify any long-term side effects. However, some rare side effects may lead to longer-term health problems if they're not treated. These include:
Pancreas damage from untreated pancreatitis: Severe pancreatitis can cause permanent damage. Some people may also experience pancreatitis again in the future.
Kidney damage from severe dehydration: Ongoing vomiting or diarrhoea can affect kidney function. In very rare cases, this can lead to kidney failure.
These long-term issues aren't caused directly by Mounjaro, but may happen if serious side effects are left untreated.
As Mounjaro is a relatively new medication, information about its long-term effects is still being collected.
Nonetheless, serious long-term problems are considered very rare, and Mounjaro is approved for long-term use when prescribed and monitored by a healthcare professional.
Can Mounjaro cause cancer?
Cancer hasn't been reported as a side effect of Mounjaro in human clinical trials.
Some studies in rats found an increased risk of certain thyroid tumours, but rats can respond differently to medicines than humans. These findings don't mean that Mounjaro increases the cancer risk in people.
keeping your head slightly raised when sleeping if needed
Antacids like Gaviscon and Rennie can also help to relieve heartburn by neutralising stomach acid.
Headaches
To help with headaches:
drink plenty of water
eat regular meals
get enough rest
try to maintain good sleep habits
Speak to your doctor if your headaches are severe, persistent, or unusual for you.
Hair loss
"If you're experiencing hair loss on Mounjaro, it's important to know that this is usually linked to rapid weight loss rather than the medication itself.
"Significant weight loss can sometimes trigger a temporary condition called telogen effluvium, which causes increased hair shedding. In most cases, this improves with time.
"I recommend maintaining a balanced diet with adequate protein intake and monitoring your symptoms. If your hair loss is severe, persistent, or causing concern, please seek further medical advice."
If you're experiencing side effects while taking Mounjaro, you don't have to manage them alone.
Our clinicians can offer advice on common side effects, recommend suitable treatments to help ease symptoms, and discuss whether staying on your current dose for longer or temporarily moving to a lower dose may help.
Most side effects improve as your body adjusts to treatment, but if they're affecting your daily life or you're concerned about your symptoms, we're here to help you find the best way forward.
Remember, always speak to your prescriber before making any changes to your Mounjaro dose.
Frequently asked questions about Mounjaro side effects
Are Mounjaro side effects worse than Wegovy?
Mounjaro side effects aren’t usually worse than those with Wegovy. In fact, both medications have very similar side effects. Some are just more common depending on which one you’re taking.
For example, with Wegovy, headaches and fatigue are a lot more common, but with Mounjaro, you’re more likely to get skin reactions or low blood pressure.
It’s also important to understand that medications affect everyone differently. Some people may react more to Mounjaro than Wegovy or the other way around.
Our clinicians will work with you to find the treatment that works best, while minimising any side effects.
Will I get headaches on Mounjaro?
You might get headaches on Mounjaro, but it’s much less likely, as only one in 20 people report headaches. They tend to happen due to dehydration or not eating enough.
Headaches typically appear in the first few weeks of treatment or after dose increases, then fade as the body finds its rhythm.
Why do I get trapped wind with Mounjaro?
As Mounjaro slows digestion, gas can become trapped more easily in your intestines. This causes discomfort that ranges from mild to quite intense.
Trapped wind can be recognised by those sharp, sometimes severe abdominal pains, and can be alarming when you first experience them.
Why am I feeling cold on Mounjaro?
You might notice you’re more sensitive to temperature or feeling slightly drained as your body adjusts to taking in fewer calories and working hard to burn stored fat.
Cold sensitivity may continue throughout your journey, particularly as you lose insulating body fat, but this is a sign that your treatment is working well.
Can I get insomnia when taking Mounjaro?
You can get insomnia while taking Mounjaro, but it’s quite rare.
Sleep disruptions can happen due to changes in your metabolism, mild nausea, or simply your body adjusting to eating less. Most people find that their sleep patterns return to normal once their body settles into the medication.
Why do I get body aches on Mounjaro and how do I manage them?
Body aches typically happen for several reasons:
your body is losing water quickly as it adjusts
changes in essential minerals like sodium and potassium
your body's natural reaction to significant changes in how it processes food and energy
You can ease body aches by:
Looking after your muscles and joints: stay hydrated and try to maintain plenty of gentle movement.
Making sure you’re getting the right nutrition: include plenty of protein in your diet and make sure you’re eating leafy greens, bananas, nuts, and seeds. These provide you with magnesium and potassium, which help stop muscle cramps and aches.
Trying hot and cold therapy: a hot bath or shower, or even using an icepack, can help to get rid of aches. Find what works best for your body.
Can Mounjaro cause a sore throat?
A sore throat isn’t a common side effect, but it can happen, usually as a symptom of a different side effect. For example, Mounjaro may cause acid reflux or dehydration, which can irritate the throat.
Fortunately, symptoms often ease with hydration, soothing drinks, or throat lozenges.
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