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Mounjaro side effects

Robbie Puddick
Written by

Robbie Puddick

Medically reviewed by

Fiona Moncrieff

7 min read
Last updated September 2023

Jump to: What is Mounjaro? | Nausea and vomiting | Constipation and diarrhoea | Rare side effects | Take home message

The most common side effects of Mounjaro (tirzepatide) are nausea, constipation, vomiting, and other symptoms affecting the gastrointestinal (GI) tract, like diarrhoea. You might also experience less common side effects like tiredness or headaches.

These symptoms most commonly occur around the time of the injection, during dose escalation, and are typically transient and will ease after you’ve adapted to the medication.

However, everyone responds differently to these medications; some people won’t experience any side effects, while others will have to discontinue their medication due to the discomfort the side effects cause.

Previous research on GLP-1 medications has suggested a possible link to rare and more severe side effects such as pancreatitis and thyroid cancer.

However, no research has found a connection between tirzepatide and pancreatitis or thyroid cancer.

What is Mounjaro?

Mounjaro is a dual-action type 2 diabetes medication. It’s a GLP-1 receptor agonist (like Wegovy and Ozempic) that reduces appetite to support weight loss and improve blood glucose control.

Mounjaro is also a glucose-dependent insulinotropic polypeptide (GIP) mimetic– GIP is a hormone in the body that supports blood glucose levels and helps our fat cells store more excess energy we might consume in our diets.

So, Mounjaro has a dual function that mimics two hormones in the body that support blood glucose levels and weight loss.

Mounjaro is manufactured by Eli Lilly; the active ingredient in Mounjaro is tirzepatide.

Mounjaro is approved for individuals living with type 2 diabetes in the U.S. They’re available via prescription in the U.S. depending on your insurance coverage, as well as being available for private purchase. Mounjaro isn’t available in the UK.

Not a magic pill

Weight loss injections are designed as additional tools for weight management interventions and shouldn’t be considered lifelong medications.

Instead, they should be used to help you kickstart your weight loss and healthy journey while you commit to living a healthier lifestyle.

Consider the use of antidepressants for people living with depression. They’re not designed to cure the condition. Instead, they’re designed to allow room for therapy to treat the underlying cause of the condition.

Mounjaro, and other weight loss injections, like Ozempic and Wegovy, are similar. They can give you room to make lifestyle changes to support long-term weight loss maintenance.

At Second Nature, we’ve been trusted by the NHS since 2017 to support people with type 2 diabetes and obesity to lose weight and manage their blood sugar levels and lead healthier lifestyles for the long term.

In 2022, the NHS published data in the BMJ that compared the weight loss outcomes of five providers in the National Diabetes Prevention Programme.

Second Nature was more than twice as effective at supporting weight loss than the other four providers.

1) Nausea and vomiting

Nausea is the most common side effect of Mounjaro and occurs in 13-25% of individuals on the medication. The incidence is dose-dependent, so the higher the dose, the more likely you will experience nausea.

However, it’s typically transient and eases after the body has adapted to the medication or the higher dose you’ve started.

Vomiting is also common when taking tirzepatide, affecting 5-14% of patients on the medication.

Both of these side effects are mainly attributed to the impact of GLP-1 medications to slow down gastric emptying and cause gut distension (enlargement). GLP-1s also communicate with our nervous system, which can disrupt signalling pathways.

These physiological effects mean food takes much longer to pass through our digestive system.

Nausea and vomiting appear to be most common when people overeat or potentially overeat fat which also slows down gastric emptying and could further worsen symptoms.

Here are some tips to reduce nausea and the risk of vomiting on Mounjaro:

  • Eat smaller meals
  • Eat slowly and mindfully
  • You might need to include snacks in-between your three main meals to ensure you’re eating enough
  • Stay hydrated
  • Avoid foods that previously made you nauseous
  • Record a food diary of foods you notice trigger symptoms
  • Eat simple meals based on a good source of protein, complex carbohydrates, fruit or vegetables, and some fat
  • Don’t lie down after your meal; maybe opt for a short walk
  • Avoid eating too close to bedtime
  • Speak to your doctor or prescribing physician if symptoms persist or become too uncomfortable

2) Constipation and diarrhoea

Diarrhoea is the second-most-common side effect of Mounjaro, affecting 8-21% of participants. Constipation is less common, occurring in 6-9%.

Constipation may occur due to reduced muscle contractions in the colon when taking the medication. The causes of diarrhoea on Mounjaro aren’t fully understood.

However, these side effects are typically transient and ease after the body has adapted to the medication or the higher dose you’ve started.

Here are some tips to reduce constipation and diarrhoea on Mounjaro:

  • Stay hydrated
  • Eat a wide variety of fibre sources (nuts, seeds, fruits, vegetables, legumes)
  • Exercise and move regularly
  • Go to the toilet when needed (don’t try to hold it in)
  • Adopt a proper toilet position and place your feet on a stool to help your bowel movements
  • Experiment with probiotics and fermented foods like yoghurt, kimchi, fermented beetroot, or kombucha
  • Speak to your doctor or prescribing physician if symptoms persist or become too uncomfortable

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3) Rare side effects

Thyroid cancer and thyroid-related disorders

A recent meta-analysis (a study reviewing many studies on one topic) analysed the risk of thyroid cancer (and other thyroid-related conditions) associated with the use of GLP-1 receptor agonists like semaglutide (Wegovy) and liraglutide (Saxenda).

The results indicated that liraglutide (Saxenda) slightly increases the risk of thyroid-related disorders compared to placebo.

This is well-documented in the literature, and individuals with a family history of thyroid cancers and related disorders shouldn’t take liraglutide.

Another GLP-1 dulaglutide was also shown to increase the risk of thyroid cancer and related disorders. It’s essential to note that these are rare side effects, and the overall risk is still minimal.

However, a systematic review and meta-analysis of 10 trials in 6,836 participants reviewed the adverse events and most common side effects of tirzepatide.

There were no reported cases of thyroid cancer, or thyroid-related conditions, related to tirzepatide.

Pancreatitis and pancreatic cancer

Despite the current guidelines against individuals with a history of pancreatic cancer or pre-existing pancreatic conditions, there’s no evidence that tirzepatide increases the risk of pancreatitis or pancreatic cancer.

A systematic review and meta-analysis of 10 trials in 6,836 participants reviewed the adverse events and most common side effects of tirzepatide. There were no reported cases of acute pancreatitis related to tirzepatide.

Still, a potential link between pancreatic function and tirzepatide can’t be completely ruled out. But based on the currently available data, it doesn’t seem to increase the risk of these conditions.

Additionally, using tirzepatide has been shown to significantly improve insulin sensitivity and the ability of pancreatic beta-cells to produce and secrete insulin.

So, it’s likely that the benefits to glucose management and insulin sensitivity observed with tirzepatide outweigh any potential risks associated with pancreatitis for most people.


A systematic review and meta-analysis of 10 trials in 6,836 participants reviewed the adverse events and most common side effects of tirzepatide. There were no reported cases of fatigue related to tirzepatide.

However, researchers conducted exit interviews with patients on Mounjaro to collect qualitative data on their experience.

Fatigue was reported by a few patients, with one saying, ‘I got real slow, like lazy. I don’t know what it was, but I was tired or whatever’.

Online community forums also suggest that fatigue seems to be a side effect people are experiencing.

There are a few possible explanations why tirzepatide may lead to fatigue (important: this is speculation at the moment; research has yet to be conducted to test these theories):

  1. GLP-1s interact with an area of the brain called the hypothalamus; this regulates our sleep-wake cycle. The interaction between tirzepatide and the hypothalamus may disrupt the regulation of our sleep-wake cycle, causing fatigue.
  2. Tirzepatide may reduce appetite to such an extent that people don’t consume enough calories; this reduces the energy available to the body for essential functions

Here are our top tips to prevent and manage fatigue on Mounjaro:

  • Eat enough in your meals, and include snacks in between if needed
  • Ensure a source of protein at each meal
  • Aim to eat 5-9 servings of fruit and vegetables a day and cook your vegetables to make them easier to digest
  • Stay hydrated
  • Exercise and move regularly
  • Consider taking a multivitamin
  • Get daylight exposure, particularly in the morning
  • Aim to sleep 7-9 hours per night
  • Avoid caffeine after midday if you’re caffeine sensitive
  • Speak to your doctor or prescribing physician if symptoms persist or become too uncomfortable

Take home message

The recent innovations in GLP-1 medications are pretty remarkable and have caused quite a stir in the world of healthcare.

We’re in new territory where obesity and type 2 diabetes medications treat an underlying cause instead of merely treating the symptoms to support better management.

However, these drugs shouldn’t be seen as miracle cures; they’re not designed to be used for life.

They’ve been designed to be taken alongside lifestyle changes that will enable you to eventually come off the medications and maintain your lower weight for the long term.

The harsh reality is that we still don’t know the long-term effects of these medications on our health. There are examples of drugs being withdrawn from the market as more long-term data comes to light.

Lorcaserin was a weight loss drug approved by the Food and Drug Administration (FDA) in the U.S. but was recently withdrawn as data indicated that it increased cancer risk.

The example of lorcaserin is why you must weigh the pros and cons when your doctor prescribes medication or before you purchase it yourself and start treatment. Report any side effects to your healthcare team.

GLP-1s are safe and effective for most people in the short to medium term, but we don’t have long-term data to be sure they’re not causing severe adverse effects elsewhere.

If you’re considering taking Mounjaro for weight loss, weighing up the pros and cons of the medications and understanding your experience will be different to others.

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