Mounjaro side effects – comparing real-life experience to the patient information leaflet

Reviewed and fact-checked by Alessandro Grenci, Superintendent Pharmacist. Read our editorial policy to see how we create informative, accurate content.

When taking a new medication it can sometimes be hard to understand what to expect. Medicines go through a rigorous process to ensure they are safe to use and manufacturers are required to list risks and side effects that have been observed. However, because all individuals are different it may sometimes be hard to interpret the listed potential side effects. In this article, we aim to help you gain an understanding of what to expect and we do this by comparing real-life experienced side effects with the mentioned side effects in the patient information leaflet to help you make an informed decision if Mounjaro for weight loss is for you.



Let’s start with the leaflet, what does it actually say about the side effects?

Very Common Side Effects (may affect more than 1 in 10 people)

  • Low blood sugar (hypoglycaemia) when used with sulphonylurea or insulin for type 2 diabetes: Symptoms include headache, drowsiness, weakness, dizziness, confusion, irritability, fast heartbeat, and sweating.
  • Nausea
  • Diarrhea
  • Vomiting (usually subsides over time)
  • Constipation

Common Side Effects (may affect up to 1 in 10 people)

  • Allergic reactions (hypersensitivity): Includes rash, itching, and eczema.
  • Dizziness (observed in weight management)
  • Low blood pressure (observed in weight management)
  • Decreased appetite (in type 2 diabetes)
  • Stomach pain
  • Indigestion (dyspepsia)
  • Bloating
  • Burping (eructation)
  • Gas (flatulence)
  • Reflux or heartburn (gastroesophageal reflux disease - GORD)
  • Hair loss (observed in weight management)
  • Fatigue
  • Injection site reactions (itching or redness)
  • Fast pulse
  • Increased levels of pancreatic enzymes (lipase and amylase in blood)

Uncommon Side Effects (may affect up to 1 in 100 people)

  • Inflamed pancreas (acute pancreatitis): Causes severe pain in the stomach and back.
  • Severe allergic reactions: Symptoms include breathing problems, rapid swelling of lips, tongue, throat, difficulty swallowing, fast heartbeat.
  • Gallstones
  • Injection site pain
  • Increased calcitonin levels in blood
  • Cholecystitis (infection of the gallbladder observed in weight management)

Review analysis

We went through hundreds of user reviews, looking for reported side effects. Here are our key findings:

  • Constipation: Many users noted constipation as a side effect, which might be due to Mounjaro's effect on gut motility. Constipation can be uncomfortable and may require adjustments in diet such as increased fibre intake, or the use of over-the-counter stool softeners to manage this symptom effectively.
  • Nausea: Nausea was commonly reported, especially following dosage increases. For some users, this side effect diminished over time as their bodies adjusted to the medication. Others found relief through dietary adjustments or by taking the medication with food.
  • Fatigue: Several reviewers mentioned experiencing significant tiredness or fatigue, particularly the day after receiving their injection. This could affect daily activities, although for many, this symptom lessened as their treatment continued.
  • Gastrointestinal Issues: Besides constipation and nausea, other gastrointestinal symptoms included abdominal pain and indigestion. A few severe cases involved conditions like pancreatitis, which is an inflammation of the pancreas that can be serious and requires immediate medical attention.
  • Appetite Reduction: While effective for weight loss, the marked decrease in appetite caused by Mounjaro sometimes led to challenges in consuming enough nutrients. Users often had to remind themselves to eat or found that they could only eat very small amounts, which could potentially lead to nutritional deficiencies if not carefully managed.
  • Injection Site Reactions: Some users experienced reactions at the site of injection, such as redness, irritation, or swelling. These reactions are typically mild and transient but can be bothersome.
  • Hypoglycemia: Particularly noted by users with diabetes, hypoglycemia (low blood sugar levels) can occur if not monitored closely. It's important for patients on Mounjaro to regularly check their blood sugar levels and adjust their diabetes medications as needed under the guidance of a healthcare provider.
  • Emotional and Psychological Effects: A few users reported changes in their mood, including the development of depression or an increase in anxiety. It's crucial for patients experiencing these symptoms to discuss them with their healthcare provider, as adjustments to the treatment or additional support may be necessary.
  • Headaches: Headaches were occasionally mentioned as a side effect, particularly during the initial stages of treatment or after a dose increase. For most, this symptom was temporary and could be managed with standard over-the-counter pain relief.


The side effects mentioned in the patient information leaflet for Mounjaro align closely with those reported in user reviews, confirming the experiences of actual patients using the medication for weight loss and diabetes management. Here's how they compare:

  • Nausea, Vomiting, and Diarrhea: Both sources confirm these as very common side effects. User reviews particularly highlighted nausea as manageable but persistent, especially after dose increases, mirroring the leaflet's mention of it being more common when starting treatment.
  • Constipation: This side effect is also consistent across the patient leaflet and user testimonials, with both sources noting it as a frequent issue. User reviews often discussed needing dietary adjustments or over-the-counter aids to manage constipation.
  • Hypoglycemia: The leaflet warns of hypoglycemia, especially when Mounjaro is used in combination with other diabetes medications like sulphonylureas or insulin. User reviews by diabetic patients confirmed occurrences of low blood sugar, necessitating careful monitoring.
  • Injection Site Reactions: Common in both user reviews and the leaflet, with reports of redness and irritation at the injection site. This side effect was generally described as mild and manageable.
  • Decreased Appetite: The leaflet mentions decreased appetite as a common side effect, which aligns with user experiences of reduced hunger and food intake, contributing to weight loss.
  • Fatigue: Mentioned in user reviews, fatigue aligns with the broader category of general side effects like feeling tired, which the leaflet notes.
  • Gastrointestinal Distress: The leaflet includes abdominal pain, indigestion, bloating, and gas, which users also frequently reported. These symptoms were often linked to dietary intake.
  • Serious Allergic Reactions: Though not commonly reported in user reviews, the leaflet advises on the possibility of severe allergic reactions like anaphylaxis, highlighting the need for caution and immediate medical attention if symptoms occur.
  • Dehydration: The leaflet warns about dehydration risks due to vomiting, nausea, or diarrhoea, suggesting users stay hydrated. Some user reviews reflected concerns about hydration, especially those experiencing gastrointestinal side effects.
  • Mental and Emotional Effects: While the leaflet does not explicitly mention depression or mood changes, user reviews did sometimes discuss emotional and psychological impacts, suggesting a broader range of potential side effects that might not be as prominently featured in official documentation.

We discussed these findings with our superintendent pharmacist, Alessandro Grenci:

When it comes to side effects what is mentioned in the patient leaflet is usually very accurate. I would however like to stress that when it comes to customer reviews it has been proven that a customer with a negative experience is a lot more likely to leave a review. That is why I would be cautious to blindly set my expectations in accordance with what has been read online from other patients. But it does serve as a general guideline of what to expect. Also as we can see in the comparison, mental effects are not discussed in the patient leaflet. Often weight-related conditions impact psychology in ways that are hard to predict and also are fairly challenging to analyse. Everyone is different and even though losing weight may help a patient feel better it is important to note that some patients have reported negative psychological effects from taking the medication.

This concludes our comparison and it shows that the patient leaflet clearly and accurately lays out what to expect in terms of side effects and that this is also in line with real-life experiences.

Sources: reviews

Patient information leaflet

Written by Christian Jakobsson