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Side effects of weight loss injections and how to manage them

Duncan Reid: Patient Safety & Professional Services Manager | minute read

The UK arrival of new treatments for weight loss has generated extensive media attention, with weight loss injection Wegovy launching in September 2023 and Mounjaro following a few months later when it was introduced to the private healthcare market. With origins in treating type 2 diabetes, semaglutide (the active ingredient in Wegovy) and tirzepatide (the active ingredient in Mounjaro) have both now been approved by the MHRA to help patients living with obesity (having a BMI of 30 or more) to lose weight. In this article, we look at the evidence gathered from studies into the effectiveness of Wegovy, what serious side effects to be aware of and what to do if you experience them, as well as common side effects that can be managed throughout the course of treatment. We’ll also provide guidance on how to get the most from weight loss injection medicines through changes to diet and activity levels.  

How effective are weight loss injection medicines?


Clinical trials have demonstrated that participants (individuals with a BMI of 30+, or individuals with a BMI of 27+ and a high-risk, weight-related health condition such as high blood pressure) given 2.4mg of semaglutide once a week lost approximately 12-15% of their starting body weight over a 68-week trial period on average, compared with participants given a placebo drug (no active ingredient) who lost approximately 2.5% of their starting body weight in the same timeframe. Both participant groups also engaged in regular physical activity and a calorie-controlled diet. The clinical trial involved 1,961 adults in total, and you can read the full details of the study in this New England Journal of Medicine publication.     


A clinical trial to assess the effectiveness of tirzepatide (the active ingredient in Mounjaro) in treating obesity demonstrated an average weight loss of 15-22% over a 72-week period in participants who received the ‘live drug’ rather than the placebo. Participants receiving the placebo, by comparison, lost approximately 3% of their body weight in the same period, with both groups following a calorie-controlled diet and engaging in regular physical activity throughout. The full results of this study are also published in the New England Journal of Medicine.  

These studies show promising results in the effectiveness of both Wegovy and Mounjaro in supporting adults with obesity to reach a healthy body weight. If you’ve been prescribed one of these weight loss injections, or you’re considering speaking to a healthcare provider about whether either Wegovy or Mounjaro might be suitable for you, it’s important to be aware of possible side effects and how you can manage common symptoms if you experience them.  

What to do if you experience any of these serious side effects

Both active ingredients in Wegovy and Mounjaro are what are known as glucagon-like peptide-1 receptor agonists (GLP-1 RAs). What this essentially means is that they mirror the action performed by our bodies’ own GLP-1 hormone, which is released in the stomach when we eat. This hormone helps to regulate how much we eat, by sending signals to the brain about our levels of fullness. This can help to reduce appetite and result in weight loss. As with all medicines, both semaglutide and tirzepatide can cause side effects although the majority of these are usually mild and temporary. However, there are some serious side effects that, although rare, are worth knowing about so you can spot the signs and take immediate action if you suspect them.  

Acute pancreatitis  

This refers to sudden inflammation of the pancreas and in rare cases, has been linked to the use of weight loss injections. It’s characterised by the sudden onset of severe tummy pain that can move into the back area. Other symptoms include a high temperature (38°C+), nausea and vomiting, indigestion, a yellow tinge to the skin and whites of the eyes (jaundice), a swollen or tender stomach, and a rapid heartbeat.  

If you suspect you may have developed acute pancreatitis, stop using your weight loss medication and contact your prescribing doctor straight away. If this isn’t possible, contact NHS 111.   

Gallbladder issues 

The gallbladder is a small organ that is responsible for storing bile produced by the liver. Gallstones are a primary gallbladder issue that can cause acute pancreatitis. They are small deposits of cholesterol and while they don’t usually have symptoms, if they travel to an opening in the gallbladder, they can cause a blockage that leads to intense stomach pain lasting between 1-5 hours. They can sometimes cause complications, including an inflamed gallbladder (known as gallstone disease). This inflammation can cause similar symptoms to acute pancreatitis. Seek medical advice straight away if you develop any of these.   

Low blood sugar 

This side effect is more likely if you have type 2 diabetes. You might notice you feel dizzy or lightheaded if you develop low blood sugar. It can be a good idea to invest in an at-home blood sugar (glucose) monitor if you’re starting Wegovy or Mounjaro so you can check your levels regularly. Your glucose levels should be at or above 4mmol/L. Other symptoms of low blood sugar include blurred vision, anxiety, feeling shaky, drowsy, or weak, developing slurred speech, a headache, or a rapid heart rate. If you notice these symptoms, eat or drink something sugary to boost your glucose levels and take a blood sugar reading after 10-15 minutes. If you’re concerned, contact your prescribing clinician or visit NHS 111 online.  

Kidney problems 

If you have an existing kidney issue, common side effects of weight loss injections such as diarrhoea and vomiting can lead to dehydration which can aggravate and worsen existing kidney conditions. To combat this, ensure you’re drinking plenty of fluids at regular intervals throughout the day.  

Severe allergic reaction (anaphylaxis) 

In rare cases, Wegovy and Mounjaro may cause a severe allergic reaction in some people. If you notice symptoms including swelling around the lips, tongue, throat, or face, difficulty breathing or swallowing, a significant increase in heart rate, fainting or dizziness, or a sudden rash and intense itching, seek immediate medical attention.  

How to manage common side effects of Wegovy and Mounjaro

Common side effects associated with weight loss injections are typically mild, temporary, and can be offset by following the scheduled dose increase as advised by your prescribing clinician. If you do find that any side effects are causing you discomfort, talk to your healthcare provider as they may recommend staying on your current dose, or dropping down your dose to make your side effects more manageable. Several of the most common side effects of weight loss injections can usually be managed at home with self-care and by following a few simple steps.  

For nausea… 

  • Practice mindful eating. Eating slowly gives your brain the chance to catch up with your body, meaning you’re less likely to eat past the point of your body feeling full. 

  • Get some fresh air. Go for a short walk outdoors if you can.  

  • Try a peppermint tea or a ginger biscuit. These ingredients are known to help ease feelings of sickness in some people.  

  • Sip water. Keep your fluid intake up, drinking little and often rather than gulping down large glasses. 

  • Avoid lying down after a meal. Instead, try to go for a gentle walk or distract yourself with a film or a book.  

  • Opt for bland, low-fat foods such as rice, toast, or crackers. These foods will be more gentle on your tummy and should help reduce the risk of feeling sick after eating when using Wegovy or Mounjaro.  

For diarrhoea and vomiting... 

  • Avoid fatty, processed, and spicy foods. Choose bland options like plain pasta, bananas, and rice to prevent further irritation. 

  • Drink plenty of water to stay hydrated.  

  • Speak to a pharmacist. They may be able to advise on an over-the-counter medicine that can help in the short term.  

For constipation… 

  • Stay hydrated. Drinking plenty of water can help to soften stools and promote more regular bowel movements. 

  • Eat a fibre-rich diet. Try to include beans and pulses, as well as plenty of fresh vegetables like broccoli and carrots in your daily food intake.  

  • Keep activity levels up. Regular exercise can help to alleviate constipation and encourage regular bowel movements.  

  • Talk to a pharmacist. They may be able to recommend over-the-counter treatments that are suitable for use with your weight loss injection medicine.  

For heartburn, acid reflux, and gas… 

  • Eat smaller amounts more often. Try to avoid large meals and opt instead for smaller portions more frequently throughout the day.  

  • Have your last meal 3-4 hours before going to bed. Lying down shortly after food can make heartburn and acid reflux more likely.  

  • Speak to a pharmacist. They may be able to offer advice on an over-the-counter antacid treatment to help prevent acid reflux and reduce unpleasant ‘eggy’ burps sometimes reported by users of weight loss injections.  

Small lifestyle adjustments that can support and accelerate weight loss

Manufacturers of both Wegovy and Mounjaro acknowledge that the most successful weight loss is achieved by patients who introduce a calorie-controlled diet and include regular physical activity in their schedules. To help you get the most from your weight loss treatment, we recommend: 

  • Setting a daily calorie goal. The NHS recommends a drop of 600 calories from the typical daily calorie intake guidance, so aim for: 

    • 1,400 calories if you’re female 

    • 1,900 calories if you’re male 

  • Your daily intake should include at least 5 fruit and veg portions, and if your diet allows, 2 servings of fish each week too. One of these should be oily, such as salmon or mackerel. There are apps available to download on your phone to help keep track of your calorie intake. Visit our healthy eating for weight loss guide to learn more about dieting for sustainable weight loss. 

  • Aim to drink between 6-8 cups of fluids (water, hot drinks, and soup all count) a day to keep you hydrated and support your weight loss goals.  

  • Get moving! Take any opportunity to get a few extra steps in, such as parking further away from the supermarket entrance when you do your shopping, or getting off the bus a stop away from work or home. As you progress with your weight loss goals, try to increase your weekly activity. Speak to a friend or family member about booking in a regular walk together each week to keep up the momentum.  

If you have any concerns or questions about your weight loss treatment, don’t hesitate to contact your healthcare provider for more information, guidance, and support.