Side Effects of Weight Loss Injections

Side effects of weight loss injections are a key consideration for anyone starting treatment with semaglutide (Wegovy) or tirzepatide (Mounjaro) in the UK. Both medicines work by acting on gut hormone receptors to slow gastric emptying and reduce appetite, which explains why digestive symptoms are the most frequently reported effects. While most side effects are mild and temporary, some require prompt medical attention. This article covers what to expect, which symptoms are serious, how long effects typically last, who is most at risk, and practical strategies for managing symptoms day to day.

What are the most common side effects of weight loss injections?

The most common side effects are gastrointestinal, including nausea, vomiting, diarrhoea, constipation, bloating and indigestion, and are usually mild to moderate and most noticeable when starting treatment or increasing the dose.

The most common side effects of weight loss injections are gastrointestinal in nature, affecting the stomach and digestive system. The two medicines licensed in the UK for weight management are semaglutide (Wegovy) and tirzepatide (Mounjaro).^[2][3] Wegovy contains semaglutide, a GLP-1 receptor agonist.^[3][7] Mounjaro contains tirzepatide, which acts on both GIP and GLP-1 receptors.^[4][6] Note that Ozempic also contains semaglutide but is licensed in the UK for type 2 diabetes, not for weight management.^[5][6] These medicines slow gastric emptying and reduce appetite, and the digestive system takes time to adjust, particularly when starting treatment or moving to a higher dose.

The side effects most frequently reported include:

• Nausea (the most commonly reported symptom)

• Vomiting

• Diarrhoea or constipation

• Bloating and excess wind

• Indigestion or heartburn

• Reduced appetite or feeling full very quickly

These effects are documented in the UK Summaries of Product Characteristics (SmPCs) for both Wegovy and Mounjaro, available via the electronic Medicines Compendium (emc), and are consistent with findings from clinical trials. They tend to be most noticeable in the first few weeks of treatment or following a dose increase, and are generally mild to moderate in severity for most people.

Less commonly, people may notice injection-site reactions such as redness, itching or mild swelling at the site of the weekly injection. Headache and fatigue have also been reported, though these are less frequent than gastrointestinal symptoms.

If you are considering a GLP-1 based weight management treatment, an online consultation with a registered provider can help clarify whether one of these options is appropriate for you. Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support, where a range of clinically appropriate options may be considered.

Which side effects are serious and when should you get help?

Seek urgent help for persistent severe abdominal pain (possible pancreatitis), signs of anaphylaxis, or inability to stay hydrated; contact your GP promptly for gallbladder symptoms, sustained heart rate increase or signs of hypoglycaemia.

Serious side effects from weight loss injections are uncommon, but certain symptoms require prompt medical attention and should not be ignored. The UK SmPCs for Wegovy and Mounjaro identify several clinically important risks that patients and prescribers should be aware of before and during treatment.

Pancreatitis is one of the most significant risks. Acute pancreatitis has been reported with GLP-1 receptor agonists, including post-marketing reports of necrotising pancreatitis and cases with fatal outcomes.^[9][3] If you develop persistent, severe abdominal pain that may radiate to your back, stop taking the medicine and contact your GP promptly. If the pain is severe and sudden, attend A&E.

Gallbladder problems, including gallstones and inflammation of the gallbladder (cholecystitis), have also been reported. Symptoms include upper right abdominal pain, fever and jaundice (yellowing of the skin or eyes). Contact your GP if you notice these.

Allergic reactions, including serious hypersensitivity reactions such as anaphylaxis and angioedema, are rare but require immediate emergency care. Call 999 if you experience difficulty breathing, swelling of the face, lips or throat, or a rapid heartbeat following an injection.

Heart rate increase has been observed with some GLP-1 medicines.^[3][4] If you notice a sustained or unexplained increase in your resting heart rate, speak to your GP.

Hypoglycaemia (low blood sugar) is an important consideration for people using these medicines alongside insulin or sulfonylureas for type 2 diabetes. Combining GLP-1 or GIP/GLP-1 medicines with these treatments increases the risk of hypoglycaemia.^[3][4] If this applies to you, discuss dose adjustments and how to recognise symptoms of low blood sugar with your diabetes care team before starting treatment.

Dehydration can become a concern if vomiting or diarrhoea is prolonged. Signs that require urgent attention include being unable to keep fluids down, passing very little urine, or feeling severely dizzy or faint. Contact NHS 111 or attend A&E if you are unable to stay hydrated.

For people with pre-existing pre-proliferative or proliferative diabetic eye disease, rapid improvement in blood glucose control may be associated with transient worsening of diabetic retinopathy. This is particularly relevant for those using these medicines to manage type 2 diabetes alongside weight.

For non-urgent concerns, contact your GP or call NHS 111 for advice. Suspected side effects can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

How long do side effects usually last?

Gastrointestinal side effects are usually temporary, improving within the first few weeks at each dose level; constipation may persist longer, while nausea and diarrhoea typically resolve more quickly.

For most people, the gastrointestinal side effects of weight loss injections are temporary and tend to improve within the first few weeks of starting treatment or after a dose increase. The dose escalation schedules used for both Wegovy and Mounjaro are specifically designed to allow the body to adjust gradually, which helps to reduce the intensity and duration of these early symptoms, as described in the respective patient information leaflets.^[3][4]

Nausea is typically most pronounced in the early weeks at a new dose level and often settles as the body adapts to the medicine. Constipation can sometimes persist for longer and may require ongoing dietary management throughout treatment. Diarrhoea, by contrast, tends to resolve more quickly.

Injection-site reactions, where they occur, are usually short-lived and resolve within a day or two of the injection. Rotating the injection site each week, as advised in the patient information leaflet, can help prevent localised skin irritation from building up.

It is worth noting that side effects can recur or intensify each time the dose is increased, following the same pattern as when treatment was first started. This is expected and does not necessarily mean the medicine is not suitable for you. If symptoms are persistent, severe or significantly affecting your quality of life at any dose level, this should be discussed with your prescriber or pharmacist, who may advise slowing the dose escalation or reviewing whether the treatment remains appropriate.

Who is more likely to experience side effects?

People with a history of pancreatitis, gastrointestinal disease or gallbladder problems, older adults, and those who escalate their dose quickly are at greater risk of experiencing side effects.

Certain groups of people are more likely to experience side effects from weight loss injections, and this is an important consideration when assessing suitability for treatment. A thorough clinical assessment before starting any GLP-1 or GIP/GLP-1 based medicine should take individual risk factors into account.

People with a history of gastrointestinal conditions, such as severe gastroparesis (delayed stomach emptying) or a history of pancreatitis, may be at greater risk of experiencing more pronounced digestive side effects or complications. The UK SmPCs for both Wegovy and Mounjaro advise caution in people with a history of pancreatitis, and caution is also warranted in those with severe gastrointestinal disease.

Those with a history of gallbladder disease should also be assessed carefully, as GLP-1 receptor agonists are associated with an increased risk of gallstones, particularly with significant or rapid weight loss.^[10][11]

People who escalate their dose quickly or who do not follow the recommended titration schedule are more likely to experience nausea and vomiting. Sticking to the prescribed dose escalation plan is one of the most effective ways to reduce side effect burden.

Older adults may be more sensitive to side effects, particularly nausea and dehydration resulting from vomiting or diarrhoea, and may need closer monitoring.

These medicines should not be used during pregnancy. If you are planning a pregnancy, speak to your prescriber about when to stop treatment, as a washout period before conception is recommended; the specific interval differs between products and should be confirmed against the relevant SmPC or with your prescriber. These medicines are generally not recommended during breastfeeding. Women of childbearing potential should use effective contraception during treatment.

Specifically, the UK SmPC for tirzepatide (Mounjaro) advises using an additional non-oral contraceptive method for four weeks after starting treatment and for four weeks after each dose increase, as the medicine may reduce the absorption of oral contraceptives by slowing gastric emptying.^[4][6]

How can you manage side effects day to day?

Eating smaller meals, avoiding fatty foods, staying hydrated, increasing dietary fibre and rotating injection sites are the most effective day-to-day strategies for managing common side effects.

Most day-to-day side effects of weight loss injections can be managed effectively with practical dietary and lifestyle adjustments, and many people find that symptoms become much more manageable after the first few weeks. The following strategies are consistent with guidance provided in patient information leaflets for Wegovy and Mounjaro.

To reduce nausea:

• Eat smaller, more frequent meals rather than large portions

• Avoid rich, fatty or heavily spiced foods, particularly around the time of your injection

• Eat slowly and stop eating as soon as you feel full

• Stay well hydrated with water or clear fluids

• Avoid lying down immediately after eating

To manage constipation:

• Increase dietary fibre gradually through vegetables, fruit and wholegrains

• Drink plenty of fluids throughout the day

• Gentle physical activity, such as walking, can help stimulate bowel movement

• Speak to your pharmacist if over-the-counter laxatives may be appropriate

To manage dehydration if vomiting or diarrhoea occurs:

• Sip fluids frequently in small amounts; oral rehydration solutions can help replace lost salts

• If you are unable to keep fluids down for a prolonged period, contact your GP or NHS 111

To reduce injection-site reactions:

• Rotate the injection site each week, using the abdomen, thigh or upper arm in turn, and avoid areas that are tender, bruised, scarred or show signs of skin thickening

• If the pen has been stored in the fridge, you may allow it to reach room temperature before injecting; do not apply heat to warm it

• Follow the injection technique described in the patient information leaflet and instructions for use for your specific device; do not reuse needles

If side effects are making it difficult to eat or drink adequately, or if you are losing weight very rapidly, contact your GP for a review. Your prescriber may recommend pausing a dose increase or temporarily reducing the dose until symptoms settle, rather than stopping treatment altogether.

Scientific References

1. MHRA authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss.
2. MHRA approves GLP-1 receptor agonist semaglutide to reduce risk of serious heart problems in obese or overweight adults.
3. Wegovy, INN-semaglutide – Product Information (SmPC).
4. Mounjaro, INN-tirzepatide – Product Information (SmPC).
5. GLP-1 medicines for weight loss and diabetes: what you need to know.
6. Tirzepatide | Drugs | BNF – NICE.
7. Semaglutide | Drugs | BNF – NICE.
8. Mechanisms of GLP-1 Receptor Agonist-Induced Weight Loss.
9. GLP-1 receptor agonists and dual GLP-1/GIP receptor agonists: strengthened warnings on acute pancreatitis, including necrotising and fatal cases.
10. Association of Glucagon-Like Peptide-1 Receptor Agonist Use With Risk of Gallbladder and Biliary Diseases.
11. Effects of Liraglutide Compared With Placebo on Events of Acute Gallbladder or Biliary Disease in Patients With Type 2 Diabetes at High Risk for Cardiovascular Events in the LEADER Randomized Trial.
12. Wegovy | European Medicines Agency (EMA).
13. Mounjaro | European Medicines Agency (EMA).
14. Semaglutide: a medicine to manage type 2 diabetes or weight.
15. Summary of NICE guidance for GLP-1 receptor agonists and tirzepatide in adults.
16. GLP-1 receptor agonists and serious adverse events: Evidence, uncertainty and clinical implications.
17. Adverse Events Associated with Incretin-Based Therapies: A Narrative Review on Mechanisms, Clinical Management, and Risk Mitigation.
18. Beyond Glycemic Control: Ocular Effects of Glucagon-like Peptide-1 Receptor Agonists.

Frequently Asked Questions