Weight loss injections, particularly GLP-1 receptor agonists such as semaglutide (Wegovy) and liraglutide (Saxenda), can indeed make you feel sick. Nausea is the most common side effect, affecting 40–50% of patients, alongside other gastrointestinal symptoms including vomiting, diarrhoea, and abdominal discomfort. These medications work by mimicking natural hormones that slow gastric emptying and regulate appetite, which explains why digestive symptoms are so prevalent. Whilst most side effects are mild to moderate and improve over time, understanding which symptoms are expected and which require urgent medical attention is essential for safe use. This article explores why these injections cause sickness, how to manage symptoms, and when to seek help.

Understanding Weight Loss Injections and Their Side Effects

Weight loss injections, particularly glucagon-like peptide-1 (GLP-1) receptor agonists such as semaglutide (Wegovy, administered weekly) and liraglutide (Saxenda, administered daily), have become increasingly prescribed for weight management in the UK. These medications work by mimicking natural hormones that regulate appetite and blood glucose levels, leading to reduced food intake and subsequent weight loss. Whilst these injections can be effective tools for weight management when used alongside lifestyle modifications, they are associated with a range of side effects that can indeed make patients feel unwell.

In the UK, these medications are licensed for weight management in adults with obesity (BMI ≥30 kg/m²) or those who are overweight (BMI ≥27 kg/m²) with at least one weight-related comorbidity. However, NHS access is more restricted. According to NICE guidance (TA875 for semaglutide and TA664 for liraglutide), these treatments are typically only available through specialist weight management services, for people meeting specific criteria, and usually for a limited treatment duration.

It is crucial to understand that feeling unwell whilst taking weight loss injections is relatively common, especially during dose escalation phases. The majority of side effects are gastrointestinal in nature and tend to be mild to moderate in severity. However, understanding which symptoms are expected and which warrant medical attention is essential for patient safety. NICE guidance emphasises the importance of appropriate patient selection, monitoring, and education about potential adverse effects before initiating treatment with these medications.

Common Side Effects That May Make You Feel Unwell

The most frequently reported side effects of weight loss injections are gastrointestinal symptoms, which affect a significant proportion of patients. Nausea is the most common adverse effect, occurring in approximately 40-50% of patients taking GLP-1 receptor agonists, though rates vary by product and dose according to clinical trials (STEP trials for semaglutide; SCALE trials for liraglutide). This sensation of queasiness can range from mild discomfort to severe nausea that interferes with daily activities. Many patients also experience vomiting, particularly during the first few weeks of treatment or following dose increases.

Other common gastrointestinal side effects include:

• Diarrhoea – loose or frequent bowel movements affecting 20-30% of patients

• Constipation – paradoxically, some patients experience reduced bowel frequency

• Abdominal pain or discomfort – cramping or generalised abdominal discomfort

• Dyspepsia – indigestion, bloating, or upper abdominal fullness

• Decreased appetite – whilst this is part of the therapeutic effect, it can feel uncomfortable

Beyond gastrointestinal symptoms, patients may experience other effects that contribute to feeling unwell. Fatigue and tiredness are reported by some individuals, particularly during the adjustment period. Headaches occur in approximately 10-15% of patients and can range from mild to moderate in intensity. Some patients report dizziness or light-headedness, especially when standing quickly, which may be related to reduced food intake or dehydration secondary to gastrointestinal symptoms.

Injection site reactions, including redness, itching, or mild swelling at the injection site, are generally mild but can cause local discomfort. These symptoms typically resolve within a few days and become less frequent with continued use as patients become accustomed to the injection technique.

It's important to stay well-hydrated, especially if experiencing diarrhoea or vomiting. Consider oral rehydration solutions if fluid losses are significant.

Serious Adverse Reactions and When to Seek Medical Help

Whilst most side effects of weight loss injections are manageable, certain serious adverse reactions require immediate medical attention. Acute pancreatitis is a rare but potentially serious complication associated with GLP-1 receptor agonists. Patients should seek urgent medical help if they experience severe, persistent abdominal pain that may radiate to the back, often accompanied by nausea and vomiting. The MHRA has issued guidance highlighting the importance of recognising this complication, which typically requires hospital admission and discontinuation of the medication.

Gallbladder disease, including cholecystitis and cholelithiasis, has been reported more frequently in patients taking weight loss injections compared to placebo. Symptoms include severe right upper abdominal pain, particularly after eating fatty meals, fever, and jaundice (yellowing of the skin or eyes). If these symptoms develop, patients should contact their GP promptly or attend A&E if symptoms are severe.

Other serious adverse reactions requiring immediate medical assessment include:

• Severe allergic reactions – Call 999 immediately if experiencing difficulty breathing, facial or tongue swelling, widespread rash, or rapid heartbeat

• Acute kidney injury – significantly reduced urine output, swelling of legs or ankles, or confusion (often secondary to severe dehydration from vomiting or diarrhoea)

• Severe hypoglycaemia – particularly in patients taking concurrent diabetes medications; symptoms include confusion, sweating, trembling, or loss of consciousness

• Changes in vision – blurred vision or other visual disturbances, which may indicate worsening of pre-existing diabetic retinopathy (particularly relevant for patients with diabetes)

• Persistent vomiting – inability to keep down fluids for more than 24 hours, leading to dehydration

• Depression or suicidal thoughts – patients and carers should be vigilant for mood changes and seek urgent help if such symptoms occur; the medication should be discontinued in this situation. While regulatory reviews have not established a causal link, monitoring is advised.

Patients should contact their GP or NHS 111 if they experience symptoms that are severe, persistent, or significantly impact their quality of life, even if these symptoms are not listed as serious adverse effects.

If you experience any suspected side effects, you can report them via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

Why Weight Loss Injections Cause Nausea and Gastrointestinal Symptoms

Understanding the mechanism behind gastrointestinal side effects can help patients appreciate why these symptoms occur and why they often improve over time. GLP-1 receptor agonists work by mimicking the action of naturally occurring GLP-1, a hormone released by the intestine in response to food intake. This hormone has multiple effects throughout the body, including the gastrointestinal tract, which explains the high incidence of digestive symptoms.

The primary mechanisms causing nausea and gastrointestinal symptoms include:

Delayed gastric emptying – GLP-1 receptor agonists significantly slow the rate at which food leaves the stomach and enters the small intestine. This delayed gastric emptying contributes to feelings of fullness and satiety, which is therapeutically beneficial for weight loss. However, it also leads to prolonged distension of the stomach, triggering nausea and sometimes vomiting. Food remains in the stomach longer than usual, which can cause discomfort, bloating, and reflux symptoms.

Central nervous system effects – GLP-1 receptors are present in areas of the brain that control appetite and nausea, including the area postrema (the brain's 'vomiting centre'). Activation of these receptors directly stimulates nausea pathways, particularly during initiation and dose escalation phases.

Altered gut motility – these medications affect the movement of the entire gastrointestinal tract, not just the stomach. Changes in intestinal motility can result in either diarrhoea (if transit is accelerated in the intestines) or constipation (if colonic motility is reduced). The variable effects on different parts of the digestive system explain why some patients experience diarrhoea whilst others develop constipation.

The good news is that gastrointestinal side effects typically diminish over time as the body adapts to the medication. Most patients find that nausea and other symptoms are most pronounced during the first 4-8 weeks of treatment and gradually improve thereafter. The dose escalation protocols recommended by manufacturers are specifically designed to minimise these effects by allowing gradual physiological adaptation.

Managing Side Effects and Reducing the Risk of Feeling Sick

Several evidence-based strategies can help patients manage side effects and reduce the likelihood of feeling unwell whilst taking weight loss injections. Dietary modifications are particularly important in minimising gastrointestinal symptoms. Patients should eat smaller, more frequent meals rather than large portions, as this reduces gastric distension and accommodates the delayed gastric emptying caused by the medication. Avoiding high-fat, greasy, or spicy foods can significantly reduce nausea, as these foods are more difficult to digest and remain in the stomach longer.

Practical dietary recommendations include:

• Eating slowly and chewing food thoroughly

• Stopping eating when feeling comfortably satisfied, not overly full

• Avoiding lying down immediately after meals

• Staying well-hydrated with small, frequent sips of water throughout the day

• Choosing bland, easily digestible foods during periods of nausea (toast, crackers, rice)

• Limiting carbonated beverages and alcohol, which can exacerbate bloating and nausea

Timing of injections can also influence side effects. Some patients find that administering their injection at different times of day affects their tolerance of the medication. Patients should work with their healthcare provider to identify the optimal timing for their individual circumstances.

Adherence to dose escalation schedules is crucial. The gradual increase in dose allows the body to adapt to the medication's effects. Patients should never increase their dose more rapidly than recommended, even if they are tolerating the current dose well, as this significantly increases the risk of adverse effects.

If side effects are severe or persistent, patients should consult their prescriber about potential dose adjustments. Sometimes, maintaining a lower dose for a longer period before escalating, or even reducing the dose temporarily, can improve tolerability whilst still providing therapeutic benefit.

Anti-nausea medications may be prescribed for short-term use in patients experiencing severe nausea, though these should only be taken under medical supervision. If prochlorperazine or cyclizine is prescribed, follow your doctor's instructions carefully. If domperidone is considered, it should only be used at the lowest effective dose for the shortest possible duration (usually no more than 7 days) due to cardiac safety concerns, and is contraindicated in people with certain heart conditions or those taking medications that affect heart rhythm. All antiemetic use in this context is off-label and requires clinician oversight.

Who Should Avoid Weight Loss Injections Due to Health Risks

Weight loss injections are not suitable for everyone, and certain individuals should avoid these medications due to increased health risks. According to UK product information, these medications should not be used in people with a personal history of severe allergic reactions to the active substances or any of the excipients.

UK SmPCs note that GLP-1 receptor agonists have been associated with thyroid C-cell tumours in rodent studies, though a causal relationship in humans has not been established. Patients with personal or family history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia syndrome type 2 (MEN 2) should discuss these concerns with their specialist before considering treatment.

These medications should not be used during pregnancy or breastfeeding. For semaglutide specifically, the medication should be discontinued at least two months before a planned pregnancy. Women of childbearing potential should use effective contraception during treatment.

Patients with a history of pancreatitis should use GLP-1 receptor agonists with caution, and alternative treatments may be preferable. Those with severe gastrointestinal disease, including significant gastroparesis (delayed stomach emptying), should discuss risks and benefits with their specialist, as these medications may affect gut motility.

Other groups who should exercise caution or may not be suitable for these medications include:

• Patients with severe renal impairment – particularly those at risk of dehydration from gastrointestinal side effects

• Individuals with diabetic retinopathy – rapid glucose lowering may temporarily worsen retinopathy in people with diabetes

• Those taking certain medications – particularly insulin or sulfonylureas, which increase hypoglycaemia risk

• Patients with eating disorders – the appetite-suppressing effects may exacerbate disordered eating patterns

• Elderly patients – may be more susceptible to dehydration and adverse effects

In the UK, NICE guidance (TA875 for semaglutide and TA664 for liraglutide) specifies that these treatments should only be available through specialist weight management services. They are typically prescribed for people with a BMI of 35 kg/m² or more (or 32.5 kg/m² or more for people from certain ethnic backgrounds) with at least one weight-related comorbidity, and who have committed to a weight management programme. Treatment is generally time-limited and subject to specific continuation criteria.

A thorough medical assessment, including review of medical history and current medications, is essential before initiating treatment to ensure patient safety and optimise outcomes.

Frequently Asked Questions