Semaglutide Side Effects: A Balanced Guide
Semaglutide has transformed the treatment of type 2 diabetes and obesity, and in the UK it is prescribed as Ozempic and Rybelsus for diabetes and as Wegovy for weight management. High demand has also created a market for unlicensed and counterfeit products. Our editorial team summarises the known side effects, from the very common to the rare but serious, so you can discuss them knowledgeably with your clinician.
How semaglutide works and why side effects arise
Semaglutide is a long-acting analogue of glucagon-like peptide-1 (GLP-1), a gut hormone released after meals. It stimulates insulin release when glucose is high, reduces glucagon, slows stomach emptying and acts on appetite centres in the brain to increase fullness.
The same mechanisms that deliver its benefits also explain most of its side effects. Slower gastric emptying causes nausea and fullness, appetite suppression reduces food intake, and rapid weight loss has knock-on effects on the gallbladder and on muscle mass.
In the STEP 1 trial, published by Wilding and colleagues, adults with obesity using weekly 2.4 mg semaglutide lost on average around 15% of body weight over 68 weeks, compared with a few per cent with placebo. Gastrointestinal events were reported by the majority of participants in both groups but considerably more often with semaglutide.
The doses mentioned here are those used in published trials and official labelling; treatment in the UK is prescription-only and started at a low dose that is increased gradually.
The common ones: the digestive system
Nausea is the signature side effect. Diarrhoea, vomiting, constipation, abdominal pain, bloating, belching and heartburn are also very common. Most are mild to moderate and transient.
Practical measures that clinicians often suggest include eating smaller meals, stopping when comfortably full, limiting fatty and very sweet foods and avoiding lying down straight after eating. Staying hydrated is particularly important if vomiting or diarrhoea occur.
Severe or persistent vomiting is not just unpleasant: dehydration can lead to acute kidney injury, especially in people taking diuretics, ACE inhibitors or metformin. Anyone unable to keep fluids down should seek medical advice.
Slowed gastric emptying has another consequence: food may remain in the stomach longer than expected before surgery or endoscopy. Patients should always tell the anaesthetist that they use a GLP-1 medicine.
Gallbladder, pancreas and other serious risks
Gallbladder problems, including gallstones and inflammation, occur more often with semaglutide, partly because rapid weight loss itself promotes gallstone formation. Pain in the upper right abdomen, fever or yellowing of the skin need prompt assessment.
Acute pancreatitis has been reported with GLP-1 receptor agonists. It is uncommon, but severe abdominal pain spreading to the back, with or without vomiting, is a reason to stop the medicine and seek urgent care.
| Side effect | Approximate frequency category | Warning signs |
|---|---|---|
| Nausea, diarrhoea, vomiting, constipation | Very common | Inability to keep fluids down |
| Gallstones | Common | Right upper abdominal pain, jaundice |
| Hypoglycaemia (with insulin or sulfonylureas) | Common in those combinations | Shakiness, sweating, confusion |
| Acute pancreatitis | Uncommon | Severe pain radiating to the back |
| Worsening diabetic retinopathy | Seen in people with existing retinopathy | Vision changes |
In the SUSTAIN-6 trial, Marso and colleagues observed more complications of diabetic retinopathy with semaglutide, mainly in people with pre-existing eye disease and a rapid fall in HbA1c. People with diabetes should keep up with retinal screening. European regulators have also added a very rare risk of an optic nerve condition known as NAION to the product information.
In rodents, semaglutide caused thyroid C-cell tumours. The relevance to humans is uncertain, but the medicine is not recommended for people with a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia type 2.
Muscle, nutrition and the active person
A portion of the weight lost on semaglutide is lean mass, which includes muscle. This is true of most substantial weight loss, but the speed and magnitude with GLP-1 medicines have drawn particular attention, especially for older adults.
Resistance training and adequate protein are the main tools for preserving muscle. Because appetite is suppressed, it can be difficult to eat enough protein, and many people benefit from dietitian support.
Signs that nutrition may be falling short include:
- persistent fatigue and poor training performance;
- dizziness when standing;
- hair shedding several months into treatment;
- frequent cramps or constipation that does not improve.
Weight regain is also common after stopping. The STEP 1 extension found that participants regained a large part of the lost weight within a year of discontinuation, which highlights that semaglutide is generally considered a long-term treatment rather than a short course.
Psychological effects, fakes and access in the UK
Regulators have reviewed reports of suicidal thoughts in people using GLP-1 medicines. The European Medicines Agency’s review in 2024 did not find evidence of a causal link, but anyone experiencing low mood or suicidal thoughts should seek help immediately.
In the UK, Wegovy is available on the NHS through specialist weight management services according to NICE guidance, and privately through registered pharmacies after a clinical consultation. Ozempic is licensed for type 2 diabetes, and supply shortages led to guidance prioritising patients with diabetes.
The MHRA has seized fake semaglutide pens and warned about products sold through social media and unregulated websites. Counterfeits may contain the wrong dose, insulin or no active ingredient at all, and some people have needed hospital treatment after using them.
Using semaglutide purely for rapid weight cutting before a competition, without medical supervision, combines the drug’s gastrointestinal risks with dehydration and inadequate nutrition, and is not advisable.
Editorial conclusions
Semaglutide is effective, but digestive side effects are very common, especially early on and after dose increases.
Less common risks include gallbladder disease, pancreatitis, worsening retinopathy in susceptible people and loss of lean mass alongside fat.
Use should be prescribed, monitored and supported by nutrition and strength training; counterfeit or unlicensed products carry serious additional risks.
You might also like our articles on metformin side effects and contraindications, on blood tests during metformin therapy and on who should not take synephrine.
References
- Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med. 2021;384(11):989–1002.
- Marso SP, Bain SC, Consoli A, et al. Semaglutide and cardiovascular outcomes in patients with type 2 diabetes. N Engl J Med. 2016;375(19):1834–1844.
- Lincoff AM, Brown-Frandsen K, Colhoun HM, et al. Semaglutide and cardiovascular outcomes in obesity without diabetes. N Engl J Med. 2023;389(24):2221–2232.
- Wilding JPH, Batterham RL, Davies M, et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: the STEP 1 trial extension. Diabetes Obes Metab. 2022;24(8):1553–1564.
- Rubino D, Abrahamsson N, Davies M, et al. Effect of continued weekly subcutaneous semaglutide vs placebo on weight loss maintenance in adults with overweight or obesity: the STEP 4 randomized clinical trial. JAMA. 2021;325(14):1414–1425.
- National Institute for Health and Care Excellence. Semaglutide for managing overweight and obesity (TA875). London: NICE; 2023.
