Semaglutide is the most widely prescribed GLP-1 receptor agonist in the United Kingdom. At 2.4mg weekly, the dose approved for weight management under the brand name Wegovy, it produces average body weight reductions of 14.9 to 17.4 percent in adults without diabetes, based on the STEP programme of trials published between 2021 and 2023. At the 0.5mg to 1mg dose used in diabetes management under the brand name Ozempic, modest weight loss also occurs. Understanding its side effect profile in the context of health and longevity is essential for adults considering it.
Gastrointestinal Side Effects
The most common side effects of semaglutide are gastrointestinal. In the STEP 1 trial, nausea affected approximately 44 percent of participants, vomiting 24 percent, diarrhoea 30 percent, constipation 24 percent, and abdominal pain approximately 20 percent. These effects are most pronounced in the dose escalation phase, typically the first twelve to twenty weeks of treatment. They generally diminish with time but do not fully resolve in all patients. Nausea is the primary reason for dose reduction and treatment discontinuation. The dose escalation protocol that starts at 0.25mg and increases every four weeks is designed specifically to reduce GI intolerance.
Slowed gastric emptying, a deliberate mechanism of the drug, produces a sustained sensation of fullness but also increases the risk of aspiration in patients undergoing general anaesthesia. Anaesthetic guidance in the UK and Europe now recommends treating semaglutide patients as if they have a full stomach regardless of the fasting period, with implications for anyone planning surgical procedures.
Gallbladder Disease
The STEP 1 trial found cholelithiasis, the development of gallstones, in 1.6 percent of semaglutide participants compared to 0.7 percent in the placebo group. Rapid weight loss is a known risk factor for gallstone formation, as it increases cholesterol concentration in bile. The risk is not unique to semaglutide but is amplified by the rate and extent of weight loss the drug produces. Adults with existing gallbladder disease or a family history of gallstones should discuss this risk specifically with their prescribing clinician.
Thyroid, Pancreatitis, and Other Warnings
Semaglutide carries an FDA and MHRA boxed warning regarding thyroid C-cell tumours, based on observations in rodent studies. The clinical relevance in humans is not established, and the drug is contraindicated in people with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2. Pancreatitis is a rare but serious adverse event, affecting fewer than 0.3 percent of patients. Symptoms of pancreatitis including severe persistent abdominal pain radiating to the back warrant immediate medical evaluation and cessation of treatment.
A modest increase in resting heart rate of one to four beats per minute has been observed across clinical trials of semaglutide. This is a consistent finding across the GLP-1 drug class and its long-term cardiovascular significance is under investigation. The SELECT trial of semaglutide, published in the New England Journal of Medicine in 2023, found a 20 percent reduction in major cardiovascular events in adults with established cardiovascular disease and overweight or obesity, which places the heart rate increase in the context of net cardiovascular benefit in this population.
- Nausea affects approximately 44% of patients, most pronounced in the first 12 to 20 weeks
- Vomiting (24%), diarrhoea (30%), constipation (24%), and abdominal pain (20%) are all common
- Gallstone formation risk is increased: 1.6% vs 0.7% in placebo group in STEP 1
- Pancreatitis is rare at under 0.3% of patients but requires immediate medical attention if suspected
- Thyroid C-cell tumour warning: contraindicated in those with personal or family history of medullary thyroid carcinoma
- Lean mass loss: approximately 38% of total weight lost is lean tissue without resistance training
- Resting heart rate increases modestly by 1 to 4 beats per minute