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Ozempic and Wegovy (semaglutide) side effects: what the licence lists, what the trials found, and when to speak to a prescriber
Every semaglutide side effect on the UK licences for Ozempic and Wegovy, how common each was in the trials, why they cluster around dose increases, the serious ones (pancreatitis, gallbladder, low blood sugar, eye disease, dehydration), "Ozempic face", hair loss, mood reviews, how it compares with Mounjaro, and when to get help.
Semaglutide, sold in the UK as Ozempic for type 2 diabetes and Wegovy for weight management, causes side effects in most people who take it, and for most of them the effects are stomach and gut symptoms that appear as the dose is increased and fade as the body adapts. That is what the trials recorded and what the licences say. This article sets out the full list carried by the UK product information, how common each effect was in the trials, when in treatment they appear, the serious but uncommon ones, the questions people search for most, including "Ozempic face", hair loss and mood, how semaglutide compares with tirzepatide on tolerability, and when a symptom should send someone to a prescriber. It describes the licensed medicines. It is not a recommendation to take, stop or continue anything; those decisions belong with the prescriber.
Where this information comes from
The trial programmes behind the two licences, SUSTAIN for diabetes and STEP for weight management, recorded every adverse event in thousands of participants over one to two years; STEP 1 alone followed 1,961 adults for 68 weeks. The UK Summaries of Product Characteristics and patient leaflets for Ozempic and Wegovy list the effects with standard frequency bands and are updated as post-marketing reports accumulate; the NHS medicines page summarises them in plain language. The frequencies below are for the licensed products at licensed doses. They do not describe unlicensed "semaglutide" sold online, whose contents and doses nobody has verified.
The common side effects and how often they occurred
The licences use standard bands: very common is more than one in ten, common between one in a hundred and one in ten, uncommon between one in a thousand and one in a hundred, rare below that. In STEP 1, gastrointestinal effects affected around three-quarters of people on semaglutide 2.4 mg at some point during 68 weeks, against under half on placebo; nausea affected roughly two in five, diarrhoea around three in ten, vomiting and constipation around a quarter each. Most events were mild or moderate and transient.
- Very common (more than 1 in 10), Wegovy: nausea, diarrhoea, vomiting, constipation, abdominal pain, headache and tiredness.
- Very common, Ozempic: nausea, diarrhoea, and low blood sugar in people also taking insulin or a sulfonylurea. Vomiting, constipation and headache are in the common band for Ozempic.
- Common (1 in 100 to 1 in 10): indigestion, acid reflux, gastritis, bloating, belching, wind, dizziness, gallstones, and worsening of existing diabetic eye disease in people with type 2 diabetes (common in both licences, mainly seen in people on insulin). Ozempic also lists reduced appetite, tiredness and raised pancreatic enzymes here; Wegovy also lists hair loss, altered taste, altered skin sensation, injection-site reactions and low blood sugar in people with type 2 diabetes.
- Uncommon (1 in 1,000 to 1 in 100): acute pancreatitis, delayed stomach emptying, increased heart rate; for Wegovy also low blood pressure and dizziness on standing, raised pancreatic enzymes and kidney stones; for Ozempic also injection-site reactions, altered taste and allergic reactions.
- Rare, very rare and frequency not known: anaphylaxis (rare, both); angioedema (rare for Wegovy, frequency not known for Ozempic); hip fracture (rare, Wegovy); a very rare optic-nerve condition causing sudden vision loss (NAION, both); bowel obstruction (frequency not known, both, from post-marketing reports).
Nearly everything in the first two bands is gastrointestinal, which follows from the mechanism: semaglutide slows stomach emptying and acts on gut-hormone receptors, so the gut is where it is felt. "Common" spans a wide range, and gallstones at a few in a hundred and hair loss at a few in a hundred are not the same kind of event; the sections below separate them.
Why side effects cluster around dose increases
Both licences start at 0.25 mg weekly for four weeks. Ozempic then moves to 0.5 mg, which is a maintenance dose; a prescriber may increase to 1 mg and later 2 mg, each after at least four weeks on the previous dose. Wegovy escalates through 0.5, 1 and 1.7 mg to a 2.4 mg maintenance dose over 16 weeks, and since 2026 the licence allows a further optional step to 7.2 mg for obesity after at least four weeks on 2.4 mg. The slow start exists because of the side-effect pattern: gastrointestinal effects were most frequent after starting and after each increase, and they were more frequent when escalation was faster. In the trials, most people found a symptom appearing after a step eased within days to a couple of weeks. The Wegovy licence says that with significant gastrointestinal symptoms a prescriber should consider delaying the next increase or returning to the previous dose until symptoms improve, and the Ozempic steps are minimum intervals, so an increase can be held. That is the opposite of the "push through" advice that circulates online.
What to expect over the first months
The trials did not publish a week-by-week symptom diary, so this is a synthesis of the frequency data and the schedule, not a timetable to expect exactly. In the first weeks on 0.25 mg, reduced appetite and mild nausea are the most commonly reported effects, and some people notice little. Each four-weekly increase tends to bring a few days of more noticeable nausea or altered bowel habit that settles. By the time the maintenance dose has been held for a month or two, most people report gastrointestinal effects as occasional rather than daily. A minority do not adapt: in STEP 1, about 7% of those on semaglutide stopped because of adverse events, mostly gastrointestinal, against about 3% on placebo.
The serious side effects
Acute pancreatitis is uncommon but serious; its warning sign is severe, persistent upper abdominal pain, sometimes spreading to the back, often with vomiting, and it needs urgent medical attention. Gallbladder disease, including gallstones and inflammation, was more frequent on semaglutide than placebo and is associated with rapid weight loss in general; pain under the right ribs, especially after eating, is the typical sign. In people with type 2 diabetes who already have diabetic retinopathy, a rapid improvement in blood sugar can temporarily worsen the eye disease, which is why the licence advises monitoring and why new vision changes should be reported promptly. Separately, both licences were updated to warn that epidemiological data may indicate an increased risk of non-arteritic anterior ischaemic optic neuropathy, a very rare cause of sudden vision loss; the licence says sudden loss of vision, including partial loss, needs urgent ophthalmological review.
Severe vomiting or diarrhoea can cause dehydration, and dehydration can injure the kidneys, particularly in people with existing kidney disease. Low blood sugar is a risk mainly for people also taking insulin or a sulfonylurea, whose doses may need reducing. Severe allergic reactions are rare but described. Because the drug slows stomach emptying it can change how other oral medicines are absorbed, and the Ozempic licence advises caution with oral medicines that need to be absorbed quickly, while the Wegovy licence notes no clinically relevant effect on absorption of paracetamol. Both licences also warn that delayed stomach emptying has been linked to cases of aspiration during general anaesthesia or deep sedation, so anyone due an operation should tell the anaesthetist they take semaglutide. These are the reasons the medicines come with a prescriber attached.
"Ozempic face", hair loss and mood: the questions people search
"Ozempic face" describes the hollowing and loosening of facial skin that follows rapid loss of facial fat. It is not a listed side effect and not specific to the drug; it is what losing a lot of weight quickly does to a face by any method, and the semaglutide medicines simply produce that loss in more people than previous treatments did. Hair loss was reported more often on semaglutide than placebo in the weight-management trials and is listed as common for Wegovy; it is generally attributed to the well-described temporary shedding that follows rapid weight loss and usually recovers over months. Mood changes and suicidal thoughts were reviewed by the European Medicines Agency and the MHRA across the GLP-1 class in 2023 and 2024 after case reports; the reviews did not establish a causal link, and neither regulator required any change to the UK product information, which does not list mood changes; anyone experiencing low mood or thoughts of self-harm should seek help regardless. Altered taste is a listed side effect (common for Wegovy, uncommon for Ozempic). Vivid dreams and reduced interest in alcohol appear in post-marketing reports and are being studied; they are not established trial findings.
Semaglutide versus tirzepatide on side effects
The two share a mechanism and a side-effect family. Across their separate trials, rates of nausea, diarrhoea, vomiting and constipation are broadly similar at comparable stages, and discontinuation for adverse events sits in the same low single-digit range. The SURMOUNT-5 trial, an open-label head-to-head in 751 adults with obesity but not diabetes over 72 weeks, reported more weight loss on tirzepatide (20.2% against 13.7%); in both groups the most common adverse events were gastrointestinal, mostly mild to moderate and mainly during dose escalation. Individual response varies more than the drugs do, and someone who tolerates one poorly may tolerate the other well; that is a conversation for a prescriber, not a reason to self-switch. Our Mounjaro side-effects article covers tirzepatide in the same format.
Unlicensed "semaglutide" and why these frequencies do not apply to it
Everything above describes pharmaceutical semaglutide at licensed doses in monitored people. Products sold online as "research" semaglutide, or as filled pens from unknown sources, are unlicensed: no regulator has checked what they contain, at what concentration, or whether they are sterile. Their side effects cannot be predicted because their dose cannot be known, and the MHRA has issued repeated warnings and made seizures of counterfeit and unlicensed weight-loss injections. If a symptom follows an unlicensed product, the relevant question is what was in the vial, and medical help should be sought.
When to contact a prescriber, and how to report a side effect
- Severe or persistent abdominal pain, especially with vomiting: urgent medical attention, because of the pancreatitis and gallbladder risks.
- Signs of dehydration: very little urine, dizziness on standing, confusion, particularly with ongoing vomiting or diarrhoea.
- Any sudden loss of vision, including partial loss, in anyone: urgent eye assessment, because both licences now warn of a very rare optic-nerve condition (NAION). New or worsening vision changes in anyone with diabetes should also be reported promptly.
- Symptoms of low blood sugar in anyone also taking insulin or a sulfonylurea.
- Any allergic reaction: swelling of the face or throat, difficulty breathing, widespread rash.
- Side effects that are not dangerous but make treatment hard to continue: the prescriber can delay or reverse a dose step, which the licences allow for.
- Any suspected side effect can be reported to the MHRA through the Yellow Card scheme.
This article is educational content only and is not medical advice or a recommendation to start, stop or change any medicine. Ozempic and Wegovy (semaglutide) are prescription-only medicines; the frequencies described are from clinical trials and the UK product information and are not predictions for any individual. Anyone experiencing side effects should speak to their prescriber or pharmacist, and seek urgent care for the serious symptoms described. Vivera Education sells nothing.
Frequently asked
- What are the most common Ozempic and Wegovy side effects?
- Nausea, diarrhoea, vomiting and constipation, each affecting more than one in ten people in the trials, along with abdominal pain, indigestion, reduced appetite, tiredness and headache. Most are mild to moderate and most frequent around dose increases.
- How long do semaglutide side effects last?
- In the trials, gastrointestinal effects peaked in the weeks after starting and after each dose increase and lessened as people stayed on a dose. For most they settle within days to a few weeks of each step; a minority stopped because of them.
- What are the serious side effects of Ozempic and Wegovy?
- The licences list acute pancreatitis, gallbladder disease including gallstones, worsening of diabetic eye disease in people who already have it, a very rare optic-nerve condition causing sudden vision loss (NAION), low blood sugar when combined with insulin or a sulfonylurea, severe gastrointestinal effects leading to dehydration and kidney injury, and rare severe allergic reactions.
- What is "Ozempic face"?
- A popular name for the change in facial appearance that follows rapid loss of facial fat: hollowing and loosened skin. It is an effect of losing a lot of weight quickly by any method, not a specific drug effect, and it is not a listed side effect.
- Does semaglutide cause hair loss?
- Hair loss was reported more often on semaglutide than placebo in the weight-management trials and is listed in the Wegovy product information as common. It is generally attributed to rapid weight loss rather than a direct drug effect and is usually temporary.
- Does Ozempic cause depression or suicidal thoughts?
- Regulators in the UK and EU reviewed reports of mood changes and suicidal thoughts for GLP-1 medicines in 2023 and 2024 and did not establish a causal link; neither regulator required a change to the product information, and the MHRA says it continues to monitor. Anyone experiencing low mood or thoughts of self-harm should seek help from a GP or NHS 111 regardless of what medicine they take.
- Is Wegovy worse for side effects than Mounjaro?
- The profiles are the same family, dominated by gastrointestinal effects, with broadly similar rates in trials and similar discontinuation rates. The SURMOUNT-5 head-to-head trial reported more weight loss on tirzepatide, with gastrointestinal effects the most common adverse events in both groups. Individual response varies more than the drugs do.
- What should I do if I get side effects?
- Speak to the prescriber or pharmacist before deciding to stop or skip doses; a dose increase can be delayed or the dose reduced. Severe or persistent abdominal pain, signs of dehydration, vision changes in people with diabetes, or symptoms of low blood sugar need prompt medical attention. Suspected side effects can be reported through the MHRA Yellow Card scheme.
- Do you gain weight back after stopping Ozempic or Wegovy?
- In the extension of the STEP 1 trial, people who stopped semaglutide 2.4 mg (the Wegovy dose) and the trial lifestyle support regained on average about two-thirds of the weight they had lost within a year. Our article on stopping GLP-1 medicines covers the withdrawal trials in detail.
- What are the long-term side effects of Ozempic and Wegovy?
- The largest long-term trial, SELECT, followed about 17,600 people aged 45 or over with existing cardiovascular disease and overweight or obesity, but without diabetes, on semaglutide 2.4 mg or placebo for a mean of a little over three years. It recorded only serious adverse events, events that led people to stop and a set of pre-specified events, so it cannot describe milder effects. Within those limits its authors reported no new safety concerns: gastrointestinal effects were the main reason for stopping, gallbladder disorders were slightly more frequent on semaglutide (2.8% against 2.3%), and 16.6% stopped for adverse events on semaglutide against 8.2% on placebo. Very rare effects are picked up after licensing through safety monitoring and large observational studies, which is how the warning about a very rare cause of sudden vision loss (NAION) was added.
- What organs does Ozempic affect?
- Semaglutide acts on GLP-1 receptors in the pancreas, where it increases insulin release when blood sugar is high, and in the brain, where it reduces appetite; it also slows stomach emptying. The warnings section of the licences names the pancreas (acute pancreatitis), the kidneys (through dehydration), the eyes (diabetic eye disease and, very rarely, NAION) and the stomach (delayed emptying, relevant to anaesthesia and to people with gastroparesis). Gallstones are listed among the side effects rather than in the warnings.
- Why do people stop taking Ozempic or Wegovy?
- In STEP 1, 7.0% of people on semaglutide stopped because of adverse events, mostly gastrointestinal, against 3.1% on placebo. Outside trials, people may also stop for reasons the trials did not measure, such as cost or supply. On the NHS in England, NICE TA875 recommends Wegovy for weight management for a maximum of two years within a specialist weight-management service (checked September 2026).
The studies and sources behind this page
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Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.