Nausea, diarrhoea, vomiting, constipation and abdominal pain are among the side effects people most often experience with Wegovy. They are particularly common while the dose is being gradually increased.
That makes the beginning of treatment a common source of questions. There is no standard set of “first-week side effects” that everyone experiences, and some people have few problems at all. Wegovy starts at a low dose precisely so the body has time to adjust before higher doses are introduced.
Most digestive effects are manageable, but Wegovy also has important less common risks. These include pancreatitis, gallbladder disease, dehydration and kidney injury, serious allergic reactions and, in European safety information, the very rare eye condition non-arteritic anterior ischaemic optic neuropathy (NAION).
At a glance
- Digestive problems account for many of the most common Wegovy side effects.
- Nausea was reported by 44% of adults receiving Wegovy in US weight-reduction trials, compared with 16% receiving placebo.
- Gastrointestinal reactions occur particularly often during dose escalation.
- The first week cannot predict how someone will tolerate later Wegovy doses.
- Severe and persistent abdominal pain can indicate acute pancreatitis.
- Vomiting and diarrhoea can cause dehydration and contribute to kidney problems.
- European regulators classify NAION as a very rare side effect of semaglutide.
What can happen during the first week of Wegovy?
Some patients notice digestive symptoms after their first Wegovy injection, particularly nausea, a feeling of fullness, diarrhoea, constipation or stomach discomfort. Others may notice little or no difference during the first week.
There is no reliable timetable saying that a particular side effect should begin on a particular day. Clinical-trial and prescribing information describe adverse reactions across treatment periods rather than providing a separate incidence rate for days one to seven.
Wegovy treatment is deliberately started at a low dose and increased gradually. The usual injection schedule begins at 0.25 mg once weekly, with dose increases at four-week intervals before the maintenance dose is reached.
The gradual schedule is intended to reduce gastrointestinal reactions. A person who tolerates the first week well can still develop symptoms after a later dose increase, while someone who initially experiences nausea may find that it improves.
The side effects reported most often
Wegovy contains semaglutide, a GLP-1 receptor agonist. Its effects on appetite and the gastrointestinal system are closely connected with many of its common adverse reactions.
In US clinical trials of adults with obesity or overweight receiving Wegovy 2.4 mg for weight reduction, frequently reported adverse reactions included:
| Side effect | Wegovy | Placebo |
|---|---|---|
| Nausea | 44% | 16% |
| Diarrhoea | 30% | 16% |
| Vomiting | 25% | 6% |
| Constipation | 24% | 11% |
| Abdominal pain | 20% | 10% |
| Headache | 14% | 10% |
| Fatigue | 11% | 5% |
These figures describe clinical-trial populations and should not be read as a prediction of what will happen to an individual patient.
Why dose increases can bring symptoms back
Gastrointestinal reactions were reported by 73% of Wegovy-treated adults in US weight-reduction trials compared with 47% of patients receiving placebo.
Nausea, vomiting and diarrhoea were particularly prominent, and gastrointestinal reactions were reported most frequently during dosage escalation.
European product information similarly describes nausea, vomiting, diarrhoea, constipation, stomach pain and indigestion as very common effects and notes that these are mainly seen during dose escalation and usually subside over time.
This means tolerating one dose does not guarantee that the next dose will feel identical. Equally, developing nausea after an increase does not necessarily mean it will persist for the remainder of treatment.
When stomach symptoms are more than an inconvenience
Ordinary nausea or temporary digestive discomfort is different from severe or persistent gastrointestinal illness.
Current US prescribing information warns that severe gastrointestinal adverse reactions can occur with Wegovy. In adult weight-reduction trials, severe gastrointestinal reactions were reported in 4.1% of Wegovy-treated patients compared with 0.9% receiving placebo.
Wegovy is not recommended in patients with severe gastroparesis.
Persistent vomiting or diarrhoea also matters because significant fluid loss can cause dehydration and contribute to acute kidney injury.
Pancreatitis: the abdominal pain that should not be ignored
Acute pancreatitis has been observed in people receiving semaglutide.
The important warning symptom is severe, persistent abdominal pain, sometimes spreading to the back and sometimes accompanied by vomiting.
This is not the same as ordinary nausea or mild stomach discomfort. Suspected pancreatitis requires prompt medical assessment.
Current European information lists acute pancreatitis as an uncommon Wegovy side effect, meaning it may affect up to 1 in 100 people according to the frequency category used in the product information.
Wegovy and the gallbladder
Gallstones and inflammation of the gallbladder can occur during Wegovy treatment.
Symptoms can include upper abdominal pain, fever, jaundice or pale stools. These require assessment because gallbladder disease can cause complications and cannot be distinguished reliably from other abdominal problems without appropriate medical evaluation.
Dehydration and kidney injury
Vomiting and diarrhoea can lead to substantial fluid loss. Kidney problems associated with Wegovy have occurred particularly in the context of gastrointestinal adverse reactions and dehydration.
US prescribing information recommends monitoring kidney function in patients experiencing reactions that could cause volume depletion, especially when Wegovy is being started or the dose increased.
Difficulty keeping fluids down, prolonged vomiting or diarrhoea, or symptoms of significant dehydration warrant medical advice.
A rare but important eye risk: NAION
European regulators classify non-arteritic anterior ischaemic optic neuropathy (NAION) as a very rare side effect of semaglutide, the active ingredient in Wegovy. The same European regulatory finding also applies to Ozempic; our Ozempic side-effects guide covers the safety information in the diabetes-treatment context.
NAION occurs when blood flow to the optic nerve is disrupted and can cause loss of vision.
After reviewing clinical-trial, post-marketing and epidemiological evidence, the European Medicines Agency’s safety committee concluded that NAION may affect up to 1 in 10,000 people taking semaglutide.
Several large epidemiological studies reviewed by European regulators suggested approximately a two-fold relative increase in NAION risk among adults with type 2 diabetes exposed to semaglutide compared with people not taking the medicine.
Sudden loss of vision or rapidly worsening eyesight requires urgent medical attention. European guidance states that semaglutide should be stopped if NAION is confirmed.
Diabetic retinopathy is a separate issue
NAION should not be confused with diabetic retinopathy.
People with type 2 diabetes can already be at risk of diabetic retinopathy, and rapid improvement in glucose control can temporarily worsen the condition in some patients.
Current US Wegovy prescribing information therefore advises monitoring patients with type 2 diabetes who have a history of diabetic retinopathy.
Other warnings worth knowing
Low blood sugar
Hypoglycaemia is particularly relevant for people with type 2 diabetes who use Wegovy alongside insulin or an insulin secretagogue such as a sulfonylurea. Their diabetes treatment may need to be reviewed by the prescribing clinician.
Serious allergic reactions
Anaphylaxis and angioedema have been reported with Wegovy. Swelling of the face, lips, tongue or throat, difficulty breathing or swallowing, or other signs of a severe allergic reaction require emergency medical attention.
Thyroid C-cell tumour warning
US Wegovy prescribing information carries a boxed warning concerning thyroid C-cell tumours.
Semaglutide caused thyroid C-cell tumours in rodents. It is not known whether Wegovy causes these tumours, including medullary thyroid carcinoma (MTC), in humans.
In the United States, Wegovy is contraindicated in people with a personal or family history of MTC or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2).
Before surgery or deep sedation
Wegovy delays gastric emptying. Rare reports of pulmonary aspiration have occurred in patients receiving GLP-1 receptor agonists during general anaesthesia or deep sedation despite following pre-operative fasting instructions.
Patients should make sure the healthcare team responsible for a planned operation or procedure knows that they use Wegovy.
Signs that need prompt attention
| What you notice | What it could indicate |
|---|---|
| Severe, persistent abdominal pain, possibly spreading to the back | Acute pancreatitis |
| Sudden loss of vision or rapidly worsening eyesight | NAION or another urgent eye condition |
| Persistent vomiting or diarrhoea with difficulty maintaining fluid intake | Dehydration and possible kidney complications |
| Upper abdominal pain with fever, jaundice or pale stools | Gallbladder disease |
| Swelling of the face or throat or difficulty breathing | Serious allergic reaction |
The table is not exhaustive. A new symptom that is severe, rapidly worsening or otherwise concerning may require medical assessment even if it is not listed here.
Wegovy side effects: common questions
What are Wegovy side effects in the first week?
Nausea, diarrhoea, constipation, vomiting or stomach discomfort can occur after starting Wegovy, but there is no standard first-week pattern. Some people experience few symptoms. Clinical-trial data do not provide a separate side-effect rate specifically for the first seven days.
When are Wegovy side effects usually worst?
Gastrointestinal reactions are particularly associated with dose escalation. Symptoms may therefore appear or become more noticeable after the dose is increased rather than only when treatment first begins.
Do Wegovy side effects eventually go away?
Many gastrointestinal effects become less troublesome with time, and European product information notes that common digestive effects seen during dose escalation usually subside over time. Persistent or severe symptoms still require medical advice.
Can Wegovy cause pancreatitis?
Yes. Acute pancreatitis is a recognised risk of semaglutide treatment. Severe and persistent abdominal pain, particularly if it spreads to the back, needs prompt medical assessment.
Can Wegovy affect eyesight?
Yes. European regulators classify NAION as a very rare semaglutide side effect. Diabetic retinopathy is a separate concern, particularly for people with type 2 diabetes who already have retinopathy.
Can Wegovy cause hair loss?
Hair loss is listed as a common side effect in current European Wegovy product information and has also been reported in clinical trials. Hair loss during substantial weight reduction can have several possible contributing factors, so persistent or significant hair loss should be discussed with a healthcare professional.
Should you skip a dose because of side effects?
Patients should not independently change a prescribed dosing schedule simply because a side effect appears. Severe symptoms may require prompt assessment, while tolerability problems can be discussed with the clinician managing treatment.
Continue your research
- Wegovy medication reference
- Ozempic vs Wegovy comparison
- Wegovy vs Mounjaro comparison
- Wegovy vs Zepbound comparison
- Ozempic side effects
- Mounjaro side effects
Reviewed by: Dr Mattias Hagman, PhD
Editorial review: MediSearcher checks medication safety content against current regulatory and prescribing information. US and European warnings are identified separately where the regulatory wording differs.
Last reviewed: 29 October 2026
How this guide was checked
This article was reviewed against current US and European Wegovy prescribing and regulatory information available on 29 October 2026, including updated European information concerning NAION. Safety information can change as regulators assess new evidence.
- US Wegovy Prescribing Information — US Food and Drug Administration / Novo Nordisk
- Wegovy EPAR — European Medicines Agency
- Wegovy Product Information — European Medicines Agency
- PRAC conclusion on NAION and semaglutide — European Medicines Agency
This article provides general information and is not a substitute for individual medical advice. Wegovy is a prescription medicine. Side effects, treatment suitability and changes to prescribed treatment should be discussed with an appropriate healthcare professional.