By Dr. Asnat Raziel, bariatric surgeon specializing in obesity treatment, metabolic health and healthy longevity; Director of the Multidisciplinary Center for Obesity Treatment, Assia Medical, Assuta Ramat HaChayal, Tel Aviv
Wegovy side effects: the full picture
Wegovy side effects are one of the first things my patients ask about. Researchers have studied Wegovy, whose active ingredient is semaglutide, in tens of thousands of patients, both in controlled trials and in the real world. Most side effects are common, mild and temporary. On the other hand, there are also rare effects worth knowing about. In this article, I sort it all out, based on the data.
Why do side effects occur?
Semaglutide mimics GLP-1, a hormone the gut releases after a meal. This hormone slows stomach emptying and activates the brain’s satiety centers. As a result, appetite drops and people eat less. However, the same slowing of the stomach also causes nausea, a sense of fullness and changes in bowel habits. That is why most side effects involve the digestive system.
Digestive Wegovy side effects
A pooled analysis of the STEP 1 to 3 trials gives the most accurate picture. Nausea affected 43.9% of patients on semaglutide, compared with 16.1% on placebo. Diarrhea affected 29.7%, compared with 15.9%. Vomiting affected 24.5%, compared with 6.3%. Finally, constipation affected 24.2%, compared with 11.1%.
These numbers sound high, but context matters. In fact, 98.1% of these events were mild to moderate, and they occurred mainly during dose escalation or shortly after. In addition, most of them faded over time. Only 4.3% of patients stopped treatment permanently because of them.
In the STEP 1 trial itself, published in NEJM, researchers also described nausea and diarrhea as temporary effects that eased over time. In that trial, 4.5% of patients on semaglutide stopped treatment because of digestive side effects, compared with 0.8% on placebo.
Nausea is not the secret to success
Many people believe the drug “works” because of the nausea. However, researchers tested this directly. Digestive side effects explained less than one percentage point of the weight loss. In other words, people without digestive symptoms lose weight at a similar rate.
What the SELECT trial teaches us about safety
The SELECT trial followed 17,604 patients for more than three years on average. Its safety analysis revealed a surprising finding: serious adverse events were actually less common with semaglutide, at 33.4% compared with 36.4% on placebo. This difference came mainly from fewer heart problems.
Even so, patients on semaglutide stopped treatment because of side effects twice as often: 16.6% compared with 8.2%. Most of this gap came from digestive symptoms, 10.0% compared with 2.0%. By contrast, the rate of stopping because of serious events was similar in both groups.
Less common Wegovy side effects
The gallbladder
Rapid weight loss raises the risk of gallstones, whatever the method. According to the US prescribing information, gallstones occurred in 1.6% of patients on Wegovy, compared with 0.7% on placebo. Gallbladder inflammation occurred in 0.6%, compared with 0.2%. Pain in the upper right abdomen, fever or yellowing of the skin calls for a visit to your doctor. I cover this in more detail in my article on gallstones during GLP-1 treatment.
Pancreatitis
Pancreatitis is rare. In the SELECT trial, it occurred in 0.2% of patients on semaglutide, compared with 0.3% on placebo. In addition, an observational study of US insurance data compared GLP-1 drugs with another weight loss drug, bupropion-naltrexone. The researchers found a higher relative risk of pancreatitis, bowel obstruction and gastroparesis, an abnormal slowing of stomach emptying. Bowel obstruction events occurred only with liraglutide. However, the absolute rates were low, and the statistical uncertainty was wide. Severe, persistent abdominal pain that spreads to the back means you should stop treatment and get checked urgently. You can read more in my article on weight loss drugs and pancreatitis.
NAION: rare damage to the optic nerve
NAION (non-arteritic anterior ischemic optic neuropathy) is damage to the optic nerve caused by a poor blood supply. It has made headlines in recent years. In June 2025, the European Medicines Agency concluded that it is a very rare side effect of semaglutide. It may affect up to 1 in 10,000 patients. Sudden vision loss requires immediate medical attention, and if doctors confirm the diagnosis, you should stop treatment.
Concerns that researchers examined closely
In recent years, some people raised concerns about a link between GLP-1 drugs and suicidal thoughts. Then, in January 2026, the FDA asked to remove this warning from the drug labels. The decision rested on an analysis of 91 controlled trials with 107,910 patients, which found no increased risk. Similarly, in the SELECT trial, the event rate was identical in both groups: 0.11%. Still, any change in mood is worth discussing with your doctor.
As for the thyroid, the label carries a warning about thyroid tumors that appeared in rodents. Their relevance to humans is unknown. For this reason, you must not take the drug if you or a family member has had medullary thyroid cancer or MEN 2 syndrome, a genetic syndrome that causes tumors in the endocrine glands.
Protecting your muscle mass
With any weight loss, part of the weight you lose is lean body mass. In a STEP 1 substudy, fat mass fell by 19.3% and visceral fat fell by 27.4%. Lean body mass fell by 9.7%. In other words, the proportion of lean mass relative to body weight actually increased.
Even so, preserving muscle is important. A joint advisory from four professional organizations sets targets of 1.2 to 1.6 grams of protein per kilogram of body weight per day during weight loss. However, it warns that calculating by actual body weight may overestimate the need. As a simple alternative, it suggests a target of 80 to 120 grams of protein a day. It also stresses that protein alone is not enough, so you need to add strength training. In my clinical experience, it helps to eat your protein first, before fullness sets in. I discuss this further in my article on preserving muscle mass during weight loss.
How to manage Wegovy side effects
Here are the practical tools, based on the research and on professional recommendations:
Increase the dose gradually: most side effects appear during dose escalation. If they are severe, your doctor can postpone the next increase.
Eat small meals: small, frequent meals, eating slowly and stopping at the first sense of fullness all reduce nausea.
Avoid fatty and fried food: such food stays in the stomach longer and makes you feel worse.
Drink enough: diarrhea and vomiting can cause dehydration and even kidney damage.
Treat constipation: fluids, fiber and movement help, and sometimes a mild medication does too.
Tell your doctors before surgery: the label warns of a rare risk of inhaling stomach contents under anesthesia. According to joint guidance from five medical societies, most patients can continue treatment. However, the guidance recommends postponing planned surgery during dose escalation or when digestive symptoms are present. For high-risk patients, it also recommends a liquid diet the day before. Be sure to tell your anesthesiologist.
Pregnancy: stop treatment at least two months before a planned pregnancy.
Diabetes medications: combining Wegovy with insulin or a sulfonylurea raises the risk of low blood sugar, so doses sometimes need adjusting. In addition, the label warns about an increased heart rate and a worsening of diabetic retinopathy, so regular follow-up matters.
You will find more practical advice in my nutrition guide for weight loss injections.
When to see a doctor right away
Most side effects are not dangerous, but some signs you must never ignore. First, severe, persistent abdominal pain, especially if it spreads to the back. Second, repeated vomiting that keeps you from drinking. In addition, pain in the upper right abdomen with fever or yellowing of the skin. Sudden vision loss, signs of dehydration such as dizziness and reduced urine, or an allergic reaction also require immediate care. In these situations, skip the next injection until your doctor has checked you.
Questions and answers about Wegovy side effects
How long does the nausea last?
For most patients, nausea appears mainly in the weeks of dose escalation. Over time, the body adapts, so the nausea eases or disappears. If it persists, your doctor can delay the next dose increase.
Do side effects mean the treatment is working?
No. As we saw, weight loss hardly depends on digestive side effects. Patients without these symptoms lose weight at a similar rate.
Are there long-term side effects?
The SELECT trial followed patients for more than three years and found no new safety signals. On the other hand, rare effects such as NAION came to light only after millions of people had used the drug. Therefore, regular medical follow-up matters throughout treatment.
Does Ozempic cause the same side effects?
Ozempic contains the same active ingredient at lower doses, and doctors prescribe it for diabetes. As a result, its side effect profile is similar. The main difference is the maximum dose.
The bottom line: Wegovy side effects in perspective
Wegovy side effects are mostly common, mild and temporary digestive symptoms. Serious drug-related effects are rare. In fact, in the SELECT trial, the overall rate of serious adverse events was slightly lower with the drug, at 33.4% compared with 36.4%. The keys are medical follow-up, gradual dose increases, good nutrition and early recognition of warning signs. This way, you can enjoy the benefits of treatment with minimal discomfort.
The information in this article is general and does not replace personal medical advice.
References
Once-Weekly Semaglutide in Adults with Overweight or Obesity
Safety profile of semaglutide versus placebo in the SELECT study: a randomized controlled trial
Wegovy Prescribing Information
New Multi-Society GLP-1 Clinical Practice Guidance Released
Novo Nordisk funded the STEP and SELECT trials and their secondary analyses.
