Semaglutide side effects: what's common and what's serious

What most people actually get

In the 2.4 mg weight-management trials, the common effects were nausea in roughly 44% of participants, diarrhoea in about 30%, vomiting and constipation each around 24%, abdominal pain around 20% and headache around 14%. Fatigue, indigestion, burping and reflux also appeared.

Placebo groups reported gastrointestinal symptoms at meaningful rates too, which is worth holding in mind before attributing everything to the drug.

Roughly 7% of participants stopped because of adverse events, against about 3% on placebo. Most people did not stop, and most effects were mild to moderate.

The pattern matters more than the list

These effects are not spread evenly across treatment. They cluster in the days after a dose increase, typically peaking two to four days later rather than on injection day, and they attenuate with continued exposure at the same dose.

That pattern is the single most useful thing to know, because it turns an alarming week into a predictable one and it explains why holding a dose longer is often the whole solution.

Managing the gastrointestinal effects

• Smaller volumes per meal. The immediate problem is mechanical, with volume sitting in a stomach that is emptying slowly.
• Less fat per meal, since fat slows gastric emptying further on top of what the drug is already doing.
• Stop at the first real signal of fullness. It arrives earlier than you are used to and is easy to eat straight past.
• Fluids between meals rather than with them.
• Stay ahead of dehydration. Vomiting or diarrhoea plus poor fluid intake is the mechanism behind a meaningful share of the serious adverse events in this class.
• If a dose is genuinely intolerable, holding it or stepping back down is a normal clinical decision, not a failure.

Constipation gets less attention than it deserves

Nausea dominates the conversation, but constipation is close behind it in frequency and it lasts longer, because it does not attenuate the way nausea does.

Fibre, fluid and movement are the first line. What matters is knowing where ordinary constipation ends. Severe abdominal distension, vomiting, inability to pass gas or a complete stop in bowel movements is a different problem and needs assessment, not a laxative. Analyses of adverse event reporting have found a raised signal for bowel obstruction in this class.

The serious risks, and what to watch for

• Pancreatitis. Severe persistent abdominal pain, classically radiating through to the back, often with vomiting. This one is assessed urgently rather than waited out.
• Gallbladder and biliary disease. These occur more often than on placebo, and risk rises with faster weight loss. Pain under the right ribs, especially after fatty meals.
• Acute kidney injury, almost always downstream of dehydration from vomiting or diarrhoea. Largely preventable with fluids.
• Hypoglycaemia, which is uncommon on semaglutide alone because its insulin effect is glucose-dependent, but becomes a real risk combined with insulin or a sulfonylurea. Those usually need adjusting.
• Diabetic retinopathy complications. In the SUSTAIN-6 cardiovascular trial there was an increase in retinopathy complications in participants with pre-existing retinopathy, thought to relate to the speed of glucose improvement. Anyone with known retinopathy needs this raised before starting.
• Aspiration under general anaesthesia. Retained stomach contents raise the risk, and anaesthetic guidance changed as a result. Tell any surgical or endoscopy team you are on it, well in advance.
• Thyroid C-cell tumours carry a boxed warning based on rodent studies. Human relevance is not established, but a personal or family history of medullary thyroid carcinoma, or MEN2, is an absolute contraindication.

The eye signal

A 2024 analysis reported a higher rate of non-arteritic anterior ischaemic optic neuropathy, a rare condition causing sudden painless vision loss, among patients prescribed semaglutide. European regulators reviewed it and concluded it is a very rare side effect.

Very rare is genuinely very rare, and this should not be a reason to avoid treatment on its own. But sudden vision loss or a sudden change in vision is an emergency whatever the cause, so it is worth knowing the association exists rather than discovering it afterwards.

The ones people ask about that are not quite what they seem

• Hair thinning. Commonly reported and rarely dangerous. It is generally attributed to rapid weight loss triggering telogen effluvium rather than to a direct drug effect, and it is usually temporary. Adequate protein and iron are worth checking.
• Facial volume loss, which gets called Ozempic face. This is what losing fat does to a face. It is not a specific drug effect and it happens with equivalent weight loss by any route.
• Muscle loss. Real, not unique to this drug, and the reason resistance training and adequate protein belong in the plan from the start rather than as an afterthought.
• Mood changes and suicidal thoughts. Reports prompted formal reviews by both the FDA and the European Medicines Agency, and both concluded the available evidence does not support a causal association. That is not a reason to ignore a change in your own mood. Report it.

When to call, and when to wait

Wait and manage: nausea in the days after a dose increase, mild diarrhoea, constipation responding to the usual measures, fatigue, headache, burping, injection site soreness.

Call your prescriber: vomiting or diarrhoea that stops you keeping fluids down, severe or persistent abdominal pain, signs of dehydration, hypoglycaemia symptoms if you take insulin or a sulfonylurea, a neck lump or hoarseness, or any new or worsening vision problem.

Emergency care: severe abdominal pain with vomiting, sudden vision loss, or signs of a severe allergic reaction.

What actually reduces side effects

In order of how much difference they make: slowing the titration, holding at a dose that works rather than climbing to the maximum, reducing meal volume and fat content, and keeping fluid intake up.

Not on that list is pushing through. There is no evidence that tolerating a worse experience produces a better result, and the maximum dose is a ceiling rather than a target.

Sources

• Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021;384(11):989-1002.
• Sodhi M, et al. Risk of Gastrointestinal Adverse Events Associated With GLP-1 Receptor Agonists for Weight Loss. JAMA. 2023;330(18):1795-1797.
• He L, et al. Association of Glucagon-Like Peptide-1 Receptor Agonist Use With Risk of Gallbladder and Biliary Diseases. JAMA Intern Med. 2022;182(5):513-519.
• Marso SP, et al. Semaglutide and Cardiovascular Outcomes in Patients with Type 2 Diabetes (SUSTAIN-6). N Engl J Med. 2016;375(19):1834-1844.
• Hathaway JT, et al. Risk of Nonarteritic Anterior Ischemic Optic Neuropathy in Patients Prescribed Semaglutide. JAMA Ophthalmol. 2024;142(8):732-739.
• American Society of Anesthesiologists. Consensus-based guidance on preoperative management of patients on glucagon-like peptide-1 receptor agonists.
• US Food and Drug Administration. Update on FDA's ongoing evaluation of reports of suicidal thoughts or actions in patients taking a certain type of medicines approved for type 2 diabetes and obesity.
• European Medicines Agency. PRAC review of GLP-1 receptor agonists and suicidal ideation.
• Wegovy (semaglutide) injection, prescribing information. Novo Nordisk.