What Are the Side Effects of Semaglutide?

Which side effects does semaglutide most often cause?

Nausea is the one people ask about most. In the STEP 1 trial, published in the New England Journal of Medicine, the majority of participants taking 2.4 mg semaglutide reported at least one gastrointestinal event, with nausea at the top of the list. Vomiting, diarrhoea, constipation, indigestion, and burping follow. Fatigue and headache are also common, particularly after a dose increase.

The timing matters. These effects are typically most pronounced in the first few days after a new pen is started or after titrating to a higher dose. For many people they ease off noticeably within one to two weeks as the body adjusts to the medicine slowing gastric emptying. Eating smaller portions, avoiding very fatty or spicy meals, and staying hydrated all help — though a prescriber or your Patient Information Leaflet is the right place for personalised advice on managing them.

Injection-site reactions, including mild redness or itching, are also reported. Rotating between the abdomen, thigh, and upper arm each week reduces the chance of localised irritation. If you want a thorough picture of what semaglutide side effects look like across the full range of users, our dedicated page brings together the clinical evidence in one place. You can read a broader overview of what people experience on our Wegovy side effects page.

Dizziness and low blood pressure on standing have been noted in some users, so getting up slowly, particularly in the early weeks, is sensible. These are rarely severe but worth being aware of from the start.

Are there more serious side effects that should prompt medical attention?

Yes, and knowing what to watch for matters more than worrying in general. The clearest urgent signal is severe, persistent stomach pain that spreads towards the back, with or without vomiting. This pattern can indicate pancreatitis, an uncommon but potentially serious inflammation of the pancreas. In January 2026 the MHRA issued a Drug Safety Update for GLP-1 medicines specifically drawing attention to this risk, advising patients to seek urgent medical help if they experience it. Do not try to manage this kind of pain at home.

Gallbladder problems (gallstones, or inflammation of the gallbladder) are another recognised risk, particularly with rapid weight loss. Symptoms can include sharp pain under the right ribs, especially after eating, and should be assessed promptly by a doctor.

Severe vomiting or diarrhoea that continues beyond a day or two raises a separate concern: dehydration and the possibility of kidney strain. This is more likely if someone is also taking medicines that affect kidney function, so staying hydrated and contacting a prescriber early is sensible rather than waiting it out.

Allergic reactions are rare but can be serious. Signs to look out for include widespread rash, swelling of the face or throat, or difficulty breathing. Seek emergency help immediately if these occur. Our page covering side effects from semaglutide goes into detail on which reactions warrant the most caution, and for a more detailed breakdown of which effects are considered severe, our page on bad side effects of semaglutide covers these in full.

Do semaglutide's side effects change over time, or at different doses?

The short answer is yes. Side effects on semaglutide are strongly linked to the dose escalation process. Treatment starts at 0.25 mg weekly, a dose whose primary purpose is to let the body adapt, before stepping up roughly every four weeks. Each increase can briefly reawaken the GI effects that settled at the previous level. This is why the titration schedule exists: moving too quickly tends to worsen tolerability.

Most people find that by the time they reach the maintenance dose, the side effects they experienced early on are considerably milder or have resolved altogether. Hair thinning is one effect worth mentioning separately; it is not caused directly by the medicine but by the stress rapid weight loss places on the hair growth cycle, a condition called telogen effluvium. It usually reverses over time. If that is a concern, our page on hair loss and Wegovy explains what the evidence shows.

Some people experience mood changes or low energy, particularly early in treatment when calorie intake falls significantly. These are worth mentioning at a follow-up review rather than stopping treatment abruptly. Understanding each side effect of Wegovy and how it tends to evolve can help you have a more informed conversation with your prescriber at that point. A prescriber can help distinguish a genuine adverse effect from an adjustment effect. If you have questions about a specific symptom or are unsure whether something you are experiencing is typical, our support team is available seven days a week.

When should you report a side effect, and how?

The UK has a straightforward system for this. Any suspected side effect from semaglutide (whether it is something you expected or something that feels unusual) can be reported through the MHRA's Yellow Card scheme at yellowcard.mhra.gov.uk. Patients, carers, and healthcare professionals can all submit reports. This reporting directly shapes how regulators monitor medicines in real-world use, and the January 2026 pancreatitis update is a direct example of that system working as intended.

Beyond Yellow Card, talk to your prescriber promptly if side effects feel unmanageable, are getting worse rather than better after two weeks, or are affecting your ability to eat and drink normally. At nume, our prescribers review every repeat order before it is dispensed — there is no auto-renewal. That clinical re-review is a natural point to raise anything that has changed since you last made contact. You can also reach our aftercare team directly through our FAQs page for general guidance on what to expect during treatment.

If you are considering semaglutide and want to discuss side effects and suitability with a GPhC-registered prescriber, a real clinician reads every consultation the same day it is submitted. Start your free consultation to have those questions answered before you begin.