Ozempic and Semaglutide Side Effects: What the Evidence Shows

Which side effects do most people experience on semaglutide?

The most common semaglutide side effects are digestive. Nausea tops the list, followed by vomiting, diarrhoea, constipation, stomach discomfort and burping. Fatigue, headache and dizziness are reported less often but are well-recognised. Injection-site reactions (redness or mild irritation at the abdomen, thigh or upper arm) can occur with injectable semaglutide.

These effects are not random. Semaglutide slows the rate at which the stomach empties, which is partly why it suppresses appetite but also why the gut takes time to adjust. According to the NHS medicines information for semaglutide, most gastrointestinal symptoms are mild to moderate and tend to ease within days to a couple of weeks, particularly after the body has settled at a given dose. The full picture of semaglutide side effects (including what drives them and how long they tend to last) is covered in more detail on a dedicated page.

Appetite loss is considered part of the mechanism rather than a side effect per se, but it can become clinically relevant if it leads to inadequate protein or fluid intake. Small, protein-rich meals and consistent hydration are the practical advice clinicians give most often. Personal dietary adjustments, though, are a conversation for your prescriber rather than a general article.

For people switching from Ozempic to Wegovy, or curious whether the products behave differently day to day, the side-effect profile of Wegovy compared with Ozempic is worth reading directly.

How long do the side effects of semaglutide actually last?

Timing matters here, and it is one of the questions our prescribers hear most frequently. The short answer: most GI effects are worst in the first one to two weeks after a dose increase, then subside as the body adapts. They rarely persist at the same intensity throughout treatment.

Semaglutide is titrated gradually for precisely this reason: starting at a low dose and stepping up over months gives the gut time to accommodate. People who rush titration or restart at a higher dose after a break tend to experience more pronounced symptoms. If nausea or vomiting becomes severe enough to prevent eating or drinking for more than a day, that warrants contact with a prescriber rather than waiting it out.

There is a separate question about what happens when treatment stops. Some people notice a return of appetite and, over time, gradual weight regain. A dedicated look at stopping Wegovy and the effects that can follow covers that transition honestly. How long specific side effects last during treatment is explored in more depth on the page on semaglutide side-effect duration.

There is no universal timeline. Body weight, dose level, individual metabolism and whether someone is eating alongside treatment all influence the experience. That variability is exactly why ongoing prescriber review matters.

When should you get medical help urgently?

Most side effects are uncomfortable rather than dangerous. A few are not. Knowing the difference is important.

Seek urgent medical attention if you develop severe, persistent stomach pain that spreads to your back, with or without vomiting, this can be a sign of acute pancreatitis. In January 2026, the MHRA issued a Drug Safety Update reinforcing that acute pancreatitis is a known, if infrequent, risk with GLP-1 medicines; the MHRA Drug Safety Updates page carries the full communication. Gallbladder problems (pain in the upper right abdomen, fever, jaundice) also need prompt assessment.

Severe vomiting or diarrhoea that prevents fluids going down can lead to dehydration and in turn affect kidney function. Symptoms of a serious allergic reaction (face, lip or throat swelling, difficulty breathing, rapid rash) are a 999 situation. Low mood or thoughts of self-harm should be raised with a GP or mental health service promptly.

Any suspected side effect, however minor, can be reported to the MHRA's Yellow Card scheme. Reporting builds the national safety evidence base and is genuinely useful, not just a box-ticking exercise.

Detailed practical notes on the side-effect pattern across Wegovy's dose schedule are covered on the Wegovy side effects page.

Ozempic is not licensed for weight loss, so what is available in the UK?

This is worth stating plainly. Ozempic (semaglutide) holds a UK licence for type 2 diabetes management, not weight loss. Prescribing it off-label for weight loss is outside its MHRA authorisation, and the MHRA has been clear that GLP-1 medicines should not be used for cosmetic or quick weight loss. Any service offering Ozempic specifically as a weight-loss product is operating outside the licensed indication.

Two medicines are licensed in the UK for weight management in adults:

Wegovy (semaglutide): The weight-management formulation of the same active ingredient, licensed for BMI of 30 or above, or 27 or above alongside a weight-related condition. It is available privately through a regulated prescriber. A summary of the treatment, including eligibility and what to expect, is on the Wegovy overview page, and current private pricing is outlined on the Wegovy prices page.

Mounjaro (tirzepatide): A dual GIP and GLP-1 receptor agonist, made by Eli Lilly, also licensed for weight management. The SURMOUNT-1 trial, published in the New England Journal of Medicine, reported an average body-weight reduction of around 20–21% at the highest dose over 72 weeks, the strongest results seen in a weight-management trial to that point.

Both are prescription-only medicines requiring clinical assessment. The treatment options page outlines both, and our prescribers discuss side effects, suitability and dose as part of every free consultation. If you have questions before starting, the FAQs and contact page are both there.

A broader look at the licensed weight-loss medicines available in the UK may also be useful if you are still comparing options.