Does Wegovy cause side effects — and what should you expect?

Step 1: what your body goes through in the first four weeks

Wegovy works by activating GLP-1 receptors in the gut and brain, slowing gastric emptying and reducing appetite signals. That slowing is clinically useful (it is how the medicine helps you eat less) but it is also the direct cause of most early side effects.

In the first pen (0.25 mg, the starting dose), nausea is the most frequently reported complaint. It usually arrives within a few hours of an injection, often in the evening if that is when you choose to dose. Loose stools or constipation can also appear in week one; people experience opposite ends of the spectrum depending on their digestive baseline. Burping and a feeling of fullness after very small amounts of food are common and, while uncomfortable, are signs the medicine is doing what it is meant to.

Fatigue and mild headache show up in some patients during this opening phase too, likely linked to reduced calorie intake rather than the drug itself. Staying well-hydrated matters more than people expect, our guide on electrolytes and Wegovy covers why fluid and mineral balance deserve attention from the start.

The practical point: many people find the first two weeks the roughest. If you can eat smaller meals, avoid fatty or spicy food and give the injection time to settle, most GI effects soften considerably.

Step 2: what changes as doses increase through the titration ladder

Wegovy's dose schedule climbs from 0.25 mg through 0.5, 1.0, 1.7 and ultimately 2.4 mg, with a new pen every four weeks, each increase signed off by your prescriber. The pattern of side effects tends to repeat at each step: a brief flare of nausea or gut discomfort for a week or so, then relative calm before the next increase.

This is not a bug; it is why the titration exists. The gradual ramp-up lets your digestive system adapt without the shock of starting at a full therapeutic dose. Most people who discontinue do so in the first eight weeks; those who persist through the early steps tend to tolerate later doses better.

Worth knowing: the new 7.2 mg maintenance dose, approved by the MHRA in April 2026, sits above the previous 2.4 mg ceiling. It is available for adults with a BMI of 30 or above, started from 0.25 mg and titrated upward by a prescriber, not jumped to directly. Whether a specific dose or pen format is right for you is confirmed at consultation, not assumed in advance.

Gut-specific concerns like wind and bloating sometimes persist even after the early phase settles. If that is a central worry, our detailed page on Wegovy and wind works through what helps.

Step 3: effects that can appear at any point, and when to act

Some side effects are not tied to early weeks or dose increases, they can occur at any stage of treatment and range from minor to urgent.

On 29 January 2026 the MHRA issued a Drug Safety Update for GLP-1 medicines specifically calling out acute pancreatitis as an infrequent but serious risk. The symptom to act on immediately is severe, persistent stomach pain that radiates into the back, with or without vomiting. This requires same-day medical attention, not a call to your online pharmacy, but 999 or A&E if the pain is severe. The MHRA's Yellow Card scheme exists so that patients can report suspected side effects directly, and it is genuinely useful for tracking rare but serious events.

Gallbladder problems are another known but less common effect: symptoms include upper-right abdominal pain, particularly after eating fatty food. Dehydration can creep up when persistent nausea or diarrhoea reduces what you eat and drink, easy to underestimate, potentially significant for kidney function.

Mood and mental wellbeing are worth monitoring too. If low mood, anxiety or any thoughts of self-harm develop after starting treatment, tell your prescriber promptly. It is not a common reaction, but it matters.

Some patients on Wegovy ask specifically about effects in areas like sexual function, questions our prescribers hear regularly. There is a separate page addressing whether Wegovy can cause erectile dysfunction for anyone who wants to explore that in detail, and a related page on whether semaglutide causes erectile dysfunction looks at the evidence for the active ingredient more broadly.

Who is most likely to experience side effects, and what the evidence shows

The STEP 1 trial, published in the New England Journal of Medicine, followed 1,961 adults over 68 weeks on semaglutide 2.4 mg. Gastrointestinal side effects were the most common reason for discontinuation, affecting a meaningful minority, though the majority of participants completed the trial. That is a realistic picture: tolerable for most, challenging for some.

People with a history of reflux, irritable bowel or motility problems tend to find GI effects sharper. Those who are older, or who start on a higher caloric deficit alongside the medicine, may notice fatigue more. Injecting into the same site repeatedly rather than rotating between abdomen, thigh and upper arm can cause localised redness or swelling.

There is no way to know in advance exactly which effects will affect you, biology varies. What a prescriber can do is flag higher-risk scenarios before you start, adjust your titration plan, and be available when questions arise. At nume, aftercare runs seven days a week, so a concern on a Saturday is not left until Monday. If you are thinking about starting, check your eligibility through a free consultation and discuss your health history with a prescriber who will actually read it.

For a fuller picture of how Wegovy causes weight loss and what the clinical evidence shows, as well as what semaglutide does beyond side effects, our Wegovy overview page covers the mechanism, licensed use and trial results in one place.