The dark side of Wegovy: what the side effects look like in practice

Why do so many people feel unwell in the first few weeks?

Semaglutide works by activating GLP-1 receptors in the gut and brain, slowing gastric emptying and reducing appetite, and you can read a full overview of how semaglutide works and what it is if you want the broader context before diving into side effects. That slowing of the stomach is useful for weight management, but it is also the direct cause of the most common complaints: nausea, burping, reflux, and a feeling that food has nowhere to go. Most people who experience these symptoms find them worst in the 48 hours after an injection, then gradually easing as the week progresses.

The escalation schedule exists precisely because of this. Starting at a low dose and stepping up slowly gives your digestive system time to adjust. That adjustment period is real, though, and it can be uncomfortable. Tiredness, headache and dizziness also appear in trial data and on the NHS medicines page for semaglutide, and they tend to follow the same early-heavy, later-lighter pattern.

Practical tip: most people who take a once-weekly injection settle into keeping it in the fridge door next to everyday items, that visibility helps prevent missed doses, which are a common trigger for a repeat of the adjustment symptoms. The full side-effect picture for Wegovy covers how each reaction typically unfolds over the titration period.

What does the MHRA's pancreatitis warning actually mean for you?

In January 2026 the MHRA published a Drug Safety Update for GLP-1 receptor agonists, including semaglutide, drawing attention to acute pancreatitis as a known but infrequent risk. "Infrequent" is a regulatory term meaning it appears in fewer than one in a hundred users in trial data, rare enough that most people will never experience it, but serious enough that the regulator felt a formal communication was warranted.

The symptom to watch for is severe, persistent abdominal pain, often described as reaching through to the back, sometimes accompanied by vomiting. This is distinct from ordinary Wegovy-related nausea. Ordinary nausea fluctuates, tends to peak and pass, and does not typically produce that radiating, gripping quality. If you develop pain that fits the serious description, stop the injection and get urgent medical help, do not wait for your next scheduled consultation.

A related concern involves the gallbladder. Rapid weight loss of any kind, including medicine-assisted loss, can increase the risk of gallstones. Symptoms include pain in the upper right abdomen, particularly after eating fatty food. The semaglutide side effects guide covers both pancreatitis and gallbladder signs in more detail, and the MHRA's Yellow Card scheme allows patients to report any suspected side effect directly to the regulator.

Are dark stools or dark urine a reason to stop?

Changes in stool or urine colour come up regularly in patient questions, and they deserve a direct answer rather than vague reassurance. Dark or tarry stools can, in some cases, point to bleeding in the upper gastrointestinal tract, a rare but serious complication. More often they reflect dietary changes (iron-rich foods, certain vegetables) or the GI slowdown that comes with treatment. The distinction matters, which is why the rule is simple: dark, tarry or bloody-looking stools, or stools with a distinctly unusual consistency, warrant a call to your GP or 111 rather than a wait-and-see approach. There is specific guidance on Wegovy and dark stool that goes through the possible causes and what each one calls for.

Dark urine tells a different story. It is most commonly a dehydration signal, and dehydration is a genuine risk when vomiting or diarrhoea is persistent. GLP-1 medicines reduce appetite and sometimes thirst alongside it; people on treatment can under-drink without realising. Kidney function can be affected by prolonged dehydration, which is why the SmPC lists this as something to monitor. More on this on the dark urine and Wegovy page. If your urine is persistently dark and you are also feeling faint, very tired or producing less urine than usual, get same-day medical advice.

Who should think carefully before starting, and what's the honest picture on Wegovy overall?

Wegovy is not suitable for everyone, and a prescriber's job is partly to identify the people for whom the balance of benefit and risk tips the wrong way. Those with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2, a history of pancreatitis, or certain gastrointestinal conditions need a careful clinical conversation before starting. Wegovy is not recommended during pregnancy, while breastfeeding, or for those actively trying to conceive; and it is not licensed for under-18s. The MHRA also advises using effective contraception during treatment and for a period afterwards, for women taking oral contraceptives alongside semaglutide, discuss with your prescriber whether additional precautions are needed.

The honest overall picture: semaglutide has a robust evidence base, including the STEP 1 trial published in the New England Journal of Medicine, showing around 15% average weight reduction over 68 weeks. For many people, that benefit is meaningful and the side effects are manageable. For some, the GI effects are persistent enough to affect daily life, and a clinical conversation about dose adjustment or switching is the right next step. The broader picture of semaglutide's less-discussed effects is worth reading alongside this page. If you want to explore whether Wegovy or another licensed treatment suits your situation, our clinical team at nume's prescribing service is here to work through that with you. Check your eligibility through a free consultation, no commitment, reviewed the same day by a real clinician.