Wegovy Side Effects Explained: What the Efeitos Colaterais Evidence Actually Shows

The first few weeks: when side effects tend to hit hardest

Picture this: it's Monday morning, your Wegovy pen is in the fridge door, and you've just done your first injection. By Wednesday afternoon, you feel queasy. This is the scenario most people describe, and it is entirely expected. Semaglutide slows the rate at which the stomach empties, which is part of how it reduces appetite, but that same mechanism is what causes nausea and, for some people, vomiting or loose stools in the early stages of treatment.

The NHS medicines information for semaglutide lists the most common reactions as nausea, vomiting, diarrhoea, constipation, stomach pain, indigestion, burping, and fatigue. Headache and dizziness also appear in the common category. None of these are cause for alarm on their own, but they are worth tracking. The pattern is fairly consistent: symptoms are worst at the start of each new dose level, then ease over the following week or two as the body adapts. Eating smaller portions, avoiding fatty meals, and staying well hydrated all help; how the Wegovy dose ramp works explains why the slow titration schedule exists partly to keep these reactions manageable.

If nausea is stopping you from keeping fluids down for more than a day, contact your prescriber. Dehydration on top of GI upset can cause kidney strain, which is a separate concern the Wegovy side effect overview covers in more detail.

Side effects that need prompt medical attention, not just patience

Most semaglutide side effects are uncomfortable rather than dangerous. A smaller number are not. In January 2026, the MHRA published a Drug Safety Update for GLP-1 medicines that specifically flagged acute pancreatitis: sudden, severe abdominal pain that persists and may spread to the back, with or without vomiting. This is infrequent, but it can be serious, and it warrants going to A&E or calling 999 rather than waiting to see if it passes.

Other reactions that should not be managed with watchful waiting include signs of a significant allergic response (swelling of the face, throat or tongue, difficulty breathing, rapid heartbeat), gallbladder symptoms such as severe pain in the upper right abdomen that comes on suddenly, and marked changes in urination alongside dehydration. People who experience unusual fatigue, yellowing of the skin, or sudden changes in vision should contact a clinician promptly.

Wegovy is also not recommended during pregnancy, while breastfeeding, or for anyone who is actively trying to conceive. It is not licensed for under-18s. Anyone with a personal or family history of medullary thyroid carcinoma or MEN2 syndrome should not take semaglutide. These are not conditions for discussion in a FAQ; they are questions the prescriber works through before treatment starts.

The MHRA's Yellow Card reporting tool exists precisely so rare reactions get captured. The link is yellowcard.mhra.gov.uk and it is open to patients as well as clinicians.

Less-discussed side effects: what people sometimes forget to mention

A question our prescribers hear fairly regularly is whether Wegovy can affect mood, sleep, or energy beyond simple fatigue. The licensed side effect profile doesn't list depression as a common effect, but changes in appetite and eating patterns can affect how some people feel day to day, and anyone noticing low mood, irritability, or thoughts of self-harm should contact their GP or prescriber without delay. The NHS has a helpful overview of semaglutide's full profile at nhs.uk/medicines/semaglutide.

There is also the question of what happens when you're under the weather. Some people find that being ill with a cold or stomach bug on Wegovy feels more disruptive than it would otherwise, partly because GI symptoms compound. Being ill while on Wegovy is worth reading if that situation arises. Separately, some people notice shifts in food preferences and cravings, not a side effect in the clinical sense, but something that can feel disorienting. Chocolate cravings on Wegovy gets into the practical detail of why appetite changes can be uneven.

Hair thinning (telogen effluvium) is occasionally reported anecdotally on significant weight-loss treatments; it is attributed to rapid weight loss and caloric reduction rather than to semaglutide itself, and it is usually temporary. Mention it to your prescriber if it concerns you.

Managing side effects practically, and when to review the plan

Semaglutide's tolerability is real, and the majority of people who stick with the titration schedule find the worst of the GI effects settle well before they reach the maintenance dose. The key variable is how quickly the dose is increased. Prescribers can pause or slow the schedule if someone is struggling, and this is an entirely legitimate clinical decision, not a failure. If your usual injection day falls at an awkward time, it is also worth knowing that taking Wegovy a day later than planned is something many people wonder about and there is clear guidance on when that is acceptable.

The cost of treatment is a fair question when you're weighing whether to continue through the initial discomfort. The treatment options and pricing page sets out what is included in a private prescription service without the hidden-fee structure some providers use. For anyone deciding whether Wegovy or another licensed treatment is the right fit, the weight loss treatment overview gives a balanced starting point.

If you're already on Wegovy and have a specific concern, the aftercare team at contact is available seven days a week. And if you're still at the decision stage, a free consultation with a prescriber is the right next step. Check your eligibility and start your free consultation when you're ready.