Wegovy 2.5mg Side Effects: What Happens in the First Weeks

Step 1: Understanding why 2.5mg comes first

Wegovy's prescribed schedule begins at 2.5mg for a reason that has nothing to do with weight loss and everything to do with tolerability. The lowest dose gives your gut time to adapt to a medicine that slows gastric emptying, signals fullness to the brain, and adjusts the pace at which food moves through the digestive system. Starting lower means the side-effect burden tends to be lighter than at later doses. A common misconception is that people who feel fine at 2.5mg have somehow avoided semaglutide's effects, in reality, the dose is doing its job; it is simply designed to settle your system rather than drive maximum weight loss from week one.

Because semaglutide works on GLP-1 receptors in the gut and brain, even this starting dose can produce noticeable changes in appetite and digestion. The NHS medicines page for semaglutide lists the full side-effect profile across the treatment schedule. At 2.5mg specifically, the effects that people most commonly describe are digestive, nausea being the most frequent, followed by changes in bowel habit. These tend to peak in the first few days after each injection and then settle. If you are also curious about what changes as the dose increases, our page covering what to expect from Wegovy 0.5mg side effects and the full Wegovy dosing schedule put this step in broader context.

Step 2: The side effects most people notice at 2.5mg

Nausea sits at the top of the list. It is usually mild, arrives a few hours after the injection, and fades within a day or two for most people. Eating smaller portions and avoiding very fatty or rich food around injection day can help. Constipation and loose stools both appear in the trial data, they sometimes alternate, which can feel odd, but both typically settle once the body has adjusted to the new digestive rhythm. Burping, indigestion, and a general sense of fullness faster than usual are also common, especially in the first two weeks.

Fatigue and mild headaches are reported by some people, often in the first week. These are not specific to 2.5mg but can be more noticeable when the body is adapting. Injection-site reactions (redness, itching, or minor discomfort at the abdomen, thigh, or upper arm) are generally brief and tend to resolve on their own. Rotating the injection site each week reduces the chance of localised irritation. For those wondering how the GI profile shifts as the dose climbs, our guide to Wegovy 1mg side effects shows how symptoms can change before you reach the higher maintenance doses, while our page on Wegovy 2.4mg side effects covers the stage where the GI profile is typically most pronounced.

Step 3: When to get medical help

Most people move through the 2.5mg period without anything alarming. A smaller number do experience symptoms that need prompt attention. Severe stomach pain (particularly pain that is persistent and spreads toward the back, with or without vomiting) should be assessed the same day. This pattern can indicate acute pancreatitis, which the MHRA's January 2026 Drug Safety Update identified as an infrequent but serious risk with GLP-1 medicines. Do not wait to see if it passes.

Signs of a significant allergic reaction (swelling of the face, lips or throat, difficulty breathing, or a widespread rash) also need emergency attention. Symptoms of gallbladder problems (upper right abdominal pain, jaundice) should be discussed with a doctor promptly. If you experience severe vomiting or diarrhoea that you cannot keep fluids down from, dehydration can develop quickly and your GP or 111 can advise on next steps. Anyone taking Wegovy who develops low mood or thoughts of self-harm should contact their prescriber or a mental health service without delay. You can report any suspected side effect to the MHRA at yellowcard.mhra.gov.uk, this helps build the national safety picture for medicines like Wegovy.

Step 4: What to tell your prescriber before and after starting

A clinician assessing you for Wegovy will ask about your medical history for good reason. A personal or family history of medullary thyroid cancer or a condition called MEN2, a history of pancreatitis, or certain gastrointestinal conditions all affect whether semaglutide is appropriate. These are not bureaucratic questions, they are the filter that keeps the medicine safe for the people who take it. At nume, every consultation is personally read by a GPhC-registered Independent Prescriber on the day it is submitted. No algorithm makes the call.

Once treatment starts, your prescriber wants to know if you are struggling. Side effects that do not settle, that are affecting your ability to eat or drink, or that feel disproportionate are worth flagging rather than tolerating. The schedule typically stays at 2.5mg for four weeks before a prescriber considers moving upward, and our page on Wegovy 1.7mg side effects is a useful reference for understanding what to expect at the next stage of the journey. If you have questions about what the journey looks like from this point, our overview of how Wegovy works covers the full picture, and our frequently asked questions address common concerns. If you are ready to talk through your own situation, our prescribers discuss suitability and side effects as part of the free consultation.