Side Effects of Weight Loss Injections: What to Expect and When to Act

The most likely side effects and why they happen

GLP-1 and dual GIP/GLP-1 medicines slow the rate at which food leaves the stomach. That mechanism is precisely what reduces appetite and supports weight loss — but it also explains the gut-related symptoms that many people notice early on. Nausea is the most commonly reported, followed by vomiting, diarrhoea, constipation, burping and acid reflux. Fatigue, headache and dizziness also appear in trial data, and some people notice mild reactions at the injection site itself.

The NHS medicines page for tirzepatide sets out the full frequency breakdown for Mounjaro; the equivalent page for semaglutide covers Wegovy's profile. Both are worth reading before you start. The pattern across both medicines is similar: symptoms are most noticeable in the first four to eight weeks, or in the days after stepping up to a higher dose. For the majority of people they ease considerably once the body has adapted.

A few practical things reduce the intensity. Eating smaller portions, avoiding fatty or spicy food during the first weeks, and staying well hydrated all help. If you find nausea reliably strikes at the same time each week, some people choose to inject on a Friday evening so the worst of it falls over the weekend rather than a busy workday, worth considering if your life runs to a predictable weekly rhythm. None of this replaces advice from your prescriber, who can also discuss whether to slow titration if symptoms are persistent.

For a fuller picture of what these injections do beyond side effects, the effects of weight loss injections page covers both the therapeutic and the adverse side of the picture.

Side effects that need prompt medical attention

Most side effects are manageable. A smaller number are serious enough to warrant contacting a doctor the same day or calling 999.

Pancreatitis is the one the MHRA specifically highlighted in its January 2026 Drug Safety Update for GLP-1 medicines. The warning sign is severe, persistent stomach pain that may spread through to your back, sometimes with vomiting, sometimes without. If that happens, stop eating and drinking and get medical help straight away. Do not wait to see if it settles.

Gallbladder problems are another documented risk. Symptoms include sudden pain in the upper right abdomen, which can come and go or build steadily. Severe vomiting or diarrhoea that prevents you keeping any fluids down can lead quickly to dehydration and kidney strain, which also merits a call to NHS 111 rather than a wait-and-see approach. Signs of an allergic reaction (rash, swelling of the face or throat, difficulty breathing) require emergency help immediately.

People with type 2 diabetes who are also taking insulin or a sulphonylurea should be aware that hypoglycaemia risk may increase; blood sugar monitoring and medication review with a prescriber matters here. For a detailed look at the more serious end of the spectrum, this page on more serious side effects goes into each in more depth.

Who is more likely to experience side effects, and what affects severity

There is no reliable way to predict exactly how any individual will respond, but certain factors shift the probabilities. Starting at the lowest dose and titrating slowly (as the licensed schedule for both Mounjaro and Wegovy is designed to do) meaningfully reduces GI severity for most people. People who have had pancreatitis before, or who have specific gallbladder or thyroid conditions, require careful prescriber assessment before starting; the same applies to anyone with a personal or family history of medullary thyroid carcinoma or MEN2 syndrome.

Pregnancy, breastfeeding and actively trying to conceive are all situations where these medicines are not recommended. Under-18s fall outside the licensed population. The prescriber's role (before any treatment is issued) is to map your health history against these considerations and make a clinical judgement. This is not a formality; it is the part that actually keeps you safe.

If you are considering treatment and want to understand what you are likely to experience, this plain-language overview of weight loss injection side effects answers the most common questions our prescribers hear. You can also explore weight loss injections in the UK more broadly, including how the medicines differ from one another, or read about Saxenda injections for weight loss if you are weighing up which option suits your situation.

Reporting side effects and buying safely

Both Mounjaro and Wegovy carry a Black Triangle (▼) designation in the UK, which means the MHRA is actively gathering post-market safety data. If you experience something unexpected, our guide to the side effects of weight loss injections can help you decide whether what you are noticing is typical or something to raise with a clinician, and reporting through the Yellow Card scheme contributes to that safety picture for everyone who takes these medicines. You do not need to be certain something was caused by the medicine, a suspicion is enough to file a report.

One safety point that cannot be overstated: fake weight loss pens have been seized in large quantities across the UK, and some have been found to contain insulin rather than the expected medicine. The only way to be confident in what you are injecting is to obtain it through a pharmacy you can verify on the GPhC register. At nume, every medicine is sourced through the licensed UK supply chain and dispensed from our own GPhC-registered pharmacy. Our prescribers (you can read about our clinical team on the clinical team page) review every consultation personally before anything is dispatched.

If you have questions that are not answered here, our FAQs cover a wide range of practical topics, and our aftercare team is available seven days a week if you are already a patient.