What Are the Real Risks of Weight Loss Injections?

Which side effects do most people actually experience?

The majority of people taking weight loss injections notice gastrointestinal effects first. Nausea is the most frequently reported, followed by vomiting, loose stools, constipation, and reflux. Fatigue, headache and dizziness also crop up, particularly in the first few weeks of treatment or after moving to a higher dose. Injection-site reactions (redness, mild bruising or soreness at the site) are common with any subcutaneous injection and generally settle quickly.

These effects are real and can be disruptive, but for most people they ease as the body adjusts. Eating smaller portions, avoiding rich or fatty foods, and staying well-hydrated are the practical steps that tend to help. Slowing titration (spending longer at a lower dose before increasing) is something to discuss with your prescriber if symptoms feel unmanageable rather than just uncomfortable.

It is worth knowing that the GI profile differs slightly between tirzepatide and semaglutide, and between individual patients. What one person experiences at 5mg, another may barely notice at 10mg. This variability is one reason a prescriber reviews your response throughout treatment, not just at the beginning. If you want to compare the three weight loss injections currently available, our overview covers the licensed options in more detail before you start. Our overview of weight loss injections covers the two licensed options in more detail if you want to compare them before starting.

What are the serious risks that warrant urgent medical attention?

Most side effects are unpleasant rather than dangerous, but a handful of symptoms should prompt you to stop and seek help immediately rather than waiting to see whether they pass.

Acute pancreatitis is the most significant safety concern the MHRA has highlighted. In January 2026, following a review of real-world data, the MHRA updated its Drug Safety Update to flag pancreatitis as an infrequent but serious risk associated with GLP-1 medicines as a class. The signal to act on is severe, persistent pain in the abdomen or upper stomach that radiates to the back, with or without vomiting. That combination, especially if it does not ease, means stopping treatment and getting medical assessment the same day.

Other symptoms requiring prompt attention include: signs of a serious allergic reaction (swelling of the face, lips or throat, difficulty breathing, rash); significant dehydration from persistent vomiting or diarrhoea (dizziness on standing, reduced urination, rapid heartbeat); and new or worsening symptoms of low mood or thoughts of self-harm, which should be discussed with a GP or mental health professional without delay.

Gallbladder problems (gallstones and cholecystitis) are also reported more often in people losing weight rapidly, regardless of the method. Right-sided or central upper abdominal pain, particularly after eating, warrants investigation. There is a broader discussion of risk patterns on our full risks page.

Are there people for whom these injections carry a higher risk?

Yes, and this is exactly why clinical assessment before treatment is not a formality. Both Mounjaro and Wegovy are contraindicated or require particular caution in a range of situations.

Neither medicine is recommended during pregnancy, for anyone who is breastfeeding, or for people actively trying to conceive. Women using the combined oral contraceptive pill who start tirzepatide are advised to add a non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase, because tirzepatide can slow gastric emptying in a way that reduces pill absorption. Semaglutide does not carry the same evidence-based warning, though a prescriber may still want to discuss contraception options.

A personal or close family history of medullary thyroid carcinoma or a condition called MEN2 (Multiple Endocrine Neoplasia type 2) is a contraindication for both medicines. A history of pancreatitis, or certain other digestive-system conditions, also requires careful prescriber review before treatment is started.

Anyone taking other medicines (including blood sugar-lowering drugs, anticoagulants or medicines with a narrow therapeutic window) should have those interactions assessed. The NHS tirzepatide information page lists key interactions, as does the product's SmPC. If you are considering switching from a different treatment, our team at the clinical team reviews transfer patients' dose history and current medications before any prescription is issued.

How should risks be balanced against the evidence for benefit?

Weighing risk against benefit is the honest work of clinical medicine, and it looks different for different people. For someone with a BMI of 38 and high blood pressure, the documented cardiovascular burden of long-term excess weight is itself a risk, one that these medicines have a meaningful evidence base for addressing. For someone in good metabolic health seeking a modest reduction, the calculation shifts.

This is not a theoretical point. NICE's recommendation for tirzepatide, published in late 2024 and detailed at NICE TA1026, explicitly weighs the clinical benefit data against the side-effect profile before concluding that treatment is appropriate for eligible patients. That recommendation does not cover everyone, and nor should it. It is a prompt, not a blanket endorsement.

At nume, our prescribers discuss suitability and side effects as a core part of the consultation, not an afterthought. The conversation is private, there is no pressure, and no prescription is issued if the clinical picture does not support it. If you want to understand how that process works before committing to anything, starting a free consultation costs nothing and puts the question directly to a prescriber. You can also read more about injection-related cancer risk questions if that is on your mind, or visit our FAQs for answers to common queries. If you are weighing up whether to proceed, our page on what the risks of weight loss injections actually involve sets out the full picture in plain language, and our guide to how a weight loss injection works in practice explains what to expect from treatment day to day.

One practical note: if your consultation falls over a bank holiday or you are planning to order before travelling, it is worth contacting us in advance so timing can be planned around your schedule.