Are weight loss injections bad for you? The real risk picture

The decision most people are really making, benefit versus managed risk

Nobody asks whether blood pressure tablets are "bad for you" in isolation; they ask whether the cardiovascular protection outweighs the side-effect profile for their situation. Weight loss injections deserve the same framing. Obesity itself carries documented health consequences (cardiovascular disease, type 2 diabetes, sleep apnoea, joint damage) and the NICE appraisals for both tirzepatide and semaglutide concluded that the clinical benefit justifies their use in adults who meet the criteria. That doesn't mean the risks are trivial. It means they were weighed systematically against meaningful health gains by independent clinical bodies.

The SURMOUNT-1 trial, involving 2,539 adults with obesity, found average body-weight reductions of around 20–21% over 72 weeks at the 15mg tirzepatide dose. The STEP 1 trial showed roughly 15% average reduction with semaglutide 2.4mg. These are not small effects, and for people with weight-related conditions, the downstream health implications can be substantial. A prescriber's job is to decide whether that calculus works for you, not to rubber-stamp a request.

What this means practically: the risk of a weight loss injection is not a fixed number. It shifts with your health history, your current medicines, and whether you're getting the genuine, correctly stored product from a regulated source. If you're unsure where you sit, exploring whether these medicines are right for your circumstances is the logical first step.

Side effects that are common, and the ones that need medical attention fast

The most frequent side effects of both Mounjaro and Wegovy are gastrointestinal. Nausea, vomiting, diarrhoea, constipation, indigestion and burping are all reported, they're uncomfortable, not usually dangerous, and for most people they ease within days to a couple of weeks as the body adjusts to a new dose. Fatigue, headache and dizziness also appear in trial data. The serious side effects are rarer but important to know before you start.

Pancreatitis is the one that clinicians watch most closely. In January 2026 the MHRA issued a Drug Safety Update specifically calling out acute pancreatitis as an infrequent but potentially serious risk across GLP-1 medicines. The warning symptom is severe, persistent stomach pain that radiates to the back, sometimes with vomiting, that warrants same-day emergency care, not a wait-and-see approach. Gallbladder problems, including gallstones, are also documented. Serious allergic reactions are rare but real.

There are also interaction-specific risks worth knowing. Women taking oral contraceptives who start tirzepatide should add a non-oral method (for example condoms) for the first four weeks of treatment and for four weeks after each dose increase, because the pill's absorption can be reduced. NHS guidance notes this is specific to tirzepatide; the equivalent interaction evidence for semaglutide is different. Anyone planning surgery should tell their anaesthetist they're on a GLP-1 medicine. These are the kinds of nuances a prescriber should be discussing with you, not something to piece together from a quick search. You can report any suspected side effect directly through the MHRA Yellow Card scheme.

The supply-chain question, why where you get it matters as much as what it is

The licensed product, stored correctly and prescribed following a proper clinical assessment, has a well-understood safety profile built over years of global trial data. Counterfeit pens have no such profile. The MHRA has seized approximately 20 million doses of illegally traded weight-loss medicines (around £45m street value) since enforcement escalated through 2025, and some of the fake pens intercepted earlier contained insulin, which has caused serious harm. Sellers offering these medicines without a prescription, via social media or at prices that look implausibly low, are red flags, not bargains.

A GPhC-registered pharmacy is verifiable in seconds at the GPhC register. nume's registration number is 9012878. The medicines come through the licensed UK supply chain, stored under temperature-controlled conditions throughout. When an order is dispatched, it travels with DPD in a plain, unbranded box (tracked to your door) which tells you something about how the logistics are handled, but the real assurance comes before it's packed: every consultation is read by a named, GPhC-registered Independent Prescriber, not automated. For more on the legal route to weight loss injections in the UK, including what the process involves and what to look for in a legitimate provider, that page covers the ground in full.

The NHS also offers access to these medicines for eligible patients, under NICE guidance TA1026 for tirzepatide, the criteria are specific and the rollout is phased. For people who don't meet NHS criteria or don't want to wait, a regulated private pharmacy is the lawful alternative, not an unregistered online seller.

Who genuinely should not use these medicines

There are absolute contraindications, and any legitimate prescriber will screen for them. People with a personal or family history of medullary thyroid carcinoma, or with multiple endocrine neoplasia type 2, should not use either medicine. Active pancreatitis or a significant history of it is another hard stop. These medicines are not licensed for anyone under 18. They are not recommended during pregnancy, while breastfeeding, or when actively trying to conceive, the MHRA advises using contraception throughout treatment and for a wash-out period before attempting conception. Certain gastrointestinal conditions also require a careful prescriber conversation before treatment begins.

Beyond absolute contraindications, some people need closer monitoring than others, people with type 2 diabetes, for example, may need their diabetes medicines adjusted as weight and blood-sugar levels change. That's not a reason to avoid treatment; it's a reason to have a prescriber involved who will review your case properly, including before each repeat. To understand more about how well these medicines perform in practice and the evidence behind the results, that page works through the trial data in plain terms. If you're ready to talk to a clinician about your own circumstances, you can start a free consultation and have your answers reviewed the same day.