The real cons of weight loss injections — and what the trial data says

What the trials recorded as the most common problems

The SURMOUNT-1 trial, which tested tirzepatide across 2,539 adults over 72 weeks, and the STEP 1 trial for semaglutide 2.4mg both reported that gastrointestinal side effects were by far the most frequent reason participants noticed problems. Nausea topped the list in both programmes. Vomiting, diarrhoea, constipation, indigestion and fatigue followed. The good news buried in the same data is that most of these effects were rated mild to moderate and tended to ease as the body adjusted, particularly after the first few weeks at a new dose level.

That adjustment window matters in practice. The titration schedule for both medicines starts at a low dose specifically to reduce GI disruption; the early weeks are not representative of what treatment feels like long-term. Still, for some people the nausea is enough to put them off, and that is a legitimate reason to pause and discuss alternatives with a prescriber rather than push through.

The MHRA issued a Drug Safety Update in January 2026 flagging acute pancreatitis as a less common but potentially serious side effect of GLP-1 medicines. Severe stomach pain that reaches the back, especially if it does not settle, warrants urgent medical attention. The MHRA's Yellow Card scheme exists partly so patients can report suspected reactions of this kind, it is worth bookmarking if you start treatment.

The weight-regain problem: what stopping actually means

This is probably the most clinically significant con, and it is not talked about enough. The weight-loss injections currently licensed in the UK work while you take them. When you stop, appetite typically returns to its pre-treatment pattern and weight follows. The STEP 1 extension data showed participants regained around two-thirds of the weight lost within 12 months of stopping semaglutide 2.4mg, a finding that reframed how clinicians think about these medicines. They are not a course of treatment with a defined end-point; they are closer to a chronic-disease medication for most people who benefit from them.

NICE's recommendation for semaglutide under TA875 acknowledges this by capping NHS use at two years within specialist services, which is a pragmatic funding decision rather than a clinical one, the evidence of regain after stopping applies regardless of the two-year limit. For tirzepatide there is no equivalent two-year cap in the NICE recommendation, but the same biological reality applies. Anyone starting these medicines should think clearly about what long-term use means for them financially and practically, not just what the first six months looks like.

If you want a fuller picture before deciding, the pros and cons of weight loss injections page sets both sides side by side.

Practical friction: the injection itself, storage and the prescription requirement

Not everyone minds injections, but some people do. Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are subcutaneous injections given once a week into the abdomen, thigh or upper arm, with the site rotated each time. The pre-filled pens are designed to make this straightforward (you press and hold rather than push a plunger) but the idea of a weekly self-injection is a genuine barrier for some people. If you are weighing up your options, our page on the mojo weight loss injection explains how that particular treatment works and whether it might suit you better. The oral semaglutide tablet (Wegovy tablets, approved by the MHRA in June 2026) removes that barrier, though with a different morning routine attached.

Storage is another small but real inconvenience. The pens must be kept refrigerated between 2–8°C; the Patient Information Leaflet gives the exact room-temperature window for when refrigeration is not available, and it is worth reading before any travel. When a pen arrives (typically in a plain, unbranded box with DPD tracking on your phone) it goes straight into the fridge, not a cupboard.

The prescription requirement is not a con in itself, but it does add a step. You cannot legally obtain these medicines in the UK without a clinical assessment; a prescriber has to judge whether they are appropriate for you. Sellers offering them without that step are operating outside the law, and the MHRA has documented serious harm from counterfeit pens bought outside the legitimate supply chain. The prescription requirement is, in practice, one of the protections. Our consultation process is how that step works at nume, reviewed by a real prescriber, not automated software.

Cost: the honest picture since September 2025

Private treatment is not cheap, and it became considerably less cheap after Eli Lilly raised Mounjaro's UK list price from around £122 to around £330 per four-week pen at the highest dose from 1 September 2025, a change widely reported at the time. Across providers, private pricing for the GLP-1 medicines now typically ranges from roughly £149 to £375 per month depending on the dose and the service. That figure recurs every month for as long as you continue treatment, which for most people who benefit is measured in years rather than months.

NHS access exists but is narrowly defined and phased. Tirzepatide became available to a second NHS cohort in June 2026 (BMI 35–39.9 with four or more qualifying conditions), and further cohorts are planned. Most people currently considering weight loss injections privately either fall outside those criteria or do not want to wait. For context on what private pricing includes (and what some headline prices leave out), the cheap weight loss injections page covers that ground specifically.

Price is not a reason to avoid treatment if it is right for you. But it is a reason to go in with realistic expectations, and to make sure you are paying for genuine, clinically supervised supply rather than a lower number on a website. If you are still deciding which medicine might be the best fit, our overview of 3 weight loss injections compares the main options currently available and may help you narrow things down. If you want to understand the full picture before making a decision, speaking to our prescribers is the right starting point, the consultation is free and there is no obligation to proceed.