Weight Loss Injections FAQs: Decisions, Facts and What to Expect

How do I know if I actually qualify for one of these injections?

Eligibility starts with BMI. The UK licences for both Mounjaro and Wegovy cover adults with a BMI of 30 or above, or a BMI of 27 or above where at least one weight-related condition is present, for example, high blood pressure, high cholesterol, prediabetes, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance, reflecting differences in metabolic risk. But a number on a scale is only the starting point. A prescriber also looks at your medical history, current medicines, and any conditions that would make these medicines unsuitable, a history of medullary thyroid carcinoma or MEN2 syndrome, certain gastrointestinal conditions, or pregnancy, for instance. The weight loss treatments overview sets out the licensed conditions and BMI thresholds in one place if you want to check before starting a consultation. BMI alone never guarantees approval; the full clinical picture does.

On the NHS, the criteria are considerably stricter and access is phased, most people who are eligible by licence are not yet eligible by NHS commissioning rules. If you are wondering whether GP weight loss injections might be an option for you through that route, our guidance page explains how NHS commissioning currently works and where the gaps lie. A private route through a regulated pharmacy is the alternative for those who meet the licensed eligibility but are outside the current NHS cohorts, or who would prefer not to wait.

What actually happens at the injection site, and does it hurt?

Both pens deliver a small subcutaneous dose into the fatty tissue just beneath the skin. The three recommended sites are the abdomen, the outer thigh, and the upper arm; rotating between them reduces the risk of tissue thickening at one spot. Most people describe a brief sting, sometimes nothing at all. Letting the pen reach room temperature for around 30 minutes before injecting can reduce discomfort slightly, worth knowing for mornings when the pen has been sitting in the fridge door overnight. After injecting, apply gentle pressure but do not rub; used pens go into a sharps container for safe disposal, never into household waste or recycling.

Technique matters more than most people expect. A consistent angle, a slow press of the button, and holding the pen in place for the count recommended in the leaflet all help ensure the full dose is delivered. The Patient Information Leaflet that comes with your pen covers the exact steps; our prescribers and aftercare team are available seven days a week if questions come up between doses.

Which injection is likely to be better for my situation?

This is exactly the kind of question that belongs in a clinical consultation rather than a FAQ, but the evidence gives some structure to think with. In the SURMOUNT-5 head-to-head trial published in the New England Journal of Medicine (2025), tirzepatide produced a greater average weight reduction than semaglutide 2.4mg over 72 weeks. SURMOUNT-1 reported average body-weight reductions of around 20–21% at the highest tirzepatide dose. The STEP 1 trial for semaglutide 2.4mg reported around 15% average reduction over 68 weeks, a meaningful result in its own right. Neither figure is a guarantee for any individual; biology, adherence, diet and activity all interact.

Beyond efficacy averages, the decision often turns on practical factors: whether an injection or a daily tablet suits your routine better (the three main licensed injection options page compares formats), whether you have any contraindications to one agent, or how you tolerate the titration period. For people on oral contraceptives, there is a specific interaction to be aware of with tirzepatide: absorption of the pill may be reduced during the first four weeks of treatment and for four weeks after each dose increase, so an additional non-oral method is recommended during those windows. Semaglutide does not carry the same evidence of reduced pill effectiveness, according to NHS England's guidance on weight-management injections. Your prescriber will factor in contraception, HRT, and any other medicines you take before recommending one over the other.

What should I do if I miss a dose or feel unwell after injecting?

Missed doses and side-effect management are clinical territory, the right answer for your situation depends on which medicine you are on, where you are in the titration schedule, and what exactly is happening. The general principle outlined in the leaflets is that a missed injection can be taken within a specific window after the scheduled day; beyond that window, skip it and resume on the usual day the following week. Never double up. For specific timing, the Patient Information Leaflet and your prescriber are the two authorities, not an internet search. The same applies to interactions with other medicines such as levothyroxine, which is a question our team is asked regularly.

On side effects: nausea, loose stools and indigestion are common, particularly in the early weeks of each dose. Eating smaller portions, staying well hydrated, and avoiding very fatty foods reduces the likelihood of a rough patch. What warrants urgent attention is different: severe abdominal pain that spreads to the back, with or without vomiting, should prompt same-day medical assessment given what is known about pancreatitis as an infrequent but serious risk with GLP-1 medicines, as highlighted by the MHRA Yellow Card scheme and the MHRA's Drug Safety Update from January 2026. Signs of an allergic reaction (swelling of the face, lips or throat, difficulty breathing) need emergency care. For everything else, our aftercare team is reachable seven days a week. You can also reach us through the contact page.

If you are weighing up the practical and financial side of starting treatment, our weight loss injection page covers how these medicines work, what the treatment journey looks like, and what to expect from a regulated private prescription. The weight loss injection price guide explains what legitimate private prescriptions typically include and what to watch for. When you are ready to take the next step, start your free consultation and a GPhC-registered prescriber will review your answers the same day.