Semaglutide Injection for Weight Loss: Your Questions Answered

What exactly is a semaglutide injection, and how does it reduce body weight?

Semaglutide is a synthetic version of GLP-1, a hormone your gut releases naturally after eating. The injection version (Wegovy) binds to GLP-1 receptors in the brain's appetite-regulation centres and in the stomach itself. The result is a combination of effects: you feel full sooner, the sensation of hunger between meals is quieter, and food moves through the stomach more slowly. None of this is willpower; it is physiology. Weight is shaped by biology, and GLP-1 medicines work with that biology directly.

Novo Nordisk manufactures the pre-filled Wegovy pen. Each pen delivers one weekly dose beneath the skin (typically into the abdomen, thigh, or upper arm) and the injection schedule starts low to give your system time to adjust. The semaglutide weight-loss injection was the first GLP-1 medicine to receive a UK weight-management licence, and for several years it was the only option. That changed with the approval of tirzepatide, which activates two receptors rather than one, but more on that shortly.

For anyone nervous about self-injecting: the Wegovy pen is pre-set and the needle is fine and short. Most people find the routine unremarkable after the first couple of uses. A question our prescribers hear most weeks is whether the injection hurts, the honest answer is that for the vast majority of patients, it barely registers.

What does the trial evidence say about how much weight people lose?

The clearest data comes from STEP 1, published in the New England Journal of Medicine, which followed adults with obesity over 68 weeks. Participants taking semaglutide 2.4 mg alongside lifestyle support lost an average of around 15% of their starting body weight, compared with roughly 2.4% on placebo. That is a clinically meaningful difference, and it was consistent enough that NICE recommended semaglutide (TA875) for use in specialist weight management services in England.

In January 2026 the MHRA approved a higher 7.2 mg maintenance dose, and a dedicated single-dose 7.2 mg pen followed in April 2026. Early trial data at 7.2 mg showed average losses of around 20.7% over 72 weeks, putting it much closer to the results seen with tirzepatide at its highest dose. Whether that higher-dose pen is appropriate for a given individual is a clinical conversation, not a consumer choice. Those exploring the full range of weight-loss injections available in the UK will find both semaglutide and tirzepatide are now licensed options, with different mechanisms and slightly different evidence profiles.

One practical note: NICE's recommendation for semaglutide comes with a two-year maximum duration within specialist NHS services. Private prescribers follow the licensed indications and make individual clinical decisions, which may differ in scope.

Who meets the criteria for a semaglutide weight-loss injection?

The licensed eligibility criteria for Wegovy are: adults aged 18 or over, with a BMI of 30 or above, or a BMI of 27 or above combined with at least one weight-related condition such as high blood pressure, raised cholesterol, type 2 diabetes, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. BMI alone does not confirm suitability; a prescriber also considers your medical history, current medicines, and whether there are any contraindications.

Semaglutide is not recommended during pregnancy, while breastfeeding, or when actively trying to conceive. It is not licensed for under-18s. People with a personal or family history of certain thyroid conditions, or a history of pancreatitis, will need a careful prescriber assessment before proceeding. The NHS medicines page for semaglutide lists the full set of cautions and contraindications.

On the NHS, access follows phased eligibility tied to NICE guidance, waiting lists are commonly long, and criteria are strict. For people who do not currently meet NHS thresholds or who cannot wait, a private prescription route through a regulated online pharmacy is the alternative. Our guide to weight-loss injection costs covers what private pricing typically involves, including what a legitimate service should always include.

What side effects should you know about before starting?

The most common side effects are gastrointestinal: nausea, loose stools, constipation, indigestion, and occasional vomiting. These tend to be most noticeable when treatment starts or after a dose increase, and they usually ease within a week or two as the body adapts. Fatigue, headache, and mild dizziness are also reported, particularly early on.

More serious but less common risks include gallbladder problems and acute pancreatitis. In January 2026, the MHRA issued a Drug Safety Update reminding prescribers and patients that severe, persistent stomach pain (especially pain that spreads toward the back) should prompt urgent medical attention, as it can indicate pancreatitis. This is an infrequent side effect, but it is one worth knowing. Suspected side effects can be reported directly via the MHRA's Yellow Card scheme, which also accepts reports about suspect sellers.

One interaction worth flagging specifically: women taking oral contraceptive pills should use an additional non-oral method of contraception for the first four weeks of semaglutide treatment and for four weeks after each dose increase, because GLP-1 medicines can slow stomach emptying and may reduce pill absorption. Discuss any existing medications with your prescriber before starting. If you are currently out of work and wondering whether treatment is still accessible, our page on weight loss injections for unemployed people explains the options available to you. If you want to understand how this injection compares to other options, our page on injectable peptides for weight loss sets out the landscape clearly.

At the end of a consultation, if you are clinically suitable and want to proceed, a prescriber will decide on the right starting point. Speak to our prescribers at the consultation page to have your questions answered by a real clinician, the same day.