Sema, semaglutide — what it actually is and what it does

What does semaglutide actually do inside the body?

The GLP-1 hormone is released naturally after eating. It signals fullness to the brain, slows the pace at which food leaves the stomach, and plays a role in how the body manages blood sugar. Semaglutide is engineered to bind to the same receptors but last far longer than the natural hormone — long enough to sustain those signals across a full week from a single weekly injection, or across a day from a daily tablet.

The result, in practical terms, is that most people find they feel satisfied on smaller meals and are less preoccupied with food between them. That shift in appetite is not willpower; it is pharmacology working on the biology that drives hunger. The NHS patient information for semaglutide explains this mechanism and lists the conditions it is used for in plain language worth reading before your consultation.

One point worth knowing: semaglutide is a GLP-1 receptor agonist only. A newer medicine, tirzepatide (Mounjaro), activates both GLP-1 and GIP receptors. Whether that distinction matters for you is a clinical conversation, not a marketing one. Our weight-loss treatment overview sets out what is available and what the evidence shows for each.

Which conditions and BMI thresholds make someone eligible for semaglutide?

Wegovy's UK licence covers adults with a BMI of 30 or above, or adults with a BMI of 27 to 29.9 who also have at least one weight-related health condition, for example, high blood pressure, raised cholesterol, prediabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance, typically 2.5 kg/m² below the standard figures.

A BMI number on its own does not settle the question. The prescriber also considers your medical history, any medicines you already take, and whether there are conditions that would make semaglutide unsuitable. Pancreatitis history, certain thyroid conditions, pregnancy, breastfeeding, and trying to conceive are among the reasons a prescriber might advise against it. Semaglutide is not licensed for anyone under 18.

The NICE appraisal of semaglutide (TA875) sets out the NHS eligibility criteria, which are narrower than the licensed indication and include a maximum treatment duration of two years within a specialist weight management service. Private prescribing follows the licensed indication rather than the NHS criteria, though clinical assessment is equally thorough. If you want to see where you stand, checking your eligibility through a free consultation is the clearest next step.

What does the side-effect picture look like in practice?

The most commonly reported effects are gastrointestinal. Nausea, loose stools, constipation, burping, indigestion and occasionally vomiting are listed in the trial data and on the NHS medicines page. For most people these effects are most noticeable in the first few weeks of treatment or after a dose increase, and they tend to settle as the body adjusts.

Starting on a low dose and moving up gradually (the standard titration approach) exists precisely to give the digestive system time to adapt. Going slowly is not a compromise; it is the designed approach. Our semaglutide information page covers the dosing ladder in more detail.

Less common but more serious effects include acute pancreatitis. The MHRA highlighted this in a Drug Safety Update in January 2026, advising people to seek urgent medical attention for severe, persistent stomach pain that extends to the back. Gallbladder problems, allergic reactions and significant mood changes are also listed as reasons to contact a healthcare professional promptly. None of this is a reason to avoid the medicine, but it is a reason to be under proper clinical supervision while taking it. You can report any suspected side effect through the MHRA Yellow Card scheme.

How is semaglutide for weight loss obtained legally in the UK?

Semaglutide for weight management is a prescription-only medicine. The legal route is straightforward: a registered prescriber assesses your health and, if the medicine is clinically suitable, issues a prescription that is dispensed by a GPhC-registered pharmacy. There is no shortcut around that process, any seller offering semaglutide without a prescriber's assessment is operating outside UK law.

The MHRA has seized large quantities of fake weight-loss pens sold through social media and unverified websites. Some contained substances that had nothing to do with semaglutide. The safest check is to verify any online pharmacy on the GPhC register before handing over personal or payment details. Prices far below the market rate and the absence of any clinical assessment are the clearest red flags.

At nume, every consultation is read by a GPhC-registered Independent Prescriber on the day it arrives, not reviewed by software and forwarded. Orders placed by midday on a working day are dispatched the same day, once approved, and delivered free by DPD the next working day in plain packaging. For more about how the service works and the team behind it, the about us page has the detail. If cost is a factor in your thinking, our Wegovy cost page explains what a legitimate private prescription covers and what market prices currently look like. Some people find it useful to read that before a consultation, particularly if payday or a bank holiday is affecting their timing.

If semaglutide turns out not to be the right fit after your assessment, our prescribers can discuss alternatives. The full Wegovy information page is a good place to read further, and the clinical evidence underpinning semaglutide goes into the trial data in more depth. If you are interested in the regulatory background, our page on the EMA's assessment of semaglutide covers how the medicine was evaluated for approval in Europe, and our guide to starting semaglutide at the 1 mg dose explains what that stage of treatment typically involves.