What is semaglutide and how does it work?

DoktorABC editorial team

Accurate, up-to-date health information written by our editorial team and reviewed by UK-registered prescribers.

What is semaglutide?

Semaglutide is the active ingredient, not a brand name. It is a GLP-1 receptor agonist, mimicking a gut hormone involved in appetite and blood sugar regulation. The same ingredient appears under three UK brand names for different licensed uses. Wegovy is licensed for weight management. Ozempic and Rybelsus also contain semaglutide but are licensed for type 2 diabetes, not weight loss, and should never be presented as interchangeable.

How does semaglutide work?

Semaglutide acts on appetite centres in the brain to reduce hunger and cravings, increases fullness after smaller meals, and can slow early gastric emptying. It does not burn fat directly; its effect comes from supporting a sustained reduction in food intake, which is why it is licensed alongside a reduced-calorie diet and increased activity, not as a standalone treatment.

Semaglutide injection versus tablets

Two semaglutide formats are licensed for weight management, distinct from Rybelsus (oral semaglutide for diabetes).

Who may be eligible for semaglutide in the UK?

Eligibility depends on the route. For a private prescription, the MHRA licence requires a BMI of 30 or above, or 27–29.9 with a weight-related condition, alongside diet and activity changes. NHS access is stricter: under NICE TA875, Wegovy is normally only available through specialist weight management services, for adults with a BMI of 35 or above with a comorbidity, or in some cases 30–34.9 if referred to a specialist service. Some ethnic groups qualify at lower thresholds. Meeting the private licence does not guarantee NHS funding, and local referral pathways vary. 

Evidence and realistic expectations

In the STEP 1 trial, adults taking Wegovy injection 2.4 mg weekly for 68 weeks alongside lifestyle support lost an average of approximately 14.9% of body weight. In OASIS 4, adults taking the 25 mg tablet for 64 weeks lost approximately 13.6% on average, rising to around 16.6% among full adherers. These are trial averages, not guarantees, and results vary with starting weight, adherence, dose, and lifestyle support.

How long does semaglutide take to work?

Appetite effects can appear within the first few weeks. Measurable weight change typically takes longer, building over several months as the dose reaches maintenance, with the most substantial change usually seen between six and twelve months. Regular follow-up allows the plan to be adjusted based on response.

Side effects and safety

The most common side effects, for both pill and injection formats, are gastrointestinal: nausea, diarrhoea, constipation, vomiting, and abdominal discomfort, most pronounced during dose escalation. Less common but more serious risks include pancreatitis, gallbladder problems, and dehydration-related kidney issues. It is not suitable in pregnancy or breastfeeding, requires caution alongside diabetes medicines due to hypoglycaemia risk, and should be flagged ahead of any procedure involving anaesthesia.

Tirzepatide vs semaglutide

Tirzepatide (Mounjaro) is a dual GIP/GLP-1 agonist, acting on two hormone pathways rather than one. In the head-to-head SURMOUNT-5 trial over 72 weeks, average weight loss was approximately 20.2% with tirzepatide versus 13.7% with semaglutide. Both are weekly injections with similar side effects. Tirzepatide produced greater average results, but tolerability and individual response mean neither is universally "best."

Semaglutide vs liraglutide

Liraglutide (Saxenda, now also generic) is an older GLP-1 agonist requiring a daily injection rather than weekly. In the STEP 8 trial over 68 weeks, average weight loss was approximately 15.8% with semaglutide versus 6.4% with liraglutide. The choice depends on tolerability and clinical assessment.

Final thoughts

Semaglutide is a well-evidenced GLP-1 receptor agonist licensed for weight management as Wegovy, available as both an injection and a tablet. It is not the same product as Ozempic or Rybelsus, which are licensed for diabetes, despite sharing the same active ingredient. Results vary between individuals and treatment requires ongoing clinical supervision.

DoktorABC connects UK patients with licensed prescribers who can assess suitability for semaglutide and issue private prescriptions through GPhC-registered pharmacies where appropriate.

FAQ

The DoktorABC medical advisory board

Dr. Roland Ruiken

Medical advisor, Norway

Dr. Viktor Simunovic

Medical advisor, Croatia

To the medical advisory board