Weight Loss Injections: How They Work, Who They're For, and How to Get Them Safely

Step 1, understanding what these injections do before you commit

Tirzepatide and semaglutide are not appetite suppressants in the older pharmaceutical sense. They copy the action of gut hormones released when you eat, signalling to your brain that you're full, slowing the rate at which your stomach empties, and (in tirzepatide's case) acting on a second hormone pathway (GIP) that influences how fat is stored and used. The practical result for most people is a genuine reduction in hunger and portion size, not willpower borrowed from somewhere it didn't exist before.

The clinical trial evidence is substantial. In the SURMOUNT-1 study, adults taking tirzepatide at the highest dose lost an average of around 20–21% of their body weight over 72 weeks, as published in the New England Journal of Medicine. Semaglutide 2.4mg produced an average reduction of roughly 15% over 68 weeks in the STEP 1 trial. These are averages from thousands of trial participants, not promises; individual results vary and depend on dose tolerated, adherence, and lifestyle factors running alongside treatment.

It's also worth being clear about what these injections are not. Ozempic is semaglutide, but it is licensed for type 2 diabetes, not weight management, and if you want to understand how weight loss injections are used for diabetes, that distinction becomes especially important to get right. The overlap in active ingredient causes a lot of confusion online. For a direct comparison of the two weight-loss options, the weight loss injections overview covers both side by side.

Step 2, confirming you meet the eligibility criteria

Both licences are written for adults. Tirzepatide's licence covers a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol, or obstructive sleep apnoea. Semaglutide's licence mirrors that structure. UK clinical guidance also notes that lower BMI thresholds apply for people from South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds, reflecting different cardiometabolic risk profiles at equivalent BMIs.

BMI alone is not the whole picture. A prescriber will look at your medical history, current medicines, and any conditions that might make one of these treatments unsuitable. People who are pregnant, breastfeeding, or actively trying to conceive should not use either medicine. Neither is licensed for under-18s. Certain conditions (including a personal or family history of medullary thyroid carcinoma, and some gastrointestinal conditions) require careful prescriber discussion before any decision is made.

If you're unsure where you stand, the guide to which injection is used for weight loss walks through the decision factors in plain terms. NICE's recommendation for tirzepatide (TA1026) sets the NHS eligibility bar higher than the licence; private prescribing follows the licence, assessed clinically case by case.

Step 3, starting treatment: the titration schedule and what to expect

Neither medicine starts at its full dose. Tirzepatide begins at 2.5mg, a pen whose job is giving your digestive system time to adjust, not delivering the therapeutic effect straight away. The prescriber then steps the dose up, typically every four weeks, through 5mg, 7.5mg, 10mg, 12.5mg and up to 15mg, at whatever pace your body tolerates. Semaglutide follows a similar staircase from 0.25mg to the 2.4mg maintenance dose, with a higher 7.2mg dose now also MHRA-approved.

The most common side effects are gastrointestinal: nausea is the one people mention most, followed by changes in bowel habit, indigestion, and occasionally vomiting. These tend to peak after a dose increase and ease within days to a couple of weeks for most people. Staying well hydrated and eating smaller, lower-fat meals helps. One sign that always warrants prompt medical attention is severe stomach pain that spreads to the back, that can indicate pancreatitis, which the MHRA highlighted in a Drug Safety Update in January 2026 as an infrequent but serious risk with GLP-1 medicines.

Women using oral contraceptives starting tirzepatide should add a non-oral method for the first four weeks of treatment and for four weeks after each dose increase, because the medicine's effect on gastric emptying can reduce pill absorption. The practical guide to weight loss injections covers this and other day-to-day management questions.

Step 4, getting a legitimate prescription and knowing the red flags

Because these are prescription-only medicines, the safest route is a registered online pharmacy or GP practice where a real clinician reviews your information. The GPhC publishes a public register of pharmacies at pharmacyregulation.org/registers, any legitimate pharmacy should appear there. Before placing an order anywhere, take 30 seconds to check.

The MHRA has made multiple public warnings about counterfeit weight-loss pens sold via social media and unregistered sellers. In 2025 alone, around 20 million doses of illegally traded weight-loss medicines (roughly £45m street value) were seized, and a site in Northampton producing fake tirzepatide pens was raided. Sellers offering these medicines without a prescription, or prices dramatically below market rates, are reliable red flags. If you're weighing up the cost question, the page on cheap weight loss injections explains what low prices typically leave out, and why the right metric for a prescription medicine is not the headline figure.

For context on what a legitimate private service includes, getting a weight loss injection from a registered pharmacy sets out what you should expect from any compliant provider. At nume, consultations are reviewed the same day by a named, GPhC-registered Independent Prescriber, a real clinician reads your answers. Treatment dispensed before 12pm on a working day ships that afternoon and arrives free the next working day via tracked DPD delivery, in plain packaging. All of that is covered by one transparent price: full detail is on our weight loss treatments page, where you can also check your eligibility.