Weight Loss Injections: What Generic Means and How These Medicines Actually Work

Step 1: understanding what 'generic' means for these medicines

A generic medicine contains the same active ingredient as the branded original, made by a different manufacturer once the original patent expires. For common medicines (think ibuprofen or omeprazole) generics are widely available and routinely dispensed. Weight loss injections are at an earlier stage. Tirzepatide, the active ingredient in Mounjaro, and semaglutide, the active ingredient in Wegovy, are both still under patent protection in the UK. That means no manufacturer can legally produce a licensed generic version of either medicine for weight management yet.

It is worth understanding why patents matter here. The original manufacturers invested heavily in the clinical trial programmes that proved these medicines safe and effective enough for MHRA licensing. The patent period reflects that investment. Once patents expire (likely later this decade for these specific compounds) generic manufacturers can apply to the MHRA, demonstrate equivalent performance, and bring lower-cost alternatives to market. That process takes time even after patent expiry.

If you have come across something described as a generic or compounded version of these injections being sold online, that is a different and more serious matter, covered in step three below. You can read more about the brand and generic names for weight loss injections if the terminology is causing confusion.

Step 2: how the licensed injections work and what the clinical evidence shows

Both Mounjaro and Wegovy are given as once-weekly subcutaneous injections, meaning they go just under the skin, usually in the abdomen, thigh or upper arm. They belong to a class of medicines called GLP-1 receptor agonists. Tirzepatide also activates a second receptor, GIP, making it the only dual-agonist weight loss medicine currently licensed in the UK. Both work by slowing the rate at which the stomach empties, reducing appetite signals in the brain and helping the body regulate blood sugar more steadily, all of which tend to result in eating less without the constant mental effort of counting calories. If you are curious whether treatment affects your day-to-day energy levels alongside appetite, our page on whether weight loss injections give you energy looks at what the evidence actually shows.

The clinical evidence behind these medicines is substantial. In the SURMOUNT-1 trial, participants using 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 around 15% average weight loss over 68 weeks in the STEP 1 trial. These are averages from large controlled studies, not guarantees for any individual. Results vary based on starting weight, adherence, diet, activity and a person's underlying health.

Treatment always begins at a low starter dose, not the dose that produces the biggest results, but one designed to let your body adjust gradually. The prescriber then reviews progress and titrates upward. That titration process is one reason clinical oversight matters throughout treatment, not just at the start. Our overview of how weight loss injections are prescribed and monitored covers that process in more detail.

Step 3: the risk of unlicensed and counterfeit versions

Because these medicines are in high demand and genuinely scarce in some supply chains, a market has emerged for unlicensed, compounded or outright counterfeit versions sold without a prescription through social media and unverified websites. The MHRA has issued multiple public warnings about this, including large-scale seizures of fake pens that in some cases contained insulin rather than the advertised ingredient. In 2025 alone, approximately 20 million doses of illegally traded weight loss medicines, with an estimated street value of around £45 million, were seized, and the first illicit UK site manufacturing counterfeit tirzepatide pens was raided. The MHRA's Yellow Card scheme allows anyone to report a suspect seller or a suspected adverse reaction.

The red flags are consistent: prices far below the typical market range, no clinical assessment required, promises of immediate dispatch, discount codes. For a prescription-only medicine, the legal and safe route is a prescription from a registered prescriber, dispensed by a pharmacy you can verify on the GPhC pharmacy register. If the price sounds too good, it usually is. A summary of what legitimate pricing typically looks like is available on our weight loss injections prices page, and if you are wondering why these medicines cost what they do, our page on why weight loss injections are so expensive goes through the main reasons.

Step 4: who these injections are licensed for, and what happens at consultation

Both tirzepatide and semaglutide are licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Lower thresholds apply for some ethnic backgrounds under UK guidance. Crucially, meeting the BMI threshold alone does not mean the medicine is right for you, the prescriber reviews your full medical history, current medicines, and any conditions that might make one option more or less suitable than another. Our guide to the 3 weight loss injections available in the UK is a useful starting point if you want a clear comparison of what is currently on offer before your consultation.

At nume, every consultation is reviewed by a GPhC-registered Independent Prescriber (a real clinician, not a piece of software) on the day it is submitted. Identity and current weight are verified, and for anyone transferring from another service or requesting a dose increase, clinical evidence is reviewed before supply is confirmed. Treatment is dispatched the same day for orders placed before 12pm Monday to Friday (handy if payday falls mid-month and you want your next pen without delay), and delivered free the next working day by DPD in plain packaging.

If you are still working out which option might suit your situation, our weight loss treatment overview sets out both medicines side by side. And if you have questions our FAQs do not answer, our frequently asked questions page covers the ones we hear most often. When you are ready to take the next step, you can speak to our prescribers through a free consultation, no commitment, no upfront charge, just a clinical conversation about whether treatment is right for you.