GLP-1 weight loss treatments have featured heavily in the headlines. Mounjaro and Wegovy have moved from niche medical interest to genuinely mainstream, and for many people they have produced results they had not achieved through diet and exercise alone.
Alongside that has come a great deal of misinformation about what these treatments can and cannot do. Some of it sets up expectations no medicine could meet. Some of it is actively dangerous, particularly the material encouraging people to buy outside the regulated supply chain.
This guide covers what the evidence actually shows, what the treatments require of you, and the factors that separate a short-term result from a lasting one.
How these treatments actually work
It is worth being precise about the mechanism, because the two most talked-about treatments are not the same drug class.
Semaglutide, the active ingredient in Wegovy, is a GLP-1 receptor agonist. Tirzepatide, the active ingredient in Mounjaro, is a dual agonist acting on both GIP and GLP-1 receptors.
Both are incretin mimetics. They do not act on your appetite hormones so much as imitate them, binding the same receptors that gut hormones released after eating would bind. The effects are a reduction in appetite, mediated largely through the brain, and slower gastric emptying, which is why people feel fuller for longer and stay full after smaller meals.
That distinction between the two drugs matters practically, because they have different trial data, different side effect profiles and, as covered below, at least one clinically important difference in how they interact with other medicines.
What the trials show, and what they do not
In the STEP programme, semaglutide 2.4mg produced substantial mean weight loss over 68 weeks. In the group followed into the trial extension, mean loss was 17.3 per cent of body weight.
In SURMOUNT-1, tirzepatide produced mean reductions of 16.0 per cent at 5mg, 21.4 per cent at 10mg and 22.5 per cent at 15mg over 72 weeks, against 2.4 per cent for placebo.
Two things are worth understanding about numbers like these. They are means across large groups, not entitlements, and the spread around them is wide. And they were achieved inside clinical trials, with structured lifestyle support, regular contact and monitoring built in as standard. That context is part of the result, not incidental to it.
The part most people do not plan for
This is the single most important thing to understand before starting, and the part least discussed in the coverage.
These medicines treat obesity as a chronic condition. They work while you take them. When treatment stops, appetite regulation reverts, and weight tends to follow.
The STEP 1 trial extension followed participants for a year after semaglutide was withdrawn. Having lost a mean of 17.3 per cent of body weight over 68 weeks, they regained around two-thirds of it within twelve months of stopping, ending at a net 5.6 per cent below where they started.
That is not a failure of the medicine, and it is not a failure of the people involved. It is how the treatment works. But it does mean the question “what happens when I come off this?” belongs at the start of the conversation rather than the end. Whether that means long-term treatment, a planned taper alongside established habits, or a maintenance dose, it is something to decide deliberately with a clinician rather than discover by accident.
Why the dose goes up slowly
Every treatment plan starts at a low dose and increases in steps. The reason is tolerability rather than caution for its own sake.
Nausea, vomiting, constipation and diarrhoea are common, particularly in the first weeks and after each increase. Escalating too quickly makes those effects considerably worse, and gastrointestinal intolerance is the most common reason people abandon treatment entirely. Since stopping early leads to regain, a titration schedule that feels frustratingly slow is doing real work.
If side effects are severe enough to make you consider stopping, that is a conversation to have with your prescriber rather than a decision to make alone. Holding at a lower dose for longer is often the answer.
An important interaction if you take an oral contraceptive
This deserves separate attention, because it is easily missed and the consequences are significant.
Because tirzepatide slows gastric emptying, it can reduce the absorption of medicines taken by mouth, including the combined oral contraceptive pill. The effect is greatest after the first dose and after each dose increase, and it lessens over time.
MHRA advice is to use an additional non-oral barrier method, or to switch to a non-oral contraceptive such as an implant or coil, for four weeks after starting treatment and for four weeks after every dose increase.
More broadly, these medicines should not be used during pregnancy, while trying to conceive or while breastfeeding, as there is not enough safety data to establish whether they could harm a baby. Anyone who could become pregnant should have a clear contraception plan in place before starting.
Who these treatments are not for
A proper consultation exists to identify this, but it is worth knowing in advance. These medicines are generally not appropriate for people with:
- Pregnancy, breastfeeding, or plans to conceive in the near future
- A personal or family history of medullary thyroid carcinoma, or multiple endocrine neoplasia syndrome type 2
- A history of pancreatitis, given the recognised association with pancreatitis and gallbladder disease
- Severe gastrointestinal disease, including gastroparesis
- A current or previous eating disorder, where appetite-suppressing medication can interact badly with existing patterns of thinking about food
Existing medicines matter too, not only contraceptives. Anything with a narrow therapeutic window taken orally is worth reviewing, which is one of the more useful things a pharmacist can do at the outset.
Why the source of your prescription matters
A growing number of unregulated sellers have moved into this space, often skipping medical assessment entirely. The convenience is the selling point. The risk is not theoretical.
The MHRA has seized hundreds of falsified weight loss pens in the UK. In confirmed cases, fake pens sold as Ozempic and Saxenda were found to contain insulin rather than the stated ingredient. Some recipients were hospitalised, with reported effects including hypoglycaemic shock and coma. Fast-acting insulin administered to someone without diabetes can drive blood glucose low enough to be life-threatening.
There is no way to identify a counterfeit pen by looking at it. The only meaningful protection is buying through the legal supply chain, which means a UK pharmacy registered with the General Pharmaceutical Council, a genuine clinical assessment and a prescribing decision made by a registered clinician.
Properly delivered, medicated weight loss begins with a thorough consultation covering BMI, medical history and existing conditions and medicines, followed by a prescribing decision by a registered clinician. Ongoing monitoring rather than a single sign-off is what allows doses to be adjusted safely and problems to be caught early.
“Achieving meaningful, sustained and above all safe weight loss with any prescription treatment depends on several factors. Ongoing clinical monitoring, regulated dose titration and honest conversations about diet, lifestyle and what happens after treatment all contribute to whether someone does achieve sustainable, healthy and long-term weight loss results,” says Ana Carolina Goncalves, Superintendent Pharmacist at Pharmica, a GPhC-registered UK online pharmacy.
The habits that support the medication
Even with the right prescription and good clinical oversight, the medicine rarely accounts for the whole result.
Protein intake matters more than most people expect. A meaningful proportion of weight lost in any calorie deficit can come from lean tissue rather than fat, and that applies here too. Adequate protein helps preserve muscle mass, with somewhere between 1.2 and 1.6 grams per kilogram of body weight commonly recommended during active weight loss. Because appetite is suppressed, hitting that target takes more deliberate planning than usual.
Resistance training helps, even in modest amounts. Two sessions a week is enough to make a difference to how much of the loss comes from fat rather than muscle.
Sleep and stress are less discussed and no less relevant. Sleep restriction raises ghrelin and lowers leptin, the hormones that drive and suppress hunger respectively, which works directly against the effect of the medication.
None of this is about undermining the treatment. It is about giving it the best chance of working as intended, and about the quality of the weight you lose rather than only the quantity.
