GLP-1 Weight-Loss Medications (Ozempic, Wegovy, Mounjaro): What the Science Says

What Are GLP-1 Weight-Loss Medications?

GLP-1 receptor agonists are a class of prescription medicines that were first developed to treat type 2 diabetes and are now also approved for chronic weight management. Semaglutide is the active ingredient sold as Ozempic (for diabetes) and Wegovy (for weight loss). Tirzepatide is sold as Mounjaro and Zepbound; it is a dual agonist, acting on both the GIP and the GLP-1 receptor rather than GLP-1 alone. All of them are given as a once-weekly injection under a doctor's supervision.

How They Work

These medicines mimic gut hormones (incretins) that the body releases after eating. By acting on appetite centres in the brain and slowing the rate at which the stomach empties, they increase the feeling of fullness (satiety) and reduce hunger. Most people therefore eat less without consciously trying to, which — combined with diet and physical activity — leads to weight loss over time. They are not stimulants and they do not 'burn' fat directly; they change appetite and eating behaviour.

What the Pivotal Trials Showed

Two large randomised trials, both published in the New England Journal of Medicine, define what these drugs can achieve. In STEP 1, once-weekly semaglutide 2.4 mg was tested in 1,961 adults with overweight or obesity (without diabetes) alongside lifestyle intervention. In SURMOUNT-1, once-weekly tirzepatide was tested in 2,539 adults at three doses. In both trials the results below reflect the average change from starting weight over the full study, after gradual dose titration.

How to Read the Calculator's Projection

The calculator applies the average percentage change from these trials to the weight you enter. It is an educational estimate of what the trial average would look like for someone of your starting weight — nothing more. It is not a personal prediction, not medical advice, and not dosing guidance. Real-world results vary widely from person to person: the trial figures were achieved over many months, with gradual dose increases (titration) and structured lifestyle support, and some participants lost far more or far less than the average. Your own outcome depends on factors a calculator cannot know.

Side Effects and Safety

The most common side effects are gastrointestinal — nausea, vomiting, diarrhoea and constipation — and are usually mild to moderate and most noticeable when the dose is being increased. Because these are potent prescription-only medicines with contraindications and possible serious effects, they should only be used under medical supervision, with the right candidate assessment, dose titration and monitoring. A doctor can weigh the benefits against your personal medical history.

What Happens If You Stop?

Weight loss from GLP-1 medicines is generally maintained only while the treatment continues. In the STEP 1 trial extension, participants who stopped semaglutide regained roughly two-thirds of the weight they had lost within a year of discontinuation, and much of the improvement in cardiometabolic markers reversed as well. In practice this means these drugs are used as an ongoing treatment for a chronic condition rather than a short course — a point worth discussing with your doctor before starting.

When to Talk to a Doctor

If you are considering a GLP-1 medication for weight loss, speak with a doctor or another qualified prescriber. They can confirm whether you are a suitable candidate, review your medical history and other medicines, explain the realistic benefits and risks, and arrange the titration and monitoring these treatments require. Use this article and the calculator to prepare questions — not to self-prescribe.