What is a GLP-1 medication — how Ozempic, Wegovy, and Mounjaro actually work

Ozempic, Wegovy, Saxenda, Mounjaro, Zepbound — GLP-1 receptor agonists have become one of the most widely discussed medication classes in recent years. Understanding what they actually do, why they work, and how they differ makes it much easier to use them well.

What GLP-1 is

GLP-1 (glucagon-like peptide-1) is a hormone naturally produced in the intestines in response to eating. It has several effects:

• Stimulates insulin release from the pancreas in response to rising blood glucose
• Suppresses glucagon (which would otherwise raise blood glucose)
• Slows gastric emptying — food moves more slowly from the stomach to the small intestine
• Acts on the brain's satiety centres, reducing appetite and increasing the feeling of fullness
• Has direct effects on the brain's reward and motivation systems that reduce food-seeking behaviour

The medications in this class are analogues — synthetic versions of GLP-1 that are modified to last longer in the body than the natural hormone, which degrades in minutes.

How they produce weight loss

The primary mechanism for weight loss is appetite suppression. GLP-1 medications act on receptors in the hypothalamus and brainstem that regulate hunger and satiety. People on these medications consistently report that food is less interesting, portion sizes feel satisfying at a fraction of the previous amount, and the mental preoccupation with food significantly reduces.

Slowed gastric emptying contributes — food stays in the stomach longer, which maintains satiety signals.

In people with type 2 diabetes, improved insulin regulation also contributes to weight loss.

The main medications and their differences

Semaglutide (Ozempic, Wegovy, Rybelsus)

Semaglutide is the active ingredient in both Ozempic and Wegovy. Ozempic is approved for type 2 diabetes management; Wegovy is approved for weight loss (at a higher dose). Rybelsus is an oral form.

Weekly injection. The dose is titrated up gradually over months. Clinical trials for Wegovy showed average weight loss of approximately 15% of body weight over 68 weeks — significantly higher than any previous pharmaceutical intervention.

Tirzepatide (Mounjaro, Zepbound)

Tirzepatide is a dual agonist — it activates both GLP-1 and GIP receptors. GIP (glucose-dependent insulinotropic polypeptide) is another incretin hormone with additional effects on fat metabolism and appetite. The dual mechanism appears to produce greater average weight loss than semaglutide alone — clinical trials show approximately 20–22% body weight reduction in some groups.

Mounjaro is the diabetes approval; Zepbound is the weight loss approval.

Liraglutide (Saxenda)

The original weight loss GLP-1, administered daily rather than weekly. Less effective than semaglutide or tirzepatide at matched effort (daily vs weekly injection). Still used where the newer agents are not accessible or appropriate.

What GLP-1 medications do not do

They do not preferentially burn fat. Weight lost on a GLP-1 comes from both fat and muscle in roughly the same proportions as other weight loss approaches. Without adequate protein intake and resistance training, a significant proportion of weight lost is muscle mass.

They are not a permanent fix on their own. Clinical trial data shows that most people regain a significant portion of lost weight within 1–2 years of stopping the medication. For many people, this means considering them as long-term management rather than a short-course treatment.

They do not remove the need for nutritional attention. The appetite suppression makes adequate protein intake challenging. The most common nutritional mistake on GLP-1s is not eating too much — it is eating too little protein, which causes muscle loss and makes weight maintenance harder.

Who uses GLP-1 medications

Originally developed for type 2 diabetes (where the glucose-regulation effects are the primary goal), GLP-1 agonists are now prescribed increasingly for obesity and overweight with metabolic comorbidities. Prescribing criteria vary by country and healthcare system.

They are not appropriate for everyone. Contraindications include personal or family history of medullary thyroid cancer or MEN2 syndrome. Caution applies in pancreatitis history. Pregnancy is a contraindication for weight-loss use.

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For the practical side — what to eat, how to protect muscle, and how to manage side effects: The Complete GLP-1 Guide.

For the structured resource with protein targets, training protocols, and a 12-week plan: GLP-1 Companion: Nutrition & Muscle Retention Guide.