In recent years, the medical approach to weight loss has undergone a paradigm shift, driven largely by a class of medications known as GLP-1 receptor agonists. Drugs like Wegovy (semaglutide) and Mounjaro (tirzepatide) are producing results that were previously unachievable without bariatric surgery — and the science behind them is as elegant as it is effective.
What is GLP-1?
GLP-1 stands for Glucagon-Like Peptide-1. It is a naturally occurring hormone produced in the small intestine in response to eating. Under normal circumstances, your gut releases GLP-1 after a meal to:
- Signal to the brain that you are full (satiety signalling)
- Slow the rate at which your stomach empties (gastric emptying)
- Stimulate the pancreas to release insulin in response to glucose
- Suppress glucagon, the hormone that raises blood sugar
The problem for people living with obesity is that this natural satiety system can become dysregulated — the brain’s response to GLP-1 is blunted, hunger signals override fullness cues, and the body’s “set point” for weight becomes difficult to lower through diet and exercise alone.
GLP-1 receptor agonists work by providing a pharmacological dose of GLP-1 activity — far more powerful and longer-lasting than the natural hormone — effectively resetting the body’s hunger thermostat.
How Semaglutide Works (Wegovy)
Semaglutide, the active ingredient in Wegovy, is a synthetic analogue of human GLP-1. Its molecular structure is approximately 94% identical to natural GLP-1, with key modifications that prevent it from being rapidly broken down by the enzyme DPP-4 in the bloodstream.
These modifications give semaglutide a half-life of approximately 7 days, which is why Wegovy is effective as a once-weekly injection. Natural GLP-1 has a half-life of less than two minutes.
Once injected, semaglutide binds to GLP-1 receptors throughout the body, with its most clinically significant effects occurring in:
- The hypothalamus — the brain’s appetite-control centre, where semaglutide directly reduces hunger signals and food cravings
- The brainstem — influencing reward-driven eating and food-seeking behaviour
- The gastrointestinal tract — slowing gastric emptying so that the sensation of fullness lasts longer after smaller amounts of food
- The pancreas — improving insulin secretion and blood sugar regulation
In the landmark STEP 1 clinical trial, patients taking semaglutide 2.4mg once weekly lost an average of 14.9% of their body weight over 68 weeks, compared to 2.4% in the placebo group.
How Tirzepatide Works (Mounjaro)
Mounjaro (tirzepatide) represents the next evolution of this class of treatments. It is the world’s first dual GIP and GLP-1 receptor agonist — meaning it activates two separate hormone pathways simultaneously.
GIP (Glucose-Dependent Insulinotropic Polypeptide) is another incretin hormone released from the gut after eating. Like GLP-1, it stimulates insulin release. But its additional role in Mounjaro’s mechanism is more nuanced: when GIP receptors in the brain and fat tissue are activated alongside GLP-1 receptors, the combined effect appears to suppress appetite more powerfully than either pathway alone.
This dual action produces a synergistic effect — the weight loss seen with tirzepatide is substantially greater than with semaglutide in head-to-head comparisons, even though the underlying mechanisms share significant overlap.
In the SURMOUNT-1 clinical trial — the largest weight loss trial of a pharmaceutical agent at the time — patients taking tirzepatide 15mg lost an average of 20.9% of their body weight over 72 weeks. At the 5mg dose, average weight loss was still 15%. In a subset of patients without diabetes, the highest dose produced an average loss of 22.5%.
The Role of the Brain
One aspect of GLP-1 pharmacology that is often underappreciated is the direct effect on the central nervous system. Both semaglutide and tirzepatide cross the blood-brain barrier and act directly on:
- Appetite and satiety circuits in the hypothalamus (reducing hunger independently of stomach fullness)
- Reward pathways in the mesolimbic system (reducing the reward response to highly palatable, calorie-dense foods)
- The nucleus accumbens (associated with cravings and compulsive eating behaviour)
Many patients describe this effect not simply as “not being hungry” but as a quiet disengagement from thoughts about food — the mental preoccupation with eating that is so familiar to people who have struggled with their weight. This neurological dimension is one of the reasons these medications are so clinically different from previous appetite suppressants.
How They Compare to Older Weight Loss Drugs
Previous generations of weight loss medication worked very differently:
- Orlistat blocked fat absorption in the gut — effective but with significant gastrointestinal side effects and modest results (around 3–5% body weight loss)
- Phentermine was a stimulant-based appetite suppressant — associated with cardiovascular risk and not suitable for long-term use
- Liraglutide (Saxenda) was an early GLP-1 agonist requiring daily injections with more modest efficacy (around 5–8% body weight loss)
The arrival of once-weekly semaglutide and tirzepatide represents a step-change in both the efficacy and tolerability of pharmacological weight management. The weight losses seen in clinical trials are comparable to — and in some cases exceed — those achieved with some bariatric surgical procedures.
Are the Results Sustained?
An important question. Clinical trial data from the STEP 4 extension study showed that patients who stopped taking semaglutide regained, on average, two-thirds of their lost weight within one year. This is consistent with the understanding that obesity is a chronic, relapsing condition — not a temporary state that can be resolved with a course of medication.
For many patients, GLP-1 therapy is a long-term or indefinite treatment, similar to antihypertensive or cholesterol-lowering medication. At Prescriptly, our clinical team will work with you to establish a sustainable long-term plan, including how to manage dose tapering if and when appropriate.
The science behind GLP-1 medications represents one of the most significant advances in metabolic medicine in decades. If you would like to explore whether Wegovy or Mounjaro is right for you, start your free confidential consultation with our clinical team today.
